Prolegomena [= Preliminaries]
to any Future Metaphysicthat can Present itself as a Science
Immanuel Kant

Copyright © Jonathan Bennett 2017. All rights reserved[Brackets] enclose editorial explanations. Small ·dots· enclose material that has been added, but can be read as
though it were part of the original text. Occasional •bullets, and also indenting of passages that are not quotations,

are meant as aids to grasping the structure of a sentence or a thought. Every four-point ellipsis . . . . indicates theomission of a brief passage that seems to present more difficulty than it is worth. Longer omissions are reported
between brackets in normal-sized type.
First launched: July 2004

Last amended: April 2007

Contents
Introduction

1

Preamble on the special features of all metaphysical knowledge

7

General Problems

12

Main transcendental Problem 1: How is pure mathematics possible?

16

Main transcendental problem 2: How is pure natural science possible?

25

Main transcendental problem 3: How is metaphysics possible in general?

45

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Immanuel Kant

Conclusion: Determining the boundaries of pure reason

61

Solution of the general question of the Prolegomena: How is metaphysics possible as a science?

73

Appendix: On what can be done to make metaphysics actual as a science77

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Introduction

Introduction
These Preliminaries are meant for the use not of learners
but of future teachers; and even the teachers shouldn’t
expect this book to help them by neatly laying out a readymade science. Rather, it is to help them to discover thisscience. [Throughout this work, ‘science’ means ‘branch of knowledge

knowledge to be had from metaphysics, or to show that there
isn’t, we must once and for all reach a conclusion about thenature of this would-be science, for it can’t go on as it has
been doing. It seems close to ridiculous, when every otherscience makes steady progress, that this one—claiming to
be wisdom personified, the oracle that everyone consults—
goes on circling around the same spot, never taking a step
forwards. Its fringe hangers-on have scattered; and people
who are sure enough that they can shine in other sciences
won’t be found risking their reputations in this one, where
there are no objective standards for distinguishing soundknowledge from mere chatter, so that any ignoramus canfeel entitled to pass judgment.

that is theoretically organised, highly structured, and soundly based’.]For some learned people, philosophy is just the history ofphilosophy (ancient and modern); these preliminaries aren’t
written for them. They must wait their turn. When those
who work to draw ·truth· from the well of reason itself havedone their work, then the historians can give the world thenews about their results. ·But they won’t regard it as news,
because· nothing can be said now that the historians won’t
think has been said already! And it is safe to predict thatthey’ll think the same about anything said in the future;human understanding has busied itself for centuries withcountless topics in many ways, so it is to be expected thatevery new idea will resemble something that has been saidin the past.
If you think that metaphysics is worth studying, my aim
is to convince you of the following:
It is absolutely necessary that you stop your work
for a while, regard anything that has been done as
not having been done, and face up to the preliminary
question of whether such a thing as metaphysics iseven possible.
If it is a science, why can’t it get universal and lasting
approval, like other sciences? If it is not, what enables it to
go on giving itself airs with its pretence of being a science,
keeping men’s minds in suspense with hopes that neverdie but are never fulfilled? If we are to show that there’s

There’s nothing extraordinary in the idea that whenpeople have worked hard at a science they should wonder•how much progress it has made, and be led from that towonder •whether such a science is possible at all. Humanreason so loves building that it has repeatedly built a tower·of theory· and then dismantled it to check the soundnessof the foundation. It is never too late to become reasonable
and wise; but if an insight comes late, it will be that muchharder to make use of it.
When we ask whether a certain science is possible, that
presupposes that we have doubts about whether it is actual.
That doubt will shock anyone whose whole fortune, perhaps,
consists in this supposed jewel ·called ‘metaphysics’·; andso anyone who voices the doubt can expect to be attacked
on all sides. Some of the attackers—clutching their big metaphysical books, and proudly conscious of their ·intellectual·possessions, which they think are legitimate because they
are old!—will look down on him with contempt. Others, for1

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Immanuel Kant

whom everything they see is all of a piece with something
they have seen before, won’t understand him. And for awhile things will stay as they were, as though nothing had
happened to raise the hope or the fear of an impending
change.
Nevertheless, I confidently predict that if you read thesepreliminaries and think for yourself,
not only will you come to doubt the ·supposed· science that you have practised ·under the name of
metaphysics·, but eventually you’ll become quite surethat nothing like that can exist without satisfying thedemands that I shall state here—demands on which
its possibility depends. You will also become sure thatsince the demands never have been met, there has up
till now been no such thing as metaphysics.
But the search for metaphysics will continue, because theinterests of human reason are so closely bound up with it;so you’ll agree that metaphysics is unstoppably on the roadto a total reform (or, better, a new birth) on a wholly newplan, even if people struggle against it for a while.
David Hume’s attack on •metaphysics was more decisivefor •its fate than any other event since the Essays of Lockeand Leibniz—actually, since the earliest recorded beginningsof metaphysics. Hume threw no light on this kind of knowledge, but he struck a spark from which a light could havebeen kindled if it had fallen on something flammable and theresultant smoulder had been nursed into flames.
Hume’s primary starting-point was a single important
metaphysical concept, namely that of the connection of causewith effect (including derivative concepts like those of force
and action and so on). Reason purports to have given birth
to this concept, but Hume challenged reason thus:
Explain to me what entitles you to think there could
be a thing x such that: given that there is x, there

Introduction

must necessarily also be something else y—for that’s
what the concept of cause says.
He showed beyond question that it is completely impossible
for reason to have—in an a priori way and purely throughconcepts ·with no input from experience·—the thought ofsuch a union ·of x with y·, because the thought of such a
union includes the thought of necessity. We cannot at all see
why, given that one thing exists, some other thing necessarily must exist, or how the concept of such a connection could
arise a priori. From this he inferred that reason is utterly
deluded regarding the concept of cause, wrongly thinking
it to be among her own children when really it is a bastard
child of the imagination that was got in the family way byexperience. What the imagination did ·according to Hume·was to consider certain sense-impressions that were related
to one another by the law of association—·so that after experiencing many F impressions
followed by G ones, you get into the habit of expecting
a G whenever you experience an F, the habit becomingstrong enough so that any new experience of an F
compels you to expect a G·—
and to mistake a •subjective necessity (habit) for the•objective necessity arising from grasping what must be
the case. He inferred that reason can’t form a thought of
the form x is necessarily and objectively connected with y, oreven with the general thought of that kind of connection.
If reason did produce any such thought (Hume held), theconcepts it involved would be fictitious, and all reason’s
claims to a priori knowledge would be merely the mis-labelled
deliverances of ordinary experience. He was saying in effectthat metaphysics couldn’t possibly exist. [At this point Kant has
a footnote, as follows:] Yet Hume called this destructive scienceof his ‘metaphysics’ and put a great price on it. ‘Metaphysics
and morals’, he says, ‘are the most important branches of2

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Immanuel Kant

learning. Mathematics and natural science are not half sovaluable.’ But all that this brilliant man had in his view was
the negative work involved in damping down the extravagant
claims of speculative reason and thus settling many endlessand vexatious controversies that lead mankind astray. He
lost sight of the positive harm that is done when reason isrobbed of its most important vistas—which it needs if it is
to mark out for the will its highest goal in all its endeavours.

Introduction

source of an inner truth—truths coming just from itself,
not from anything outside it given through experience—sothat it could be applied to things other than merely theobjects of experience. That was Hume’s problem. He wasn’t
challenging our indispensable need for the concept of cause,
but merely asking what its origin is. If the origin was settled,questions about the conditions governing the use of theconcept, and about the domain in which it can be validly
used, would automatically have been answered also.

[End of footnote. That last remark reflects views of Kant’s about reason’s
link with freedom, and freedom’s link with morality.]

To deal adequately with this problem, however, Hume’s
opponents would have had to dig deeply into the nature of
reason, considered as the faculty of pure thinking: not a job
to their taste! They were more comfortable with a different
approach, one that let them defy Hume without bringing any
insight to his problem, namely by appealing to common sense.
It is indeed a great gift from heaven to have plain common
sense. But this common sense must be shown in practice,through judicious and reasonable thoughts and words, notby appealing to it as an oracle when one has no rational
arguments to offer. Appeal to common sense when insightand science have failed you, but don’t appeal to it before
then!—that ·rule of intellectual conduct· is one of the devious
inventions of recent times, and it enables a shallow ranter tojoin battle with a solid thinker, and hold his own. But anyone
with a flicker of insight left to him would be careful not tograsp at this straw. If you place this appeal to common sense
in a clear light you will see that it is nothing but an appeal
to the opinion of the mob—whose applause embarrasses thephilosopher but brings joy and reassurance to the popularsmart alec. I should think that Hume had as much claim
to sound common sense as Beattie did, and he also had
something that Beattie lacked, namely a critical reason that
restrains common sense so that •it doesn’t speculate or, if
speculations are the topic of discussion, •it doesn’t crave for

His inference was hasty and wrong, but at least it was
based on investigation; and this investigation thoroughly
deserved ·a better response than it got. It ought· to havebrought together the intelligent people of the time to search
for a happier solution of Hume’s problem as he had formulated it; and if that had happened, a complete reform of thescience ·of metaphysics· would have quickly followed.
But metaphysicians have always suffered the misfortuneof not being understood by anyone, and this is what happened to Hume. It really hurts to see how totally Hume’s
opponents—Reid, Oswald, Beattie, and finally Priestley too—missed the point of his problem. They kept taking for granted
things that he had called into question, and offered furious
and often arrogant demonstrations of things he had neverthought of questioning; so they didn’t pick up the pointerhe had given to an improvement ·that metaphysics mightundergo·. In this they failed so completely that at the endof the debate the status quo was still standing: it was asthough nothing had happened!
Hume had never cast doubt on the proposition that theconcept of cause is proper, useful, and even indispensable forour knowledge of nature; that wasn’t in question. What wasin question was whether reason could think that concept
a priori. If it could, the concept of causation would be the3Prolegomena

Immanuel Kant

Introduction•experience (which is all that Hume provided for them) but
rather from •pure understanding. [By the phrase ‘the deduction ofany decision when it isn’t satisfied that it has the arguments
to support one. This is the only way someone’s common
sense can remain sound. Chisels and hammers can serve
very well in working wood, but for copperplate we need an
engraver’s needle.
Thus •sound common sense and •speculative understanding are both useful, but each in its own way: •the former
serves in judgments that apply immediately to experience,•the latter comes into play when universal judgments frommere concepts are to be made, as in metaphysics. In thelatter environment sound common sense has absolutely noright to judge.
Here is an open confession about something that happened many years ago: it was my recollection of ·the thoughtof· David Hume that broke into my dogmatic slumber, andpointed my work in speculative philosophy in a completely
new direction. I was nowhere near accepting his conclusions.
He had reached them by looking at only a part of his
problem—a part that by itself can give us no information.
·Still·, if we start from a well-founded but undeveloped
thought that someone else has left to us, we can hope thatby continuing to think it through we shall get further than
did the brilliant man to whom we owe the first spark of light.
So I tried first to see whether Hume’s objection could be
put into a general form, and I soon got a result:
The concept of the cause-effect connection is far frombeing the only idea by which the understanding has apriori thoughts about the connections of things. On
the contrary, metaphysics consists purely of suchconcepts—·i.e. concepts of the connections of things·.
I tried to find out how many such concepts there are, andsucceeded in this in the desirable way, namely by startingfrom a single principle. Then I proceeded to the deductionof these concepts, which I was now certain didn’t come fromthese concepts’ Kant refers to a theoretically grounded and justified listof the concepts in question]—something that proves and explains

why the metaphysical concepts of the connections of things
are just exactly the ones on the list. This deduction had
seemed impossible to Hume; and apart from him nobody
had even thought of it, although everyone had confidently
used the ·metaphysical· concepts, without asking what theirobjective validity was based on. The deduction was thehardest task that anyone could tackle in the service of
metaphysics; and the worst of it was that I couldn’t get help
from metaphysics as it then was, because this deduction is
what’s needed to make metaphysics possible. But ·despite
getting no help from metaphysics· I did succeed in solving
the Humean problem, not merely for a particular case ·of thecause-effect connection· but with respect to the whole facultyof pure reason. With that done, I could safely—though
always slowly—go on to map out the whole domain of purereason, establishing its boundaries and its contents. I did all
this completely, and from general principles, which is what
metaphysics needed if its system was to be securely built.
I expounded the Humean problem in its most general
possible form in my book Critique of Pure Reason; but I am
afraid that that work may go the same way as the problem did
when Hume first propounded it. The book will be misjudged
because misunderstood; and people will misunderstand it
because they are inclined to skim through the book rather
than thinking it through. That is ·admittedly· a disagreeabletask, because the work conflicts with all ordinary concepts,
as well as being dry, obscure, and long-winded! ·Despite
those drawbacks·, I confess that I didn’t expect to hear aphilosopher complain that the book isn’t a crowd-pleaser,
not entertaining, not an easy read, given that what’s at issue
4

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Immanuel Kant

in it is no less than the existence of a highly prized andindispensable kind of knowledge—a question that can’t besettled except by working strictly according to rule and withgreat precision. Such work might in the course of timeplease the crowd; but ·a concern for· popularity is quite
inappropriate at the start. Still, one of the complaints isjustified: the book’s plan is diffuse, making it hard for thereader to keep in mind the chief points of the investigation;and that contributes to a certain obscurity. I intend to
remedy that with these Preliminaries.
The earlier work, which maps out the entire faculty ofpure reason, will be the foundation to which the Preliminaries
are to be related. But the latter work—·the book you now
hold in your hands·—is only a preparatory exercise ·and
not a contribution to metaphysics itself·; because we can’t
think of letting metaphysics appear on the scene, or evenhave a faint hope of attaining it, until our critique has been
established as a science that is complete in every detail.
We have long been used to seeing dreary old knowledge
spruced up as new by being taken out of its former context
and turned into a system in fancy new clothing with new
terminology; and that’s all that most readers will initially
expect my critique to be. But these Preliminaries may help
the reader to see that it is ·not old stuff in new clothes,but· a wholly new science that no-one has ever thoughtof—indeed, the very idea of which was unknown—and towhich no previous work has made the slightest contribution.
The only exception to that is the pointer one could get from
Hume’s doubts; but even he didn’t suspect ·there could be·such a possible formal science; instead, he played safe byrunning his ship onto the shore (scepticism), and letting
it lie there and rot. I prefer to give the ship a pilot who
can safely sail it anywhere he likes, by means of secure
principles of navigation drawn from a knowledge of the globe,

Introductionand equipped with a complete chart and compass.
Suppose we are confronted by a new science that is wholly
isolated and the only one of its kind. If we start with theassumption that we can make judgments about it in terms ofknowledge that we have already gained—which is precisely
what has first to be called in question ·when consideringa new science·—all we shall achieve is to see everywhere
things we already know, with the •words sounding familiar
but everything seeming (·so far as the •content is concerned·)
to be pushed out of shape, senseless, gibberish. That’s
because we’ll be relying on •our own notions, which long
habit has made second nature for us, instead of relying on•the author’s. But the long-windedness of the work, to theextent that it comes from the science itself and not merely
from the exposition, as well as the unavoidable dryness andby-the-rules precision, are qualities that can bring credit to
the science—though not to the book!
It isn’t given to many of us to write with the subtlety andgrace of David Hume, or with the solidity and elegance ofMoses Mendelssohn. Yet I flatter myself that I could havewritten in a crowd-pleasing way if my aim ·in the Critique ofPure Reason · had been merely to outline a plan and leave it
to others to complete, rather than having set my heart on thegood of the science that had occupied me for so long. Indeed
it took a lot of perseverance and a good deal of self-denial toput •the prospect of later but more lasting applause aheadof •the enticements of an immediate success.
The making of plans is often an arrogant and boastful activity through which someone •gives himself airs as a creative
genius by demanding what he doesn’t himself supply, •finds
fault with what he can’t improve, and •makes proposals that
he himself doesn’t know how to carry out—though a soundplan for a general critique of pure reason, if it isn’t to amount
only to the usual spouting of pious hopes, will have to have5Prolegomena

Immanuel Kant

more content than one might expect. But the domain ofpure reason is so •separate from everything else, and so•inter-connected within itself, that we can’t lay a finger on
one part without affecting all the others, and can’t build
anything there without first determining where each part isand how it relates to the rest. That’s because our judgment•within this domain can’t be corrected by anything •outside
it, and so the validity and use of every part of the domain
depends on how it relates to all the rest—just as with thestructure of an organism we can work out the purpose ofeach part only from a full conception of the whole. So it canbe said that •such a critique shouldn’t be trusted unless it
is perfectly complete, down to the smallest elements of purereason, and that •in the domain of reason you must settle
everything—or you’ll settle nothing.
As for a mere plan or sketch of the critique of pure reason:
its usefulness as a sequel to the critique is a measure of
how useless—how unintelligible and unreliable—it would
be if given in advance. Taken as a sequel, it gives us avantage-point from which we can take in the whole thing,
can test—one by one—the chief points of the science, andcan make the exposition of it much better than it was thefirst time around.

Features of metaphysical knowledge

and on pages 15 and ??. Everywhere else in this work and throughout
the Critique of Pure Reason Kant uses the terms in an utterly different
sense, in which it distinguishes not •expository methods but •kinds ofproposition. This use of the terminology is the one that is still current;
Kant explains it in section 2 below.]With my critique of pure reason completed, I now offer
a plan of it as a sequel. The plan is to be laid out inthe •analytic manner, whereas the critique itself had tobe composed in the •synthetic style so that readers could
command a view of all the joints of the science—the natural
hanging-together of the structural parts of ·pure reason·,an utterly special cognitive faculty. But if you also findthis too obscure—this plan that I offer as the Preliminaries to any future Metaphysic—bear in mind •that it’s notnecessary for everyone to study metaphysics, •that many
people have the aptitude to succeed very well in sciences(even deep ones) that are closer to sense-experience, yet
can’t succeed in investigations dealing with highly abstract
concepts, •that such people should employ their talents onother subjects; •that someone who undertakes to make judgments in metaphysics—let alone to construct a metaphysicalsystem—must satisfy the demands I have made here, which
he can’t do by rejecting them, so he must either adopt mysolution or thoroughly refute it and put another in its place;and, finally, •that this notorious obscurity (·allegations ofwhich are· often a cloak to cover the accuser’s laziness or
stupidity) also has its uses ·as a defence against insolent intruders. There’s no shortage of them in metaphysics!· People
who maintain a cautious silence in relation to other sciences
approach metaphysics in a spirit of bold pronouncements
and snap judgments, because in this area their ignorance is
not contrasted with the knowledge of others.

[In the next paragraph Kant uses ‘analytic’ and ‘synthetic’ to mark a
distinction between two methods of presentation of some doctrine. An•analytic presentation starts with things we all know to be true andworks its way from those to the theory or doctrine that explains andis supported by them. A •synthetic presentation goes in the opposite
direction: it starts with the fundamental theses of the doctrine to be expounded, and works from those to various of their consequences, which
could include the things-we-already-know that are the starting-point for
the analytic format. This use of ‘analytic’ and ‘synthetic’ occurs only here6

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Features of metaphysical knowledge

Preamble on the special features of all metaphysical knowledge
1: The sources of metaphysics

have adequately described it in my Critique of Pure Reason.
So much for the sources of metaphysical knowledge.

If a domain of knowledge is to be exhibited as a science, weneed to know exactly what features are special to it, marking
it off from all other sciences. Otherwise the boundaries of
all the sciences run into one another and none of them canbe treated soundly according to its own nature. Our ideaof a possible science and of the territory it covers is basedon its special features—whether they have to do with its•subject matter, or its •sources of knowledge, or the •kind ofknowledge it involves, or of some or all of these together.
Let us consider first the sources of metaphysical knowledge. The very concept of metaphysics ensures that thesources of metaphysics can’t be empirical. ·If something
could be known through the senses, that would automatically show that it doesn’t belong to metaphysics; that’s
an upshot of the meaning of the word ‘metaphysics’·. Its
basic propositions can never be taken from experience, nor
can its basic concepts; for it is not to be physical but
metaphysical knowledge, so it must lie beyond experience.
•Outer experience is the source of physics properly so-called,and •inner experience is the basis for empirical psychology;
and metaphysical knowledge can’t come from either of these.
It is thus knowledge a priori—knowledge based on pureunderstanding and pure reason.
Mathematics also answers to that description. To markoff metaphysics from mathematics as well as from empiricalenquiries, we’ll have to call it pure philosophical knowledge.
In this phrase, ‘pure’ means ‘not empirical’; and ‘philosophical’ stands in contrast to ‘mathematical’. The difference
between these two ways of using reason—the mathematicaland the philosophical—is something I needn’t go into here; I

2: The only kind of knowledge that can be called metaphysical(a) The distinction between synthetic and analytic judgments
in general.
Because of what is special about the sources of metaphysicalknowledge—·namely, that they don’t include experience·—allsuch knowledge must consist in judgments that are made apriori. However, a priori judgments can be divided into twogroups, according to their content: (1) those that merely spellout what’s already there, adding nothing to the content of theknowledge, and (2) those that add something, and enlarge
the given knowledge. We can call (1) analytic judgments, and(2) synthetic.
Analytic judgments say nothing in the predicate thatwasn’t already thought—though less clearly—in the conceptof the subject. If I say ‘All bodies are extended’, I haven’t
added anything to my concept of body, but have merely
analysed it. Extension was already implicitly thought of in
the concept of body, before I made the judgment. So the
judgment is analytic. On the other hand the proposition‘Some bodies are heavy’ contains something in the predicate that isn’t thought—even unclearly or implicitly—in theconcept of body. It thus enlarges my knowledge in that it
adds something to my concept, and hence must be called asynthetic judgment.
(b) The common principle of all analytic judgments is the lawof contradiction.
All analytic judgments rest wholly on the law of contradiction.
7

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Immanuel Kant

The predicate of an affirmative analytic judgment has alreadybeen thought in the concept of the subject, so it can’t bedenied of the subject without contradiction. This is thecase with the proposition ‘Every body is extended’. ·That’s
equivalent to something of the form ‘Everything that is F andextended is extended’, so that to deny it would be to say thatsomething is F and extended and not extended, which is an
outright contradiction. The law of contradiction, which saysthat no contradiction is true, thus underlies the truth of theanalytic proposition that all bodies are extended·.
So all analytic propositions are a priori judgments, eventhose that contain empirical concepts as does the judgment‘Gold is a yellow metal’. ·I must have experience if I am tohave the concepts of gold, of yellow, and of metal; but· toknow that gold is a yellow metal I need no further experience;
all I need is to analyse my concept of gold, which contains
the concept of being a yellow metal.

Features of metaphysical knowledge

why go to experience when the judgment can be derived
purely from my concept? That every body is extended isa proposition that holds a priori, and not a judgment ofexperience. For before I look to experience I already have inthe concept of body all that I need for that judgment: I need
only to extract the predicate (‘extended’) from that concept
according to the law of contradiction. In doing that, I also
become conscious of the necessity of the judgment—and·that’s further evidence that this analytic judgment isn’t
based on experience, because· experience can never teachme that something is necessary.
(2) Mathematical judgments are all, without exception,
synthetic. This is certainly true and is very important, but it
seems to have escaped the notice of all previous analysersof human reason, and indeed to be directly opposed to
all their theories. Those earlier thinkers saw that all theinferences of mathematicians proceed according to the law ofcontradiction, and wrongly slipped into thinking that mathematical truths were known from the law of contradiction.
This was a great mistake. The law of contradiction canlead one to a synthetic proposition, but only from anothersynthetic proposition. (Still, it must be borne in mind thatmathematical propositions are always a priori judgments,
not empirical ones. They carry necessity with them, andthat can’t be learned about from experience. If you disagree,
I shan’t argue; I shall merely make this claim about the
propositions of pure—i.e. non-empirical—mathematics!)
One might think that the proposition 7 + 5 = 12 is analytic,
and that it follows according to the law of contradiction fromthe concept of
the sum of 7 and 5.
But if we look more closely, we find that the concept of thesum of 7 and 5 contains only
the uniting of 7 and 5 into a single number;(c) Synthetic judgments need a different principle from thelaw of contradiction.
Some synthetic judgments have an empirical origin, andcan be known only a posteriori; other synthetic judgments
have a priori certainty, and originate in pure understanding
and reason. No synthetic judgment can come from the lawof contradiction alone. Such judgments must conform tothat principle (which is just to say that they mustn’t be
self-contradictory), but they can’t be deduced from it.
·In the rest of this section four kinds of synthetic judgment will be identified and discussed. Although they are allsynthetic—meaning that none of them can be establishedmerely by analysing concepts—three of the four kinds canbe learned a priori ·.
(1) Judgments of experience are always synthetic. It
would be absurd to base an analytic judgment on experience:8

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and in thinking this we don’t have the least thought of what
this single number is in which the two are combined. I cananalyse my concept of the uniting of seven and five as long as
I please—I shall never find 12 in it. I have to go outside theseconcepts and—with the help of an intuition that corresponds
to one of them (my five fingers for instance)—add the 5 given
in intuition to the concept of 7, adding them one by one.
Thus in this proposition 7 + 5 = 12 we really amplify ourconcept ·of 7 + 5·, adding to it new concept that wasn’t
thought in it. That is to say, arithmetical propositions arealways synthetic. It will be easier to grasp this if we takelarger numbers. It is obvious that however we might turnand twist our concept of
the sum of 38976 and 45204
we could never find 84180 in it through mere analysis,
without the help of intuition.

Features of metaphysical knowledge

at the fingers. But he believes—as we’ll see in section 7—-that puremathematics involves •pure intuitions: for example, a geometer works
out the properties of circles not by merely taking the abstract concept
circle and analysing it, but by somehow giving himself a pure intuitionof a circle, and working out the properties of all circles from that. Thisis something like imagining-a-circle, but it isn’t ordinary imagination,
which is copied from sense experience.
[The basic idea is something like this: Every time you see or feelsomething circular, various aspects of your mental state are contributedby the sensations that come from outside you, and others are contributedby your understanding, i.e. the concept-using faculty. If all of that weresomehow stripped off, what would be left is a very thin, abstract intuitionof the circular thing just as a circle. That is, nothing would be left of it but
its purely spatial or geometrical properties; they will be the same for every
circular thing; so the stripped down intuition will be the same in eachcase. That stripped down intuition is what Kant calls a ‘pure intuition’of a circle. According to him, this isn’t contributed by sensation fromoutside you; rather, it is conferred on your mental state by your own
mind, specifically by your own faculty of sensible intuition. You are sobuilt, he thinks, that you have to experience the world outside yourself
as spatial, not because the outer world is spatial but because you impose
spatiality on the intuitions you have of it. Kant puts this, sometimes, bysaying that what’s represented in a pure intuition is the form of yoursensibility or of your sensible intuition.
[For the geometer to establish synthetic truths about circles, Kant
holds, he must not only have •the concept circle but must also have •apure intuition of a circle. This pure intuition, he sometimes says, exhibits
the concept; it illustrates or exemplifies it; it shows the geometrician what
a circle is, taking him from the merely conceptual thought of circles to akind of abstract non-sensory view of a circle.

[Kant’s use of the term ‘intuition’ needs to be explained. Traditionally, the word has had two meanings. •In one it contrasts with‘demonstration’—you know something intuitively if it is immediately selfevident to you, whereas demonstrative knowledge involves a series ofdeductive steps. •In the other meaning—which alone is relevant toKant—our faculty of ‘intuition’ is our ability to be mentally confronted
by individual things, to have in our minds representations of the thingsand not merely of certain features or properties of them. Kant uses
‘intuition’ to stand not just for the faculty but also for the mental representations that it involves. Thus, for example, when you see the
Lincoln Memorial you have an intuition of it, and this is an exercise ofyour faculty of intuition. That intuition is a ‘sensible’ one, meaning thatyou get it through your senses. It stands in contrast with a conceptof the Lincoln Memorial—such as the concept or abstract thought of alarge white memorial to a great American statesman. Having in yourmind a (conceptual) representation of a large white memorial etc. is quite
different from having in your mind an (intuitive) representation of the
Lincoln Memorial, that one particular individual object.
[Now, Kant holds that we are also capable of having in our minds
intuitions that don’t come from the senses; he calls them ‘pure’ or ‘apriori’ intuitions. When in the previous paragraph he speaks of theintuition of my five fingers, that is a •sensible intuition: I feel or look

[The same story can be re-told about the perceptions of events: strip
off everything empirical, and everything conceptual, and you are left witha mere, bare, pure intuition of time. As •space is a form of your sensibility in experiencing things outside yourself, •time—Kant thinks—is
a form of your sensibility in relation not only to things outside you but
also to the flow of your mental history. Just as geometry is based on pure
intuitions of space (or of spatial figures), Kant says, arithmetic is basedon pure intuitions of time; see section 10. We now return to Kant’s text.]

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Immanuel Kant

Nor is any principle of pure geometry analytic. Thata straight line is the shortest path between two points is
a synthetic proposition. For my concept of straightness
contains nothing having to do with •quantity—it is purely a•qualitative concept—so it can’t contain the thought of whatis shortest, ·because that is quantitative·. Here again, we
need help from intuition if we are to have a basis for putting
shortest together with straight.
Why are we so prone to believe that in such a judgment
the predicate is already contained in our concept so thatthe judgment is analytic? The source of this mistake is acertain ambiguity. We ought to join in thought a certain
predicate (’shortest’) to a given concept (’straight’), and thisrequirement is inherent in the concepts themselves. But thequestion isn’t what we •ought to think along with the given
concept but what we •do think in it, even if unclearly. Once
we distinguish those, we can see that while the predicate
is indeed attached to the subject concept necessarily, it is
attached only through an intuition that must also be present;
it isn’t to be found in the subject concept itself.
Some other principles that geometers use are indeed
really analytic and rest on the law of contradiction: forexample ‘Everything is equal to itself’, and ‘The whole isgreater than its part’. These identical propositions can be
useful in setting out arguments, but they don’t actually
say anything; they can be useful methodologically, but they
don’t contribute to the content of what is said. Furthermore,even these analytic propositions, though they are indeed
validated purely by our concepts, wouldn’t be allowed into
mathematics if they couldn’t be illustrated by propositions
that are connected with intuition. ·For example, ‘The wholeis greater than its part’ is allowed into mathematics because
it can be applied to numbers, areas and lengths, which are
given to us in intuition·.

Features of metaphysical knowledge

Pure mathematical knowledge differs from all other a priori knowledge in this: it never proceeds from concepts, but isalways achieved by construction of concepts. Mathematical
propositions must therefore go beyond the concept to what
the corresponding intuition contains, ·because this intuition
guides the construction of the concept·; hence they can’t
and shouldn’t come from the analysis of concepts, and are
therefore one and all synthetic.
This may seem a small and unimportant point; but
the neglect of it has done harm to philosophy. Hume
had the worthy philosophical aim of surveying the wholedomain of pure a priori knowledge—a domain in which thehuman understanding lays claim to great possessions—but
he carelessly sliced off a large part of the territory, its
most considerable province, namely pure mathematics. He
thought that mathematics rested on the law of contradiction
alone. Although he didn’t classify propositions in quite theway that I do here, or with the same names, he in effectsaid: Pure mathematics contains only analytic propositions,but metaphysics contains a priori synthetic propositions.
Now this was a great mistake, which infected his wholesystem of thought. If he hadn’t made this mistake, he would
have taken his question about the origin of our ·a priori ·synthetic judgments to cover not only •metaphysics (e.g. theconcept of causality) but also •mathematics. He had too
much insight to base mathematics on mere experience, so·if he had likened metaphysics to mathematics in the way
I have been defending· he would have spared metaphysics
from the vile mistreatment to which he subjected it, becausethat attack would have hit on mathematics as well, which
Hume can’t have wanted to do. And then, fine thinker that
he was, he would have been drawn into lines of thought
like those that I am now offering—though he would havepresented them in his own uniquely elegant style.
10

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Immanuel Kant

(3) [following (2), which began on page 8.] Natural science also
contains synthetic judgments that can be known a priori, forexample:•In all changes in the physical world the quantity ofmatter remains unchanged.
•When one body collides with another, action andreaction must always be equal.
Clearly these are not only necessary and a priori in origin
but are also synthetic. ·I shall show this of the first ofthem. It says that the total amount of matter in the universe
never changes, which is to say that matter is permanent ·.
Now, in ·thinking· the concept of •matter I do not think its•permanence but only its •presence in the space that it fills.
·Thinking that matter is permanent isn’t like thinking thatwomen are female, or that tigers are animals·. In judging
that matter is permanent, therefore, I go beyond the conceptof matter in order to add to it something that I didn’t thinkin it. So the proposition isn’t analytic but synthetic; yet itis thought a priori, as are the other propositions of the purepart of natural science—·the ‘pure’ part being the part thatowes nothing to experience·. [This paragraph on natural science

Features of metaphysical knowledge(4) Properly metaphysical judgments are all synthetic.

can be used to work towards a definition of the concept inquestion. But such a •judgment belongs to metaphysics only
because the analysed •concept does; the process of analysis
is just the same as we use when analysing empirical concepts
that don’t belong to metaphysics. The only judgments thatare really strictly metaphysical are synthetic ones.
When the a priori concepts that are the building-bricksof metaphysics have been gathered together in a systematicway, the analysis of them is of great value. The analytic
judgments that are arrived at in this way can be separated
out from the rest of metaphysics, and presented as a separate part of the whole system. The only use that theseanalyses have in metaphysics is as a useful preliminary to
the procedure of arriving a priori at synthetic propositions
involving the concepts that have been analysed.
The upshot of this section is that metaphysics is centrally
concerned with a priori knowledge of synthetic propositions.
These are what metaphysics is for. We are helped to arriveat them by analyses and analytic judgments—indeed, ones
using the very same process of analysis as we do when trying
to clarify our concepts in other branches of knowledge. But
the essential content of metaphysics is the generation ofknowledge a priori, both according to intuition and according
to concepts, leading ultimately to synthetic propositions apriori—philosophical knowledge.

The whole aim of metaphysics is to arrive at conclusions
that are synthetic. Analytic judgments are also involved, but
only as aids to constructing •arguments; what metaphysics,
properly so-called, is really about is the establishment of•conclusions, which are always synthetic. If a concept(such as that of substance) belongs to metaphysics, then the·analytic· judgments that analyse this concept also belong
there—for example the judgment that substance is thatwhich exists only as subject etc.–and a set of such judgments

The distinction between analytic and synthetic is essential in
the present kind of enquiry into the human understanding; it
isn’t much used anywhere else, so far as I know. The reason
why dogmatic philosophers overlooked this apparently obvious distinction is that they didn’t look for the sources of metaphysics in the pure laws of reason in general ·and so they
didn’t see how metaphysical truths could be known a priori

is brought across from the Critique of Pure Reason. There’s evidence
that Kant intended such a paragraph to occur here, and omitted it byaccident.]

3: A note about the analytic/synthetic distinction11

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Immanuel Kant

and yet be synthetic·. [By ‘dogmatic’ philosophers Kant means,

General Problems

narrow and almost nothing at all. ·Grudging as that is, itdoes at least admit the possibility of some synthetic a priori
knowledge·. But what he says of this kind of knowledge isso skimpy and unsystematic that it’s not surprising thatit didn’t prompt anyone—and in particular didn’t prompt
Hume—to consider propositions of this kind. It is hard tolearn universal and yet definite truths from someone who
only had them floating obscurely before him in his thought!
One needs to discover them for oneself, in one’s own thinking;then one can find them elsewhere, where one would certainly
not have found them before because the authors weren’t clear
in their own minds about what they were saying. ·That’s how
I found the analytic/synthetic distinction in Locke’s pages
when Hume didn’t find it there: the crucial point is that I
had first worked out the distinction for myself·.

broadly speaking, ones who plunge ahead doing metaphysics without
first raising the question of how—or indeed whether—metaphysics is
possible.] Thus two recent German philosophers tried to derive

the law of sufficient reason, which is obviously synthetic,from the law of contradiction. [The law of sufficient reason saysthat there’s a reason for everything that is the case, i.e. that there’s acorrect answer to every ‘Why?’-question.] Still, there is a hint of this

distinction in Locke’s Essay at IV.iii.9ff. Having previously
discussed the different kinds of judgments and how we arriveat them, including•judgments of ‘identity or contradiction’ (which are
analytic), and•judgments of ‘co-existence’ (which are synthetic),
he admits that our a priori knowledge of the latter is very

General Problems
4: The general problem of the Preliminaries: is metaphysics

However, things haven’t turned out so well for humanreason. There’s no single book that one can point to. . . .andsay, ‘This is metaphysics; here you will find knowledge ofa highest being and of a future world, which is the noblest
aim of this science, proved from principles of pure reason.’
Many propositions have been agreed without dispute to benecessary and certain, but they are all analytic, and concern
the materials and building-stones of metaphysics rather
than the enlargement of our knowledge. You may point tosome synthetic propositions (e.g. the law of sufficient reason)
which are widely accepted, though you have never proved
them through mere reason, a priori, as you ought to have.

possible at all?

If we had a real metaphysics that could claim to be ascience—if we could say ‘Here is metaphysics, all you have to
do is to learn it, and it will convince you of its truth’—then wewouldn’t have to ask whether metaphysics is possible, ·justas we don’t have to ask whether geometry, say, is possible·.
Our only question would concern how it is possible, andhow reason should set about doing metaphysics; and thiswould be •a test of our mental skills, not •a challenge to theexistence of the thing itself.
12

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Immanuel Kant

Help yourself to them; but when you want to use them forsome serious purpose you will find yourself caught up in
wrong or dubious assertions—the sort of thing that has set
metaphysical systems against one another in their doctrines
or in their arguments, destroying their claims to be believed.
Indeed, the very attempts to create a science of metaphysics were the first cause of early scepticism—a way of
thinking in which reason attacks itself so violently that it
could never have arisen except in complete despair about
our ability to carry out reason’s most important designs.
Men began to investigate reason itself, long before starting
methodically to investigate nature ·in the physical sciences·.
Even at that stage, reason had already been employed inconnection with ordinary experience; and reason is always
present to us, whereas laws of nature have to be laboriously
sought out. So metaphysics floated to the top like foam,
which dissolved the moment it was scooped off. But as soon
as one lot of foam dissolved, more came frothing up to thesurface. Some philosophers eagerly collected foam; sometried to show their wisdom by ridiculing the vain efforts ofothers; none looked for the cause of the foam down in thedepths.
We are tired of dogmatism that teaches us nothing, andjust as tired of scepticism that promises us nothing (not evenpermission to rest comfortably in ignorance). The knowledge
we need is important, and that’s a challenge to us; but wehave had centuries of bad experience with things we thought
we knew through ‘pure reason’ that turned out not to beknowledge at all, and that fact makes us suspicious. ·So weare •under pressure to push on forwards, and also •nervous
about doing so·. Where do we go from here? That depends
on the answer to the question ‘Is metaphysics possible atall?’ We should try to answer this not by picking away
sceptically at particular doctrines of this or that actual

General Problems

system of metaphysics—for we don’t yet admit that there are
any systems of metaphysics—but by considering the conceptof such a science.
In the Critique of Pure Reason I tackled this problem bylooking into pure reason itself: by establishing the natureof reason, I was able to work out what its materials andmethods must be. This is hard to do. It demands a reader
who is resolved to think himself gradually into a system
based on reason itself and on nothing else, aiming to develop
knowledge out of that alone, without help from any fact.
Because the present work is called Preliminaries, on theother hand, it ought to consist of preliminary exercises;
they should aim not to •expound the science itself but
rather to •show what’s needed for the science to be brought
into existence. Preliminaries should try to get help fromsomething that is already known to be reliable, from which
one can confidently work back to the ultimate sources thataren’t yet known.
Although we can’t take it for granted that there is any
such science as metaphysics, we can—fortunately—say withconfidence that some pure synthetic a priori knowledge isreal and that we already have it. I refer to pure mathematics and pure natural science. Each of these contains
propositions that are everywhere recognized—partly throughreason that shows them to be necessary and certain, andpartly through universal agreement arising from experience(though not actually based on experience). So we have somea priori synthetic knowledge that is, at least, unchallenged;
we don’t have to ask whether such knowledge is possible (forit is real), but only how it is possible. When we can answer
that, we’ll know how to go about showing the possibility of
all other kinds of synthetic a priori knowledge.

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Immanuel Kant

5: The general problem: how can there be knowledge based

General Problems

done any harm to use that same formulation here, for it
must be clear to every reader that when I speak here of‘knowledge based on pure reason’ I always mean knowledgeof synthetic propositions, never of analytic ones; ·and ofcourse knowledge through pure reason is always a priori ·.
[At this point Kant has a footnote commenting on the shift
from the old senses of ‘analytic’ and ‘synthetic’ (explained on
page 6 above) to his new senses for those terms.]
Metaphysics stands or falls with the solution to thisproblem. Someone may propound his metaphysical claims
as plausibly as he likes, smothering us with conclusions
piled on conclusions; but if he hasn’t first answered thisquestion properly, we are entitled to say to him:
‘This is all pointless ungrounded philosophy and false“wisdom”. You purport to be using pure reason tocreate a priori knowledge, not by merely analysing
concepts but by making new connections that don’t
rest on the law of contradiction; and you think you
have insight into these connections independently of
all experience. But how do you get such insight? How
can you justify your claims?’
He can’t answer by appealing to the common sense of
mankind, for that isn’t evidence—it’s mere hearsay. . . .
The question must be answered, but that is hard to do.
One reason why an answer wasn’t attempted long ago isthat a satisfactory answer to this one question demands
much deeper, more persistent and more careful thought
than goes into the most lengthy and ambitious metaphysicalworks ever published. (A weightier reason is that nobody
thought to ask the question!) Every reader who looks hardat the problem will initially be frightened by how hard it
is. Indeed, if it were not that there really is synthetic apriori knowledge, the thoughtful person would think suchknowledge to be impossible. This is what happened to Davidon pure reason?

We have seen the vast difference between analytic andsynthetic judgments. It is easy to see how there can beanalytic propositions:they come purely from the law of contradiction. There is also
no special problem about how there can besynthetic propositions that are known a posteriori,
i.e. known from experience: experience itself is nothing
but a continual joining together of perceptions, ·so it isn’t
surprising that it enables us to join concepts in a synthetic
way. Returning to an example used earlier, the synthetic
proposition that some bodies are heavy can be establishedthrough experiences in which •perceptions of body are joinedwith •perceptions of weight·. What we do have a problem
about is the possibility ofsynthetic propositions that are known a priori.
Whatever makes this sort of knowledge possible, it isn’t thelaw of contradiction ·and it isn’t experience·, so we mustsearch to find out what it is.
But we cannot rightly start by asking whether synthetica priori propositions are possible. For there are plenty ofthem, really given to us with undisputed certainty; and as
our present procedure involves starting with what we alreadyknow, we shall start from the premise that there is humana priori knowledge of some synthetic propositions. But thenwe still have to ask how this knowledge is possible, i.e. whatmakes it possible. When we know this, we can learn how to
use such knowledge and can learn what its limits are. Statedprecisely, then, the crucial question is this:How is it possible to have a priori knowledge of synthetic propositions?
·In the title of this section· I expressed this as a question
about ‘knowledge based on pure reason’. It wouldn’t have14

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Immanuel Kant

Hume, although he didn’t put the question to himself inthis general form (which is the form we need if we are to
get an answer that is decisive for the whole of metaphysics).
Hume asked an intelligent question: How can I arrive at a
judgment in which one concept is connected necessarily withanother, even though the one doesn’t contain the other? Hethought it couldn’t be done; which led him to conclude that
only experience can provide us with such connections. In
other words, he thought that this supposed necessity (whichis the same as this supposed a priori knowledge) is merely
a long-standing habit of accepting something as true, andhence of taking •a necessity in our thought—·a mere mental
compulsion·—to be •a necessity in the world.
If you want to complain about the toil and trouble that
I am going to give you in solving this problem, I invite you
to try solving it in an easier way! Perhaps that will makeyou grateful to the man who has taken this deep task overfor you, and you may even come to be surprised—given how
difficult the problem is—that the solution isn’t even harderthan it is. I have had to work for many years •to solve thisproblem in its full scope—i.e. covering all the cases—andfinally also •to be able to present it in the analytic form, as
you will find it here. [This is the old sense of ‘analytic’, explained on

General Problems

They may want to carry on their business not as a sciencebut as an art of swaying people with pronouncements thatare good for them and agreeable to ordinary common sense.
They are entitled to ply this trade; but then they should speak
the modest language of rational belief, admitting that they
mustn’t claim to •know—and shouldn’t even •conjecture—anything about what lies beyond the bounds of possible
experience. The most they can legitimately do is to •assume
things; and even then they aren’t making assumptions fortheoretical purposes (for they must renounce those), butsolely for practical use, assuming whatever is needed to guideour thought and behaviour in everyday life. That’s their only
chance of being useful and wise. It will be better, too, if
they give up the name ‘metaphysician’; for metaphysicians,
properly so-called, aim to be theoretical philosophers; they
try to establish judgments a priori, which means necessary
judgments; so they can’t fool around with conjectures. Whatthey assert is science or it is nothing at all. . . .
In now proceeding to the answer to the question ‘How
is a priori knowledge of synthetic propositions possible?,according to the analytic [old sense] method in which we
presuppose that such knowledge through pure reason is real,
we can appeal to only two sciences, namely pure mathematics
and pure natural science. Only these can represent objects
to us in intuition. If one of them should yield an item of apriori knowledge, it could show that this knowledge is realby showing that it fits with the intuited object; and we could
then work back from the reality of this knowledge to whatever
it is that makes it possible.
In order to move on from these kinds of pure a priori
knowledge, which are both real and grounded, to the possible kind of knowledge that we are seeking, namely tometaphysics as a science, we must take our question alittle more broadly. As well as enquiring into •the possibility

page 6]

All metaphysicians are therefore solemnly and legallysuspended from their business until they have satisfactorily
answered the question: How is a priori knowledge of synthetic propositions possible? Only an answer to this will
provide them with the credentials they must produce if weare to credit them with teaching us things in the name ofpure reason. If they can’t produce those credentials, we—as
reasonable men who have often been deceived—should flatly
refuse to listen to them, without asking any more about what
they are offering.
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Immanuel Kantof metaphysics as a science, we must also investigate •thenatural human disposition to pursue such a science. That
involves looking into the a priori thoughts that are uncritically accepted, developed, and called ‘metaphysics’. The
truth of such thoughts is under suspicion, but the thoughts
themselves are natural enough; they fall within the scope ofour question because they involve the natural conditions outof which metaphysics arises as a science.
So our main problem splits into four questions, which
will be answered one by one:(1) How is pure mathematics possible?
(2) How is pure natural science possible?
(3) How is metaphysics possible in general?

How is pure mathematics possible?

(4) How is metaphysics possible as a science?
It may be seen that the solution of these problems, thoughchiefly designed to present the core of the Critique, also
has an odd feature that is worth attending to separately.
We are looking to reason itself for the sources of certain
sciences, doing this so that from its performance we canassess reason’s powers as a faculty of a priori knowledge.
This procedure also brings benefit to those sciences, inrespect not of their content but of their proper use; andthey throw light on the higher question about their common
origin, while also giving an occasion better to explain theirown nature.

Main transcendental Problem 1:
How is pure mathematics possible?
6

knowledge—if we can track down that knowledge’s sources.

Mathematics is a great and proved domain of knowledge; italready has a large scope, and there’s no limit to how far it
can be extended in the future; and its results are absolutely
necessary and certain, which means that they owe nothing toexperience. Mathematical propositions are pure products ofreason, yet they are thoroughly synthetic. How can humanreason create such knowledge wholly a priori? Doesn’t ourmathematical faculty, which isn’t and can’t be based on
experience, presuppose some basis for a priori knowledge?
This basis must lie deeply hidden, but we might be able todiscover it through its effects—i.e. through our mathematical7
We find that all mathematical knowledge has this special
feature: it must first exhibit its concept in intuition, doing
this a priori in an intuition that isn’t empirical but pure.
[See the explanation of ‘pure intuition’ on page 9.] Without resorting to a priori intuitions, mathematics can’t take a single
step. So its judgments are always intuitive. (In contrast
with philosophy, which has to be satisfied with conceptual
judgments. Philosophy may illustrate its necessary doctrines
through intuition, but can never deduce them from it.) This
fact about mathematics points us to the absolutely basic
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Immanuel Kant

How is pure mathematics possible?

to occur without any object •being present or •having beenpresent, to which the intuition could refer; and in that case it
couldn’t be an intuition—·or so it seems·. We can form someconcepts a priori, without being related in any immediate
way to an object: we can do this with the concepts that
contain only the thought of an object in general, without any
detail—for example the concepts of quantity, cause, and soon. (Though even these have meaning for us only if we usethem concretely, applying them to intuitions through which
we confront actual instances of quantity and cause in our
experience.) But how can an intuition of an object precede
the object itself?

thing that makes mathematics possible, namely that it isgrounded in pure intuitions in which it can construct all its
concepts—that is, can represent them in a manner that is•concrete rather than abstract, and •a priori rather thanempirical. If we can discover this pure intuition and whatmakes it possible, we will then be able to explain how there
can be synthetic a priori propositions in pure mathematics,
and thus how mathematics itself is possible. Empiricalintuition provides us with experiences that enable us toconnect concepts with other concepts, forming a posteriori
judgments that are empirically certain. Pure intuition also
lets us connect concepts with other concepts, but in theircase the synthetic judgment is a priori certain and necessary,·not merely empirically certain·. Empirical judgments contain only what we •happen to have encountered in •empiricalintuition, whereas mathematical judgments contain what•must necessarily be met with in •pure intuition. ·The plates
or coins or moons that I happen to have seen or felt may
be significantly unlike the ones that you have encountered;
but there can’t be any such difference between my a priori
intuition of a circle and yours·. An a priori intuition is
inseparably joined with the concept before all experience,
independently of every particular perception.

9
If our intuition had to represent things as they are in themselves, no intuition could ever take place a priori; intuition
would be empirical every time. ·Here is why·. If an intuition
takes place a priori, then no object of it is present and given
to me; but if the object isn’t present and given to me, I can’tknow what it is like in itself. Actually, even if an object
is intuitively present to me, it is incomprehensible how I
could know a thing as it is in itself, for a thing’s properties
cannot migrate into my mind! ·Since I can’t get the thing’s
own properties into my mind, the most I can do is to havein my mind my representations of them; but that meansthat I am taking in the thing not •as it is in itself but •as I
perceive and think about it·. Never mind that just now; let us
pretend that this is possible. My present point is that suchan intuition wouldn’t take place a priori, i.e. before the object
was presented to me; for if the object were not present, there
would be nothing that connected my representation—·myintuition·—in any way with that object in particular.
There’s only one way to have an intuition that precedes
the reality of the object, and thus occurs as a priori knowl-

8
Now we seem to have made the problem worse than ever,for now we have to ask: How can one intuit anythinga priori? An intuition is a representation of a sort that·ordinarily· depends directly on the presence of the object.
·There’s no problem about an intuition of an object that ispresent to one at the time, or of an object that has beenpresent at an earlier time and is still remembered·. It seems
impossible, though, to intuit something a priori without help
from any outer stimulus. Such an intuition would have17

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Immanuel Kant

How is pure mathematics possible?

senses·. Mathematics can’t proceed analytically by dissecting
concepts, but only synthetically; so without pure intuition it
can’t take a single step, since only pure intuition provides
the material for synthetic a priori judgments. Geometry isbased on the pure intuition of space. Arithmetic forms its
own concepts of numbers by successively adding units intime. Our representations of space and time are merely
intuitions, however, ·rather than concepts·; and here is why.
Start with empirical intuitions of bodies and their changes,
and strip them of everything empirical—i.e. everything you
know about them through sensation—and what you are left
with is space and time. These are therefore pure intuitions.
They must be involved in all empirical intuitions, and cannever be omitted, because they underlie everything empirical.
But just because they are themselves pure a priori intuitions,they must be mere forms of our sensibility. They precede
all our empirical intuition, i.e. all our perceptions of real
objects; through them we can know objects a priori, though
indeed only as they appear to us ·and not as they are inthemselves·.

edge. I could have such an intuition if it contained nothing
but the form of my sensibility. ·My sensibility is my capacity
for being affected by particular real things. Through it I cometo have sensible intuitions. In any such transaction with anobject, the faculty of sensibility makes its own contribution;
the intuitions that occur in my mind depend not only on•what the objects are like but also on •the characteristic
marks left on them by my faculty of sensibility; these constitute its form ·. The form of my sensibility is available
to me in advance of any of the impressions in which I am
affected by objects. ·That’s because I know in advance that,whatever my particular experience turns out to be like, itwill reflect the form of my sensibility; which is to say that· I
can know a priori that I can intuit objects of the senses onlyin accordance with this form of sensibility. It follows •that
there can be, ·and we can know·, propositions that concern
merely this form of sensibility, •that such propositions arevalid for objects of the senses, and •that they can’t be applied
to anything except objects of our senses.

10

11

Thus it is only through the form of sensible intuition [=‘form of sensibility’] that we can intuit things a priori. Such apriori knowledge, however, concerns objects only as they
appear to us through our senses, and not as they may be inthemselves. If a priori knowledge of synthetic propositions is
to be possible, and we are to understand how it is possible,
it must be subject to this ·limitation to how things •appear
as distinct from how they •are in themselves·.
Now, space and time are the two intuitions on which puremathematics bases all its judgments that present themselves
as certain and necessary. Pure mathematics must construct
its concepts on the basis of pure intuition, ·i.e. the kind of
intuition that is conducted a priori, with no reliance on theThat solves the problem about how mathematics is possible.
Pure mathematics is possible only because it bears on mere
objects of the senses. The empirical intuition of such objects
is grounded a priori in a pure intuition of space and time,
and this pure intuition is merely the form of our sensibility.
It precedes the actual appearance of objects, since it makes
it possible for them to appear to us. ·Objects can appear
to us only through our sensibility; and anything we getthrough our sensibility bears the marks of the form ofsensibility·. Our a priori intuitions don’t involve the content
of the appearance, the element of sensation in it, for thatbelongs to the empirical realm; they involve the form of the18

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appearance, namely space and time. If you suspect thatspace and time are features of things in themselves rather
than mere features of how things relate to sensibility, thentell me: How in that case could we know a priori—in advanceof any experience of things—what the intuitions of space andtime must be like? Yet we do know this. There’s no problem
about this knowledge so long as space and time are taken
to be nothing more than formal conditions of our sensibility,
and the objects are taken to be merely appearances. For
then the pure intuition that embodies the form of sensibility
can be understood as coming from us—from our side of thetransaction with objects—which means that it can be had apriori rather than empirically.

How is pure mathematics possible?

as not bounded by anything; and this can only come fromintuition, and could never be inferred from concepts. Thus
mathematics is really grounded in pure a priori intuitions;they are what enable it to establish synthetic propositions
as necessary and certain.
[In this paragraph Kant speaks of a certain ‘transcendental deduction’ of certain concepts. A ‘deduction’ is a theoretically grounded or
justified list; it is ‘transcendental’ in Kant’s main sense of that word if
it is based on considerations about what makes some kind of a priori
knowledge possible ].

Hence our transcendental deduction of
the concepts of space and time—·i.e. our establishing thatwhatever is given to us in experience must be in space andin time·—also explains the possibility of pure mathematics.
If we didn’t have such a deduction, and couldn’t take it
for granted that whatever is presented our senses—whether
outer (space) or inner (time)—is experienced by us only as it
appears, not as it is in itself, we could still do mathematics
but we wouldn’t have any insight into what it is.

12
To clarify and confirm all this, we need only to look at how
geometers do (and absolutely must) go about proving thattwo figures are completely congruent, meaning that one canbe replaced at all points by the other. All such proofs ultimately come down to this: The two figures coincide with eachother; which is obviously a synthetic proposition resting on
immediate intuition. This intuition must be given pure and apriori, otherwise the proposition couldn’t hold as absolutely
certain and necessary. If it rested on an empirical intuition,
it would only have empirical certainty, and would mean:
So far as our experience has shown us, this proposition has
held until now. That space has three dimensions, and thatno space could have more, is built on the proposition thatnot more than three lines can intersect at right angles in apoint. This proposition can’t be shown from concepts, but
rests immediately on intuition, and indeed (because it isnecessary and certain), on pure a priori intuition. That a line
can be drawn to infinity, or a series of changes continued
to infinity, presupposes a representation of space and time

13
If you can’t help thinking that space and time are real qualities attached to things in themselves, try your intelligence on
the following paradox. When it has defeated you, you may
be free from prejudices at least for a few moments, and thenyou may be more favourably disposed towards the view thatspace and time are mere forms of our sensible intuition.
If two things are completely the same in every respectof quantity and quality that can be known about eachseparately, you would expect it to follow that each can be
replaced by the other in all cases and in all respects, without
the exchange causing any recognizable difference. That is thecase with two-dimensional figures in geometry, but not withthree-dimensional ones: it can happen that two of them havea complete inner agreement yet also have an outer relation
19

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Immanuel Kant

How is pure mathematics possible?

Note I

such that one can’t be replaced by the other. . . . What canbe more like my hand, and more equal in all points, than its
image in the mirror? Yet I can’t put such a hand as is seen in
the mirror in the place of its original: for if the original wasa right hand, the hand in the mirror is a left hand, which
could never serve as a substitute for the other. Here are no
inner differences that any understanding could think—·that
is, no differences that can be expressed in concepts·—andyet the differences are inner as far as the senses tell us, forthe left hand can’t be enclosed in the same boundaries as theright (they aren’t congruent), despite all their equality andsimilarity. For example, the glove of one hand can’t be usedon the other. So the two hands are intrinsically different ina manner that can’t be captured in concepts—it can only
be shown through the fact that a spatial region that exactly
contains one won’t contain the other.

The propositions of geometry aren’t mere fantasies that might
have nothing to do with real objects. Pure mathematics, andin particular pure geometry, is objectively valid, but only
in application to objects of the senses. When we represent
such objects through our sensibility, we represent them notas they are in themselves but only as they appear to us.
So they must have any features that are conferred on themby the form of our sensibility—and in particular they must
be in space, because space is simply the form of all outerappearances. Outer appearances are possible only throughsensibility, the form of which is the basis for geometry; soouter appearances must conform to what geometry says
about them.
If the senses had to represent objects as they are inthemselves, the situation would be quite different. For thenthe facts about our representation of space would provide no
guarantee about how things are in reality. The space of thegeometer—a mere representation—would be a fiction with noobjective validity, for there would be no reason why things
should have to conform to the picture that we make of themin advance of being acquainted with them. But if•this picture, or rather this formal intuition, comesfrom the essential nature of our sensibility, throughwhich objects must be given to us,
and if•what this sensibility represents aren’t things in themselves but only their appearances,it then becomes conceivable—indeed undeniable—that•all outer objects of the world of the senses must agree
exactly with the propositions of geometry.
It is a remarkable fact that at one time mathematicians
who were also philosophers began to have doubts about

How can this be? Well, these objects are not representations of the things as they are in themselves, but are sensible
intuitions, i.e. appearances, which come about throughthe relation to •our sensibility of •certain things that are
unknown in themselves. When this sensibility is exercised
as outer intuition, its form is space; and the intrinsic natureof any region of space is fixed by how that region relates tospace as a whole, the one big space of which it is a part.
(The part is made possible by the whole: ·a small regionof space can exist only if there’s a larger region of which it
is a part·. This never happens with things in themselves,
but it can happen with mere appearances.) Thus, to makeintelligible to ourselves the difference between similar andequal yet incongruent things (e.g. snails winding oppositeways), we must relate them to the right and the left hand.
That means that it must be done through intuition; it can’t
be done through any concept. ·That is, it must be done byshowing, and can’t be done by telling·.
20

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Immanuel Kant

their geometrical propositions—not about whether they weretrue of space, but about they held good in application tonature, that is to things in space. They feared that a line innature might consist of physical points, in which case thespace of the natural object would consists of simple parts,
although the space the geometer thinks about can’t be likethat. ·That is, they feared that the space of natural objects
might not be infinitely divisible, and might instead be madeup of atoms of space, so to speak; whereas geometrical spaceis infinitely divisible·. They didn’t realize that the spatiality ofouter things must exactly conform to the space the geometer
thinks about, because:•all objects in space are mere appearances, i.e. notthings in themselves but representations of our sensible intuition; and•the space the geometer thinks about—space inthought, we might call it—is just a form of our facultyof sensible representation.
·Putting these two together: an outer thing must be given
to us through our sensibility, so it must conform to theform—the essential nature—of our sensibility, so it mustobey the propositions of geometry·. This is the only way
to defend the objective validity of geometrical propositions
against shallow metaphysical attacks. . . .

How is pure mathematics possible?

follows is this: All bodies, along with the space that contains
them, are merely representations in us, and exist only in our
thoughts. ‘Isn’t this blatant idealism?’ ·No, it is not, and I
now explain why·. Idealism says this:Only minds exist, and the other things we think
we perceive are only representations in us, with noexternal object corresponding to them.
I say the contrary:Things are given to us as objects of our senses, existing outside us, but we know nothing of what they arein themselves; all we know are their appearances, i.e.
the representations they cause in us by affecting oursenses.
So I say that there are bodies outside us—i.e. things of whose
nature in themselves we know nothing, knowing them only
through our representations of them. We call such a thing a‘body’, meaning ‘the appearance to us of an unknown thingwhich is nevertheless real’. Can this be called idealism? It isthe very opposite of it!
Long before Locke’s time, but more so afterwards, it
was generally accepted that although outer things are perfectly real, many of their properties belong not to things inthemselves but only to their appearances. These properties,
including heat, colour, taste etc., were agreed to have no existence of their own outside our representations. I go further.
I count also as mere appearances the remaining qualities
of bodies—the ‘primary’ qualities of extension, place, andspace in general with all that depends on it (impenetrability
or materiality, shape, etc.). I have weighty reasons for thisview, and there isn’t the slightest reason to reject it. A man
who holds that •colours are aspects of the sense of sight andnot qualities of the object in itself should not on that accountbe called an idealist. So I should not be called idealist either,merely because I hold that •all the qualities that make up

Note II
Anything that is to be presented to us as an object must be
given in intuition. But all our intuition happens throughthe senses—the understanding doesn’t intuit anything. Now,
we have just seen that the senses never ever enable us toknow things as they are in themselves; all we encounter
through the senses are the appearances of things; and theseappearances are mere representations of sensibility. What21

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the intuition of a body belong merely to its appearance. This
doctrine of mine doesn’t destroy the existence of the thingthat appears, as genuine idealism does; it merely says that
we can’t through our senses know the thing as it is in itself.
What would I have to say to stop people from accusing
me of idealism? It wouldn’t be enough for me to say:
Our representations of outer objects are perfectly
appropriate, given how our sensibility relates to thoseobjects;
for that’s what I have said ·and still the accusations continue·. I would also have to say:
Our representations of outer objects are exactly likethe objects themselves.
But that, to me, makes as little sense as the assertion thatthe sensation of red is like the property of the pigment thatcauses this sensation in me.

How is pure mathematics possible?

understanding to think about are appearances only, notthings themselves. Now that I have given this necessary
corrective, it would be an unpardonable misunderstanding—almost a deliberate one—to say that my doctrine turns all
the contents of the world of the senses into pure illusion.
When an •appearance is given to us, it is up to us tochoose how to •judge the matter. The •appearance depends
on the senses, but the •judgment depends on the understanding, and the only question is whether a given judgment
is true or not. But the difference between truth and dreaming isn’t ascertained by •the nature of the representationsin question (for they are the same whether or not one is
dreaming), but by •their inter-connections according to therules that bring representations together under the conceptof an object and settle whether or not they can co-exist ina single experience. And it isn’t the appearances’ doing if
our mind takes illusion for truth, i.e. if it takes the intuition
through which we are given an object to be a concept of thething or even to be the thing itself—these being items that theunderstanding can think ·but the senses can’t present ·. The
senses represent the planets to us as moving backwards
and forwards, and in this there’s neither falsehood nor
truth, because as long as we take this ·planetary· path tobe nothing but appearance, we make no judgment about
the objective nature of the planets’ movements. But when
the understanding isn’t on its guard against this subjective
representation’s being taken to be objective, a false judgment
can easily arise—‘They seem to be moving backward’, wemay say. The illusion mustn’t be charged to •the senses,
however, but to •the understanding, whose job it is to renderan objective judgment on the basis of the appearances.
Thus, even if we gave no thought to where our representations come from, when we connect our sensory intuitions(whatever their content) in space and in time, according toNote III‘When you admit the ideality of space and time, you turnthe whole world of the senses into pure illusion’—it is easy
to foresee that this complaint will be levelled, and easy toshow, from what I have said, that it is futile. At first allphilosophical insight into the nature of knowledge throughthe senses was tainted by taking sensibility to be a mode ofrepresentation which, though confused, lets us know things
as they are without our being able to get the whole content
of this representation clear in our minds. ·Replacing thatdisastrous mistake·, I showed •that sensibility has to be understood in terms not of this logical clear/obscure distinction
but of something genetic, having to do with where knowledge
comes from—sense-perception represents things not as they
are but only the mode in which they affect our senses—andconsequently •that what sense-perception provides for the22

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Immanuel Kant

the rules governing the way all knowledge hangs together
in experience, we will encounter illusion if we are negligent
and truth if we are careful. The difference between illusion
and truth turns on how sensory representations are handled
in the understanding, not on where they come from. In thesame way, if I
•take all the representations of the senses to be nothing
but appearances,•take space and time also to be appearances and asa mere form of sensibility that isn’t to be met withoutside its borders, and•use these representations only in relation to possible
experience,then my regarding them as appearances won’t involve me inthe slightest temptation to think in terms of error or illusion;
for appearances can hang together according to rules oftruth in experience. ·Whether they do so hang together issomething I can determine without bringing in their ultimate
status, i.e. the question of whether space and its contents areappearances·. That’s how all the propositions of geometry
hold good for space as well as for all the objects of the senses
and consequently of all possible experience, whether I •takespace to be a mere form of the sensibility or •regard it assomething that clings the things themselves; though it is
only in the former case that I can grasp how I can know apriori that these propositions are true of all the objects ofexternal intuition. Apart from that one matter of knowing
how geometry can be known a priori, all my dealings withspace and its contents are just what they would have been if
I hadn’t departed from the common view.
But there’s a way in which an error could arise. If I
pass off space and time as qualities inherent in things inthemselves, there will be nothing to stop me from thinking
that those two concepts would hold good for the same things

How is pure mathematics possible?

that they now apply to even if my senses were different
and couldn’t present those things to me; and so I shall be
led to venture to carry my notions of space and time out
beyond all possible experience; and then I can fall victim toan illusion that would generate a grave error, namely that of
passing off as valid for everything something that is merely
a subjective condition of the intuition of things and valid
only for all objects of sense, i.e. for all possible experience. I
would be led into this error by thinking of space and time as
containing •things in themselves, rather than as restricting
them to the •conditions of experience.
So my doctrine of the ideality of space and of time ·(i.e.
my doctrine that space and time are appearances) comes
nowhere near to turning the whole world of the senses
into mere illusion. I shall offer two graphic illustrationsof this. Firstly, the doctrine· is so far from •turning thesensible world into illusion that it is the only means of•saving something from being regarded as mere illusion;
what it saves is one of the most important kinds of knowledge(the kind that mathematics propounds a priori), which thedoctrine guarantees does apply to actual objects. ·Here is
why it is the only way of securing this result·. Without theideality of space and time it would be quite impossible todecide whether the intuitions of space and time—which we
don’t take from any experience, and which nevertheless lie
in our representations a priori ·so that we take them to every
experience·—aren’t mere phantoms thrown up by our brain,with nothing adequately corresponding to them, in whichcase geometry itself is a mere illusion; whereas we have beenable to prove geometry’s unquestionable validity with respect
to all the objects of the sensible world, just because they are
mere appearances.
Secondly, it is so far from being the case that these principles of mine turn the truth of experience into mere ·sensory
23

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Immanuel Kant

illusion· by making appearances of the representations of thesenses, that they are rather the only means of preventing thetranscendental illusion by which metaphysics has hitherto
been deceived, leading to an infantile snatching at bubbles
by metaphysicians who took appearances—which are mere
representations—to be things in themselves. [By ‘transcen-

How is pure mathematics possible?

well-defined notions by putting their own follies in the placeof them. I have myself given this theory of mine the name•‘transcendental idealism’, but that can’t entitle anyone tomuddle it either with the •empirical idealism of Descartes
or with the •mystical and visionary idealism of Berkeley.
(My critique ·of pure reason· contains the proper antidote tophantoms like Berkeley’s. As for Descartes: all he had was an
insoluble problem, which led him to think that everyone is atliberty to deny the existence of the corporeal world because it
could never be proved satisfactorily.) Doubting the existenceof things constitutes ‘idealism’ in the ordinary sense; but
the doctrine I have labelled as ‘idealism’—·in the phrase‘transcendental idealism’·—doesn’t concern the existenceof things, since it never entered my head to doubt thatthey exist. Rather, it concerns the sensory representationof things, especially of space and time. All I have shown
regarding space and time, and thus more generally regarding
all appearances, is that they aren’t •things but mere •featuresof how we represent things, and aren’t qualities of thingsin themselves. But the word ‘transcendental’ was meant toguard against this misconception. (For me, ‘transcendental’signifies a reference to our knowledge not •of things but
only •of our ability to have knowledge. ·I characterized myidealism as ‘transcendental’ because it offers an explanationof how we can know certain things a priori ·.) But rather
than furthering the misunderstanding, I now retract thelabel ‘transcendental’ and ask that my idealism be called‘critical’. But if it really is an objectionable idealism to convert
actual things (not appearances) into mere representations, bywhat name shall we call the idealism that goes the opposite
way and changes mere representations into things? It may,
I think, be called ‘dreaming idealism’, in contrast to theformer, which may be called ‘visionary’. Both are refuted by
my transcendental idealism—or, better, critical idealism.

dental illusion’ Kant here means something like ‘abstract philosophicalillusion’. His more special sense of ‘transcendental’, explained near theThatillusion is what brought on stage the remarkable antinomy [=‘contradiction’] of reason that I shall return to in sections 51–3.
All it takes to clear up this ·internal contradiction into which
reason falls· is a single observation: that appearance, as longas it is employed in experience, produces truth, but as soon
as it goes beyond bounds of experience and consequently
becomes transcendent [= ‘freed from any constraints having to do
end of section 12, will come up again in the next paragraph.]with the senses’; not the same as ‘transcendental’ in either of the latter’ssenses], it produces nothing but illusion.

Thus, in letting things as we confront them throughthe senses retain their actuality, and limiting our sensory
intuition of these things only by saying this:In no respect—not even in the pure intuitions ofspace and of time—do they represent anything morethan mere appearance of those things, never theirconstitution in themselves,
I am not imputing to nature a sweeping illusion. [For thephrase ‘pure intuition’, see the explanation on pages 9–9.] My rejectionof all such imputations is so obviously valid and convincingthat one might think there was no need for it. And there
wouldn’t be, if it weren’t for the existence of incompetentjudges who—liking to have an old name for everythingthat diverges from their own wrong-headed though common
opinions, and always clinging to the letter of what is said
with no thought for its spirit—are ready to deform and distort24

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Immanuel Kant

How is pure natural science possible?

Main transcendental problem 2:
How is pure natural science possible?
14·The word ‘nature’ has two senses. I shall use it in what I

If I am to bring things under ·causal· laws, these laws must
apply to them necessarily, and experience could never showme how things must be—only what there is and how it is. So
it can never teach me the nature of things in themselves.

shall later call its ‘formal’ sense in this section and the next;
and then in section 16 I shall start to use ‘nature’ in what I
call its ‘material’ sense. Both will be in play in section 36·.
Nature is the existence of things insofar as it is governed byuniversal ·causal· laws. If this meant the existence of thingsin themselves, we couldn’t know nature either •a priori or •aposteriori. ·One way of knowing things a priori is knowing
them through the analysis of concepts·. We couldn’t know
nature as it is in itself in that way, because knowledge ofwhat things are like in themselves can never come fromanalytically dissecting our concepts: we aren’t asking what
is contained in our concept of the thing, but rather about
what is added to this concept in the reality of the thing itself.
·Some synthetic propositions can be known a priori because
their truth is assured by the nature of our understanding,somewhat in the way that mathematical truths can be knowna priori because our sensibility assures their truth. But thisis also not applicable to the supposed ‘knowledge of nature as
it is in itself’, which we are discussing·. My understanding·has an effect on how things appear to me, but it· can’t
dictate what things are like in themselves. They don’t have toconform to it; so if I am to know about things in themselves,
my understanding must conform to them, ·not vice versa·.
That means that I couldn’t know about them until they
had somehow been presented to me; which is to say that I
couldn’t know them a priori.
Nor could I have a posteriori knowledge—·i.e. knowledge
through experience·—of the nature of things in themselves.

15
Yet we do have pure natural science, which discovers a priori
certain laws that govern all of nature, and discovers them tobe necessary. One part of it is what we call ‘general naturalscience’, which is a preliminary to empirical physics. In this
we find •mathematics applied to appearances ·on the basis of•intuition·, and also •conceptual principles that make up the•philosophical part of pure knowledge of nature. ·A coupleof qualifications should be mentioned·. •It isn’t strictly pure,because there are things in it that are based on experience,such as the concepts of motion, of impenetrability, of inertia.
•Nor is it ‘general’ in the strictest sense, because it concerns
only the objects of the outer senses, whereas a truly generalnatural science would apply laws to the whole of nature—not
only outer objects (physics) but also inner ones (psychology).
Still, some principles of this general physics are strictly
universal, for instance the propositions ‘Substance is permanent’ and ‘Everything that happens is determined by a cause
according to constant laws’. These really are universal lawsof nature that we can know a priori. So pure natural science
does exist, and we have to ask: How is it possible?

16
I now want to use the word ‘nature’ in a broader sense, its
material sense, in which it refers to ·every aspect of· •thetotality of all objects of ·possible· experience, i.e. the whole25

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Immanuel Kant

How is pure natural science possible?

govern our •ways of knowing also govern •the objects that
we know, as long as these are considered as objects ofexperience and not as they are in themselves. There are
two things we can say:
(1) A judgment of perception can’t count as valid for experience unless the mind in which it occurs conforms
to the following law: When any event is observed tohappen, it is connected with some earlier event that it
follows according to a universal rule.
(2) Everything that we experience as happening must becaused to happen.
It makes no difference which we say: they come down to thesame thing.
Still, we’ll do better if we start with (1). We can make apriori discoveries about what the conditions are under which
experience is possible, but we can’t make such discoveries
about laws that apply to things in themselves independentlyof our experience of them. So our only way of studying •thenature of things a priori is by studying •the conditions underwhich experience is possible, including the universal lawsof the mind that make it possible. ·What I am saying, ineffect, is that we should tackle (2) by tackling (1)·. If I choseto start with (2), I would risk falling into error by imaginingthat I was talking about nature in itself. That would set me
whirling around in endless circles, trying in vain to discoverlaws governing things that aren’t given to me ·as things are
given to me in experience·.
So our only concern here will be with experience and withwhat universal conditions have to be satisfied for experience
to be possible—conditions that we can know about a priori.
On that basis we are to establish the characteristics ofnature as the whole object of all possible experience. You
will understand, I think, that I am not talking about •rulesthat we learn by observing a nature that is already given,

perceivable world. Up to this point I have been using ‘nature’in its narrower sense, making it refer only to •the way all
things fall under the system of laws.
The perceivable world is all we have to concern ourselveswith. If we tried to learn about things that couldn’t be objectsof experience, we would have to think about them throughconcepts that couldn’t be illustrated or cashed out in termsof any possible experience. Such concepts would be empty;
we ·could play around with them in our minds, but· we
could never know whether they applied to anything rather
than being mere fictions contrived by us. Knowledge ofsomething that couldn’t be an object of experience would
be supernatural ·in the quite literal sense of being above
nature·, and the supernatural is no part of our present
concern. The knowledge that we care about is the sortwhich, although it •precedes experience ·rather than •arisingout of it·, can •be confirmed by experience.

17
It has just been shown that the laws of nature can never beknown a priori of objects considered in themselves (rather
than in terms of possible experience of them). But we aren’t
concerned here with things in themselves; their properties
don’t interest us. Our concern is with things considered as
objects of a possible experience, and the totality of thesethings is what we here call ‘nature’ in the broad sense. Now,
we are going to enquire into what enables us to have a priori
knowledge of nature, and we have to choose between two
ways of framing our problem.
•How can we know a priori that experience itself must
conform to law?
•How can we know a priori that things (considered as
objects of experience) must conform to law?
The two questions turn out to be equivalent. The laws that26

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Immanuel Kant

for such rules already presuppose experience; so I am nottalking about how through experience we can study the lawsof nature, for laws learned in that way wouldn’t be laws apriori, and wouldn’t supply us with a pure natural science.
Rather, my topic is the question of how •the conditions that
we can know a priori have to be satisfied if experience is to
be possible are at the same time •the sources from which all
the universal laws of nature must be derived.

How is pure natural science possible?

All our judgments start out by being judgments of perception, and thus as valid only for us (i.e. for our subject).
Later on we make them refer to an object, and mean them
to be valid for all people and for ourselves at all times. A
judgment’s being about an object connects with its being
universally valid, and the connection runs both ways. On the
one hand: if my judgment is about an object, then anyone
else’s judgment about that same object should agree withmine, which is to say that mine must be universally valid.
On the other hand: if a judgment of mine is universally valid,
agreeing with the judgments of all others, this agreement has
to be explained. The explanation must be that the judgments
agree with one another because they all refer to the same
object.

18
The first thing to make clear is this: although all judgments ofexperience are empirical (i.e. have their ground in immediate
sense-perception), the converse doesn’t hold: not all empirical judgments are judgments of experience. That’s because
a judgment of experience must contain more than merely
an empirical component, given through sensory •intuition.
It must also involve particular •concepts that ·don’t comefrom sense-experience, but· originate a priori in the pureunderstanding—concepts under which every •perception
must first be brought and then by means of them changed
into •experience.
Empirical judgments fall into two kinds: •judgmentsof experience and •judgments of perception. The former
are objectively valid. They are based on immediate sense
perception, but they add to it: when something is given
to sensible intuition, a •judgment of experience applies toit certain special concepts that pure understanding gives
rise to, completely independently of experience. Perceptions
are turned into experience by being brought under theseconcepts. •Judgments of perception are only subjectively
valid: all they need is that the perceptions hang together
in the right way in mind of the person concerned (thesubject); they don’t involve any of the pure concepts of theunderstanding.

19
So something’s being true of an object is equivalent to its
having to be the same for everyone: •objective validity and•necessary universal validity stand or fall together. When
we regard a judgment as universally valid and necessary,
we mean by this that it is objectively valid, even though we
don’t know the object in itself. We know the object throughthis judgment—i.e. through the judgment that anyone who
has perceptions of kind F with respect to the object mustalso have perceptions of kind G. So judgments of experience
get their objective validity not from immediate knowledge ofthe object but from how perceptions are connected with one
another; and these connections come not from anything empirical but from pure concepts of the understanding. ·They
can’t have an empirical basis because they involve necessity;
the judgments in question say that certain perceptions must
be associated with certain others; and experience nevertells us what must be the case·. The object in itself always
remains unknown; but it gives us perceptions through our27Prolegomena

Immanuel Kant

sensibility, and these are connected; and when a concept of
the understanding settles it that the connection is universally
valid, the result is an objective judgment—something that
doesn’t merely report on perceptions but says things aboutan object.
Here is an illustration. That the room is warm, sugar
is sweet, wormwood is nasty, are merely subjectively valid
judgments.1 ·In making such judgments·, I don’t expectthat I shall find the room to be warm or sugar sweet or
wormwood nasty at all times, or that everyone else will findthem to be so. All that such a judgment does is to connect
two sensations to a single subject (myself) at a particulartime; they aren’t intended to be valid of the object. I callthem judgments of perception. Matters are quite different
with judgments of experience. What experience teaches me
under certain circumstances it must teach me and everybody
always; its validity isn’t limited to one person or one time;
so its judgments are objectively valid. For example, when
I say that air is elastic, this starts out as a judgment of•perception, which merely connects two of my sensations to
one another. But if I mean it as a judgment of •experience,
I require that this connection be universally valid, i.e. that
I and everybody must always conjoin the same sensations
under the same circumstances.

is judging, which belongs entirely to the understanding.
There are two kinds of judging. (1) I may merely compare
perceptions and conjoin them in a consciousness of mystate. (2) I may conjoin them in consciousness in general.
What I have in (1) is merely a judgment of perception, a
subjectively valid connecting of perceptions in my mind,
without reference to an object. People often think that all
you need for experience is to compare perceptions and toconnect them in your consciousness by means of judgments
about them; but they are wrong. That procedure doesn’tlead to judgments that are universally valid and necessary,and that’s what is needed for objective validity and for realexperience.
To turn perception into experience, therefore, we need (2)a different kind of judging. An intuition (or perception) mustbe brought under a pure a priori concept of the understanding; this concept settles what kind or form of judgment canbe made about this intuition; thus it connects the individual
person’s intuition with a frame of mind that •anyone must
be in when making judgments about such intuitions; and in
this way it provides the empirical judgments with •universal
validity. Such a concept, I repeat, merely fixes a general way
in which judgments can be brought to bear on the intuition.
It might be the concept of cause, for instance. To bring thisto bear on one’s intuition (or perception) of air, for example,
is to be disposed to make hypothetical judgments of the form
‘If air is compressed, then. . . ’.

20
So experience is a product of the senses and of the understanding, and we have to discover how these two faculties
combine to produce it. One of them is simply intuition ofwhich I am conscious, i.e. perception, which belongs merely
to the senses. The second element that goes into experience1

How is pure natural science possible?

Before a judgment of perception can become a judgmentof experience, the perception must be brought under such a
concept of the understanding—as when air is brought under

Actually, these judgments of perception could never become judgments of experience, even if a concept of the understanding were added. They refermerely to feeling, which is incurably subjective and can never become objective. Still, they serve my immediate purpose of illustrating judgmentsthat are merely subjectively valid, involving no relation to an object. In the next footnote I shall give an example of judgments of perception that canbecome judgments of experience.

28

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Immanuel Kant

the concept of cause, yielding judgments of the form ‘If air
is. . . , then. . . ’. The judgment that air is elastic can become
universally valid, and thus be turned into a judgment ofexperience, because of certain preliminary judgments thatbring the intuition of air under the concept of cause andeffect. (An easier example is: ‘When the sun shines on
the stone, it grows warm.’ This is a mere judgment ofperception and contains no necessity, no matter how often
I and others may have perceived this. But if I say ‘The sun
warms the stone’, ·which means that the sun causes thestone to become warm·, the concept of cause is added to theperception and connects the concept of warmth necessarily
with the concept of sunshine.)
If all our objectively valid synthetic judgments are analysed, it turns out that they never consist in mere intuitions
that are brought together in a judgment through mere comparison. Always, a pure concept of the understanding has
been added to the concepts that are abstracted from intuition.
This applies even to the judgments of pure mathematics,
including its simplest axioms. The principle ‘A straight lineis the shortest path between two points’ presupposes thatthe line has been brought under the concept of size. That
concept doesn’t come from intuition; it has its seat solely
in the understanding, and serves to get the intuition (of theline) ready for quantitative judgments to be made about it.

How is pure natural science possible?

Logical table of judgments
1

2

Q UANTITY
Universal
Particular
Singular

Q UALITY
Affirmative
Negative
Infinite

3
R ELATION
CategoricalHypothetical
Disjunctive

4
M ODALITY
Problematic
Assertoric
Apodictic

Table of concepts of the understanding
1
Q UANTITY
Unity (measure)
Plurality (size)
Totality (whole)

2
Q UALITY
Reality
Negation
Limitation

21
3

If we are to prove that experience is possible insofar as it
rests on pure a priori concepts of the understanding, weneed a •list of these concepts. We arrive at this list through a•list of basic kinds of judgments that we can make, because
the pure concepts of the understanding run parallel to those
judgment kinds. . . .

R ELATION
Substance
Cause
Causal interaction

29

4
M ODALITY
Possibility
Existence
Necessity

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Immanuel Kant

How is pure natural science possible?

Pure physical table of the universal principles of natural science

1

2

Axioms of Intuition

Anticipations of Perception

3

4

Analogies of Experience

Postulates of empirical thinking generally

*****

21a

merely universal in the weak way that comes from comparing
intuitions with one another·. This added element can only
be the concept that represents the intuition as a suitablesubject for one form of judgment rather than another.

If we are to grasp all this in a single thought, I must first
remind you that our topic isn’t where experience comes frombut what experience contains. The former topic belongs toempirical psychology, though even that wouldn’t sufficewithout the latter topic, which belongs to the critique ofknowledge and especially of the understanding.
Experience consists of intuitions, which belong to thesensibility, and of judgments, which are entirely a work of theunderstanding. But the •judgments that the understanding
forms from sensory intuitions alone are not •judgments ofexperience. They merely connect the perceptions as given
in sensory intuition, while a judgment of experience must
express •what is contained in experience in general, andnot merely •what is contained in the mere perception (which
has only subjective validity). So a judgment of experiencemust •add something to •the sensuous intuition and •thelogical tie-up of that intuition to others in a judgment (after
it has been made universal by comparing ·this intuition withothers·). It must •add something implying that the synthetic
judgment is necessary and therefore universally valid—·not22
Summing up: The business of the senses is to intuit; thatof the understanding, to think. Now, thinking is unifying
representations in a consciousness, and this can be done
either in •a contingent and subjective way or in •a mannerthat is necessary and objective. Since thinking is the same
as judging, it follows that judgments are of two kinds: ajudgment may be merely•subjective (when representations are inter-related
only with respect to one person’s consciousness),or it may be•objective (when the representations are related withrespect to consciousness in general, i.e. with respect
to every possible conscious mind).
The basic kinds of judgment are simply possible ways of
unifying representations in a consciousness; and when they
serve as concepts, they are concepts of the necessary unifica30

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Immanuel Kant

How is pure natural science possible?

tion of representations in any consciousness, which meansthat the judgments that involve them are objectively valid.
In experience, perceptions are synthetically but necessarily
connected in a consciousness; for them to be connected inthis manner, they must be brought under pure concepts of
the understanding; so these concepts are required if any
judgments of experience are to be made.2

a transcendental system [meaning, roughly, ‘a system that has to
do with grounds for a priori knowledge’.] And, lastly, the principles according to which these concepts are applied to allappearances constitute a physical system, i.e. a system ofnature. This system precedes all empirical knowledge ofnature, and is what first makes such knowledge possible; so
it can properly be called universal and pure natural science.

23

24•Judgments can be seen as ways of unifying representations
in a consciousness. Looked at in this way, they are •rules.

Of the physical principles listed in section 21, the first brings
all phenomena, as intuitions in space and time, under theconcept of quantity, which makes it a principle governing
the application of mathematics to experience. The second
principle takes up the genuinely empirical element, namely
sensation, which signifies what is real in intuitions. It doesn’tbring sensation directly under the concept of quantity, because sensation isn’t an intuition that contains either spaceor time, though it places the sensed object in both space
and time. But still there’s a quantifiable difference between
sense-representation and a total absence of intuition in time,the difference between reality and zero. For we can conceiveof intermediate degrees separating
any given degree of •light from darkness,any degree of •heat from absolute cold,
any degree of •weight from absolute lightness,any degree of •fullness of space from total vacuum;

When they represent the perceptions as necessarily united,
they are a priori rules; and when they stand on their own feet,
not being derived from something more fundamental, they
can be called ‘principles’. The broad kinds of judgment thatbring intuitions under pure concepts of the understandingaren’t derived from anything; they stand on their own feet.
So they’re the a priori principles of possible experience.
Now the •principles of possible experience are at the sametime •universal laws of nature, which can be known a priori.
This solves the problem raised by our second question, ‘How
is pure natural science possible?’ Here is how. Logic offers
us only one set of basic kinds of judgment (and thus one
set of basic rules); no other is possible. These constitutea logical system. The concepts that emerge from it, whichmake synthetic necessary judgments possible, constitute2

But how does this proposition that Judgments of experience require that perceptions be brought together necessarily square with my often-madestatement that Experience as a posteriori knowledge can only provide contingent judgments? When I say that experience teaches me something, I
mean only that I learn something from the perception that lies in experience—for example, that
Heat always follows the shining of the sun on a stone—and to that extent the proposition of experience is always accidental ·or contingent·. The proposition that
This heat necessarily follows the shining of the sun
is indeed contained in the judgment of experience (by means of the concept of cause), but it is not a fact learned from experience. On the contrary,
this addition of the concept of cause to perception is what creates experience in the first place.

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Immanuel Kant

How is pure natural science possible?

just as there are intermediate degrees—as small as you
like—separating•consciousness from total unconsciousness (psychological darkness).
So there’s no perception that can prove an absolute absence;for instance, there’s no psychological darkness that can’t
be considered as a kind of consciousness, which is merely
relatively dark, by comparison with some other stronger
consciousness—and that’s how it is in all cases of sensation.
Sensation is what gives each empirical representation (eachappearance) its own particular flavour. ·It might be thought
to be all content, with no form, and so not to be somethingthat the understanding could say anything about in advance·.
But the account I have been giving shows how the understanding can •anticipate even sensations—·i.e. •say something about them in advance of their actually occurring·—bymeans of the principle Every sensation has a degree, fromwhich it follows that what’s real in all phenomena has adegree. This is the second application of mathematics tonatural science.
·In discussing those two (sets of) principles of naturalscience, I have been implicitly discussing the corresponding(sets of) concepts, listed just before the list of principles in
section 21. In the next section I shall take up the other two
sets of concepts, and their associated principles·.

another in time, they must be brought under the conceptof •cause and effect. Thirdly, judgments of experience about
things that exist together must involve the concept of •twoway causal interaction. [Kant’s word for this is Gemeinschaft, which
is usually but unhelpfully translated by ‘community’.] Thus a priori
principles are involved in objectively valid though empirical
judgments; they are needed if we are to have real experience,
which connects objects in nature. These principles are thereal laws of nature, and can be called ‘dynamic’.
Finally, judgments of experience include ·ones expressing· knowledge of correspondences and connections; but
their topic isn’t how appearances relate to one anotherin experience, but rather how they relate to experience ingeneral. This has to do with•whether they satisfy the formal conditions that theunderstanding recognizes,
or with•whether they fit with the materials of the senses andof perception,orit •brings both of those considerations together undera single concept.
So it has to do with •possibility, •actuality, and •necessity
according to universal laws of nature.

25

My third table ·on page 30·—the table of principles that thecritical method has extracted from the nature of the understanding itself—has a completeness that raises it far above
every other table that anyone ever did or ever will offer in avain attempt to extract principles by non-critical methods
from things themselves. What makes my table complete is
this: so far as the understanding is concerned, the essenceof experience lies in the •judgments that can be made about

26

In the table of the concepts of the understanding, one of
the headings is Relation. This refers not to mathematical
relations, but rather to dynamic ones (relations having to dowith how things exist in time). ·Firstly·, appearances must be
brought under the concept of •substance; this is the conceptof a thing, and any judgment about what exists must involve
it. Secondly, when appearances involve events following one
32

Prolegomena

Immanuel Kant

it, and I have used ·properties of· the faculty of •judgment asa single guiding rationale for what is included in my table ofprinciples, namely all the synthetic a priori principles. So we
can be certain that there are no more principles of that sort,and that certainty affords a satisfaction that the dogmatic
method can never achieve. [Kant’s use of ‘dogmatic’ is explained ina note on page 12.] Yet this is not all: my table of principles has
another much greater merit ·that I shall now explain·.

themselves; that prevents them from referring to how sensory intuitions are inter-connected in a possible experience,
which is the basis on which synthetic propositions can beproved a priori. So the essential constraint on the concepts
used in these principles is: It is only as objects of experiencethat things necessarily a priori satisfy the conditions laid
down in the principles.
From this it also follows that the proof of these principles has a unique feature: namely that they aren’t directly
concerned with appearances and their ·inter·-relations, butwith the possibility of experience. Appearances ·on their own
aren’t the whole story; they· constitute only the matter ofexperience, not its form. That is, the principles I’m talking
about are concerned with objectively and universally valid
synthetic propositions, in ·the context of· which we distinguish judgments of experience from judgments of perception.
·I shall now add a little detail about how this happens—how
the principles are proved—in connection with three of thefour groups of principles listed on page 30.·

We must carefully bear in mind the premise •that enables
us to infer that there can be a priori knowledge ·such as
the table of principles involves·, and •that at the same time
subjects all such principles to the constraint that they are
only about the conditions of possible experience in generalso far as it conforms to laws a priori. If we lose sight of thisconstraint, we risk the principles’ being misunderstood, andtheir being extended in use beyond the original sense thatthe understanding attaches to them. So I don’t saythat things in themselves have a quantity,that their actuality has a degree,that their existence has a connection of qualities in a
substance,(1) ·Re the Axioms of Intuition·: Appearances, as mere

intuitions occupying a part of space and time, come under
the concept of quantity, which can be used in a rule-guided
way in synthetic a priori propositions which generalize overthese intuitions.

or the like. Nobody could prove any of those propositions, because they are synthetic—·connecting things with quantity,
degree-of-actuality, and so on·—and it is utterly impossible
to prove such synthetic propositions on the basis of mere
concepts, ·because what is proved from mere concepts isalways analytic·. The above propositions have only concepts
to work with, because they purport to be about things in
3

How is pure natural science possible?

(2) ·Re the Anticipations of Perception·: Insofar as a per-ception contains not only intuition but sensation (which
always differs from its own total absence by ever-smallerdifferences), the reality of appearances must have a degree.

Small areas of heat and light can be just as great in degree—·that is, just as intense·—as large ones; similarly, brief pains or other states ofconsciousness can be equal in degree ·or intensity· to long-lasting ones. ·Where degrees of intensity are concerned·, the quantity at a point in space
and at a moment in time can be just as great as in any space or time of whatever size or duration. So degrees are quantities, but what is quantified
is not •an intuition but rather •the mere sensation which is the intuition’s content. The only way to measure them, therefore, is through the relationof 1 to 0, that is. by their capability of decreasing by infinite intermediate degrees to disappearance, or of increasing from nothing through infinite
gradations to a determinate sensation in a certain time. The quantity of a quality is a degree ·of intensity·.

33

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Immanuel Kant

Now, sensation doesn’t itself occupy any part of space or oftime,3 but it takes time to get from empty space or time to·something involving· sensation. Thus, although sensation(taken as that quality of empirical intuition that specificallydistinguishes it from other sensations) can never be knowna priori, it can nonetheless be intensively distinguishedfrom any of the same kind as a quantity of perception inany possible experience. That’s what makes it possible toapply mathematics to nature as regards the sensory intuition
through which nature is given to us.
But pay special attention to the mode of proof of (3)the principles that occur under •the title of ‘Analogies ofExperience’. Unlike the principles of applied mathematics,
these refer not to the genesis of intuitions but to how they areinterconnected, as they actually occur, in experience; which
can only be the story of how they are made to occur in time
according to necessary laws—laws that make the conditions
objectively valid and thus create experience. So the proof ofthese principles doesn’t turn on connections amongst thingsin themselves but merely amongst perceptions; and it doesn’t
involve the matter or content of the perceptions, but only
how they are related to one another in time according touniversal laws. . . .
In these preliminaries I can’t go on longer about this,except to say one thing to my reader. You have probably beenlong accustomed to regarding experience as a mere empiricalhanging-together of perceptions, and so haven’t had thethought that it must go much beyond them, conferring universal validity on empirical judgments and for that purpose
requiring a pure and a priori unity of the understanding.
So I recommend to you that you pay special attention to
my distinction between experience and a mere aggregate ofperceptions, and to judge the mode of proof from this pointof view.

How is pure natural science possible?

27
Now we have reached the place where Humean doubt can beremoved. Hume rightly said that reason can’t give us insightinto•causality, i.e. the notion that the existence of one
thing might necessitate the existence of another.
I add that we have equally little insight into the concept of•substance, i.e. the notion that the existence of things
must be based on a subject that cannot itself be a
predicate of anything else.
Indeed, we can form no concept of the possibility of sucha thing, although we can point to examples of its use inexperience. Nor have we any insight into
•two-way causal interaction, i.e. into how substancesthat have their own entirely separate existences cannecessarily depend on one another.
None of these three concepts is supplied by reason; they
have—as I have shown—their seat in the understanding.
These concepts and the principles drawn from them standa priori before all experience; they are applicable only toexperience, but within that domain they have undoubted
objective rightness. That doctrine saves me from having toconclude that the concepts in question are borrowed fromexperience, which would mean that the necessity they involveis fictitious—a mere illusion resulting from long habit.

28
I can’t conceive how (1) things in themselves could•exist as substances, or•be causes, or•be in two-way causal interaction with others as partsof a real whole.
34

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Immanuel Kant

Still less can I conceive how any of these could be true of (2)appearances considered as raw and unprocessed perceptions
or sensory states, not brought under concepts of the understanding. But we can conceive of such connections of (3)representations in our understanding. These representations
figure in one kind of judgment as•subject related to predicates,in a second kind as•source related to upshot,
and in a third kind as•parts that are inter-related to make up a knowablewhole.
We also know a priori that unless we take the representationof an object to be related in one of these ways, we can’t
have any knowledge that would be valid of the object. Of
course if we attend to the object in itself, we are lost: there
is no possible way for me to recognize that a thing in itself
is related in one of those ways, i.e. that it belongs under
the concept of •substance or of •cause or (in relation toother substances) under the concept of •two-way causal
interaction. But things in themselves aren’t my topic. What
I am concerned with is how experiential knowledge of things
involves those three types of judgment, i.e. how objectsof experience can be brought under those concepts of theunderstanding. I have perfect insight into that: I grasp not
merely that we can bring appearances under these concepts
but that we must do so, in that way using the concepts as
principles of the possibility of experience.

How is pure natural science possible?

source and another as an upshot. I may have occasion to
make such a judgment, reporting that in my perceptions
one kind of appearance is regularly followed by another,as when I say ‘If the sun shines long enough on a body,then the body grows warm’. This doesn’t connect the twonecessarily, and it doesn’t involve the concept of cause; sofar, it is merely a subjective connection of perceptions. For
it to be a proposition of experience, it must be regarded asnecessary and as universally valid, like the proposition ‘The
sun through its light is the cause of heat’. The empiricalgeneralization with which I started is now regarded as alaw, and as being valid for appearances in a manner that isrequired if experience is to be possible—for there can’t be
experience without rules that are universally and therefore
necessarily valid. So I do have insight into the concept ofcause, as a concept necessarily belonging to the possibility ofexperience. What about the concept of things ·in themselves·
as causes? I have no conception of that, because the conceptof cause doesn’t correspond to anything in •things but only
to a •fact about experience, namely that if experience is tobe objectively valid knowledge of appearances and of theirsequence in time, some appearances must be related to laterones in conditional judgments.

30
Hence the pure concepts of the understanding have absolutely no meaning if they are pulled away from objects ofexperience and applied to things in themselves (noumena).
[Kant uses ‘noumenon’ (plural ‘noumena’) to mean ‘thing that can only29

be thought’, in contrast to ‘phenomenon’ (plural ‘phenomena’), meaning

Let us apply all this to Hume’s problematic concept, namely
the concept of cause. Sheer logic tells me a priori that
there can be conditional judgments—ones of the form ‘If. . . ,then. . . ’—in which one piece of knowledge is treated as a‘thing that can be experienced’.

Things in themselves are noumena

because although we can perhaps think about them, we can’t possibly experience them.] The role of pure concepts of the understanding

is to spell out appearances, so to speak, enabling them to be35

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Immanuel Kant

read as experience. When these concepts are applied to theworld of the senses, the principles that arise from this use
help our understanding to manage our experience. Beyond
the bounds of experience they are arbitrary connections withno objective reality: there is no a priori guarantee that they
apply to anything, and no examples can be given of theirapplicability to objects. Indeed, we don’t even know whatsuch an example could be like. We have no conception ofit, because examples have to be drawn from some possible
experience. Possible experience is the proper domain of thepure concepts of the understanding.
So the Humean problem is completely solved, though ina way that would have surprised its inventor. The solution
secures an a priori origin for the pure concepts of the understanding, and for the universal laws of nature it securesa status as valid laws of the understanding; but it does
this in such a way as to limit the use of these concepts toexperience only, and it grounds them in a relation between
the understanding and experience that is the complete
reverse of anything that Hume envisaged—instead of theconcepts being derived from experience, that experience isderived from them.
My line of argument yields the following result: All
synthetic a priori principles are simply principles of possible
experience; they can never be applied to things in themselves,
but only to appearances as objects of experience. Hence puremathematics as well as pure natural science can never bearon anything except appearances.

How is pure natural science possible?

never dawned on the dogmatic thinkers that the goal of theirefforts might be so near; nor did it dawn on the philosophers
who, proud of their supposedly sound reason, set out on theirquest for results, equipped with concepts and principles ofpure reason (which were legitimate and natural, but fit only
for merely empirical use). These philosophers did not andcould not know any fixed boundaries to the territory within
which results might be gained, because they hadn’t andcouldn’t have ever reflected on the nature of such a pureunderstanding or even on its possibility.
Many a naturalist of pure reason (by which I mean
someone who thinks he can settle metaphysical questions
without any theoretical grounding in the subject) may claimthat the prophetic spirit of his sound reason enabled him,long ago, not merely to suspect but to know and understandthe doctrine I have been advancing with so much ado (or,as he may prefer to say, with long-winded pomp), namelythat with all our reason we can never reach beyond thedomain of experience. But when he is questioned about
his principles of reason individually, he must admit thatmany of them haven’t been taken from experience and are
therefore independent of it and valid a priori. But thenwhat basis will he have for putting limits on the dogmatist
who uses these concepts and principles beyond all possible
experience because he sees them to be independent of it?
And even he, this expert in sound reason, in spite of all
his assumed and cheaply acquired wisdom, risks wandering
inadvertently beyond objects of experience into the domain of
fantasies. He is often deeply enough involved in it, though hecolours his groundless claims by adopting popular language
and announcing everything as ‘mere probability’, ‘rational
conjecture’, or ‘analogy’.

31
Until now, metaphysicians have proceeded boldly enough,but always trampling over everything blindly, without making
any distinctions. My work gives us, at last, something
definite to rely on as a guide in metaphysical enterprises. It
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Immanuel Kant

How is pure natural science possible?

32

33

Since the earliest times of philosophy, enquirers into purereason have thought that in addition to the things of thesenses, or appearances (phenomena) of the world of thesenses, there are things of the understanding (noumena),
and have thought that only the latter are real. That’s because
they took the former—i.e. appearances—to be illusory; amistake, but an excusable one in a primitive age.

Indeed, there’s something seductive in our pure conceptsof the understanding, which tempts us to use them in atranscendent manner—that being my label for a use that
goes beyond all possible experience [not = ‘transcendental’; see
explanation on page 24]. Our concepts of substance, of power, ofaction, of reality, and others are quite independent of experience, containing nothing of sensory appearance, and so they
seem to be applicable to things in themselves (noumena).
And this impression is strengthened by the fact that thoseconcepts contain within themselves an element of necessity
which experience never matches up to. The concept of cause
implies a rule according to which one state follows anothernecessarily; but experience can only show us that one stateof affairs often or usually follows another, so it can’t provide
us with either strict universality or necessity.
So the concepts of the understanding seem to havecontent and significance that spreads beyond their empiricaluse, and the understanding unknowingly builds for itself amuch larger addition to the house of experience, and fills
it with merely notional entities, without once noticing that·in doing this· it has carried its otherwise lawful concepts
beyond the bounds of their ·legitimate· use.

In fact, when we (rightly) regard the objects of the senses
as mere appearances, we thereby admit that they have athing in itself as their ground—·namely, the thing of which
they are appearances·. We don’t know what this thingis like in itself; all we know is its appearance, i.e. how
this unknown something affects our senses. In accepting
appearances, therefore, we also admit the existence of thingsin themselves: the thought of such ·noumena, i.e.· ‘things of
the understanding’, isn’t merely allowed but is unavoidable.
[In this paragraph, Kant refers to the Aesthetic. That’s the first partof the Critique of Pure Reason, concerning the status of time and space
and their relation to sensibility.] My critical deduction limits theprinciples of the Aesthetic so that they hold good only forobjects of possible experience, because extending them to allthings would be turning everything into mere appearance;
my deduction doesn’t at all imply that there are no noumena.
So these beings of the understanding are allowed, subject
to this rule, to which there can be no exceptions: We don’t
and can’t know anything determinate about these beings of
the understanding. That’s because our pure concepts of
the understanding and our pure intuitions bear on objectsof possible experience—i.e. things of the senses—and on
nothing else. As soon as we move away from the senses,
those concepts are drained of all their meaning.

34
Because of all this, the Critique of Pure Reason had to contain
two important though extremely dry investigations. In one ofthem, contained in the chapter entitled ‘The Schematism of
the Pure Concepts of the Understanding’, I show that what
the senses provide for are not •concrete applications of thepure concepts of the understanding, but only the •schemas
for their use, and that the corresponding object occurs only
in experience (as something the understanding makes outof the materials of the senses). In the second indispensable
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Immanuel Kant

the •understanding’s job is to think, and it can never be
forgiven if it wanders instead, for it is our only resource forsetting limits, when they are needed, to the wanderings ofthe imagination.
The understanding begins its misbehaviour very innocently and soberly. First it brings to light the elementary
items of knowledge that it contains in advance of all experience, though they must never be applied outside experience.
It gradually discards these limits, and what’s to prevent itfrom doing so when it has quite freely drawn its principles
from itself? ·Then, having dropped the restriction to experience·, it proceeds first to newly-thought-up powers in nature,and soon after that to beings outside nature. In short, it
proceeds to a ·non-natural· world; and there can be no
shortage of materials for constructing such a world, because
fertile fiction-making provides them in abundance—andthough it isn’t confirmed by experience it is never refutedby it either. This is why young thinkers arc so partial tometaphysics of the truly dogmatic kind, devoting to it theirtime and talents that could be better employed.
But it is no use trying to damp down these fruitless effortsof pure reason by •offering all sorts of reminders of how
hard it is to answer such deep questions, by •complaining
about how limited our reason is, and by •down-playing
our assertions as mere conjectures. The only way to getthese fruitless efforts to be completely abandoned is to•show clearly that they are impossible, and to allow reason’s
knowledge of itself to become a true science in ·terms of·which the domain of reason’s right use is distinguished withmathematical certainty from that of its worthless and idle
use.

chapter, ‘On the Basis for Distinguishing all Objects into Phenomena and Noumena’, I show that, although our pure concepts and principles of the understanding are independentof experience, and despite their seemingly greater sphere ofuse, we still can’t use them to have any thoughts whatsoever
beyond the domain of experience, because their only role is to
fix the logical forms of judgments that we make about given
intuitions. But as there’s absolutely no intuition outside thedomain of the senses, these pure concepts have no meaningoutside that domain; and all these noumena, together with
the intelligible4 world that they compose, are nothing but the
representation of a problem, ·namely the problem or question: What are noumena like? What is the intelligible world
like? · What the question is about is something possible; but
answering it in terms of the concepts of our understanding
is quite impossible. That’s because of the nature of ourunderstanding, whose role isn’t to deliver intuitions but toconnect intuitions that are given in experience; ·i.e. it doesn’t
present us with real particular things, but only enables
us to inter-connect particulars that we get from elsewhere,
namely from our senses·. So experience must contain all thematerials to which we apply our concepts; and beyond it noconcepts have any significance, as there’s no intuition thatmight offer them something to grip onto.

35
The •imagination may perhaps be forgiven for sometimes
wandering, not keeping carefully within the limits of experience; for such roaming gives it life and vigour, and ·that’s
an advantage, because· it is always easier to moderate theimagination’s boldness than to rouse it from lethargy. But
4

How is pure natural science possible?

Not the intellectual world (as the usual expression is). For cognitive •operations of the understanding are intellectual, and some of them arethinkings about the world of our senses. The term ‘intelligible’ applies to •objects insofar as they can be represented by the understanding all on its
own, without our sensible intuitions coming into it in any way. . . .

38

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Immanuel Kant

36: How is nature itself possible?

How is pure natural science possible?

insight into the objects in themselves! This answer is givenin the Critique itself (in the Transcendental Logic), and inthese Preliminaries in the course of the solution ·in sections
14–32· of the second main problem ·raised at the end ofsection 5·.
Why is our sensibility like that? Why is our understanding like that? We cannot address these questions, because
we have to use our sensibility and our understanding in all
the questions we ask and all the thinking we do in looking
for answers.
There are many laws of nature that we can know only
through experience; but experience can’t teach us the generaltruth that appearances are connected in conformity withlaws, because the application of such laws is what makes
experience possible in the first place.
So the possibility of experience—of any experience—is at
the same time the universal law of nature, and the principlesof the experience are themselves the laws of nature. For weknow nature only as the sum-total of appearances, i.e. ofrepresentations in us, and so the only source from which we
can derive
the laws governing •how nature’s parts are interconnectedisthe principles governing •how they are connected inus,that is•the conditions that have to be satisfied if they are to
be united in a single consciousness.
If they weren’t united into one consciousness there couldn’t
be any experience.
The main thesis of this part of the Preliminaries, namelythat universal laws of nature can be known a priori, leads all
by itself to this conclusion:

This question is the highest point that transcendental philosophy can ever touch. [Reminder: by ‘transcendental’ Kant means‘having to do with grounds for a priori knowledge’.] It is a pointthat transcendental philosophy must reach, because it is its
boundary and completion. Really it contains two questions.
First: What makes it possible for there to be nature—in
the material sense of that word, in which it stands for the
totality of appearances? That is to ask: How are space andtime and their contents possible in general? The answer
is: What makes them possible is •the way our sensibility
is—the special way in which it is affected by objects thatare in themselves unknown and aren’t in themselves spatialor temporal. This answer has been given in the Critique (inthe Transcendental Aesthetic), and here in the Preliminaries
through the solution ·in sections 6–13· of the first problem·raised at the end of section 5·.
Secondly: What makes it possible for there to be naturein the formal sense, in which nature involves the totalityof rules that must apply to all appearances if they are to
be connected by thought in an experience? The answer
must be this: What makes nature possible is •the way ourunderstanding works. ·In the background is the crucial fact
that· all the representations of the sensibility have to berelated to a consciousness; ·for different items to be held ina single consciousness, they must be related to one anotherin certain ways, and these relations are imposed uponthem by the understanding. And so all the representations
that we are discussing must fall within the scope of ourunderstanding. And the answer to our question is that there
can be a rule-governed nature (in the formal sense) because·
our understanding demands that items that are thought
about be brought under rules. This rule-governedness iswhat makes experience possible; don’t mistake this for an39

Prolegomena

Immanuel Kant

and so· we wouldn’t be able to know any object whatever inthe sensible world), and so it depends on •the original lawsof the understanding. Because of this, we can say—though
it sounds strange at first—that The understanding doesn’tdraw its laws from nature, but prescribes them to nature.

The source of the highest laws of nature lies in ourselves, i.e. in our understanding. Rather than using•experience to find the universal laws of nature in•nature, we must go in the opposite direction. Thatis, we must look for •nature itself—as a system that
universally conforms to laws—in the features of oursensibility and understanding that make •experience
possible.
How else could the laws of nature be known a priori, giventhat they aren’t analytic but synthetic?
Why must the principles of possible experience agree withthe laws that govern what is possible in nature? We have a
choice of two answers: either (1) these laws are drawn fromnature by means of experience, or conversely (2) nature is
deduced from the conditions that make experience possible.
But (1) is self-contradictory, for the universal laws of naturemust be known independently of all experience, because all
empirical use of the understanding is based on them; so only(2) remains.5
Empirical laws of nature always rely on particular perceptions. We must distinguish such laws from the pure oruniversal laws of nature, which aren’t based on particular
perceptions and simply lay down the conditions that enable
perceptions to be unified so as to constitute experience. So
far as the laws are concerned, nature and possible experience
are one and the same. •The law-abidingness of possible
experience—·i.e. the holding of laws that are valid not justfor actual but for all possible experience·—depends on •thenecessary connection of appearances in experience (a connection without which ·there would no unified consciousness,
5

How is pure natural science possible?

37
I shall illustrate this seemingly bold proposition by an example that is meant to show that laws that we discover inobjects of sensory intuition (especially laws that we know
to be necessary) are already held by us to have been placed
there by the •understanding, even though they are otherwise
just like the laws of nature that we ascribe to •experience.
·Actually, I shall do rather more than that. I shall showthat laws that we are already willing to ascribe to ourunderstanding (namely, those of geometry) lead to one of
the laws (namely, the inverse-square law of gravitation) that
we wouldn’t think of as contributed by our understanding
unless we had been introduced to my critical philosophy·.

38
If we consider the properties of the circle, through which
this figure provides a unity for ever so many arbitrary spatialconfigurations all under a single universal rule, we can’t help
crediting this geometrical thing with having a constitution.
(·Analogously, when we think about the properties of iron,through which it enters into countless law-governed interactions with all sorts of other kinds of stuff, we can’t help
crediting it with a constitution or inner nature·.) For example,
take any two straight lines that intersect one another andintersect some circle (any circle you like):

Crusius was alone in suggesting a middle way. It could be (he said) that these laws of nature were originally implanted in us by a spirit who can’t
err or deceive. But there is so much human error—including plenty of it in Crusius’ own system!—that it seems very dangerous to rely on this lineof thought. Even if some things have been instilled in us by the Spirit of Truth, we have no reliable way of distinguishing these from ones put thereby the Father of Lies.

40

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Immanuel Kant

The rectangle constructed with the two segments of
one of the lines is equal to the rectangle constructedwith the two segments of the other.
Now I ask: Does this law lie in the circle or in the understanding? That is: Is the basis for this law something containedin the figure itself, independently of the understanding,or is the situation rather that the understanding, having
constructed the figure according to its concepts (a set ofpoints equidistant from a given point) introduces into it thislaw about the chords cutting one another in geometrical
proportion? When we follow the proofs of this law, we soonsee that it can only be derived from the equality of the circle’s
radii, which is the basis for the understanding’s constructionof this figure. But we can replace the concept on which
the circle is based by a more general one that fits every
sort of conic section (the circle being just one sort); thatwill advance the project of unifying various properties ofgeometrical figures under common laws; and if we take thatstep we’ll find that all the chords that intersect within theellipse, parabola, and hyperbola, always intersect in sucha way that the rectangles of their segments always •bear aconstant ratio to one another (the circle is the special casewhere they •are equal).
If we proceed still further, to the fundamental laws ofphysical astronomy, we find that the whole of the material
world is governed by a physical law of mutual attraction forwhich the rule is: The force of attraction decreases inversely
as the square of the distance from each attracting point, i.e.
as the spherical surfaces increase over which the force
spreads. [Kant’s line of thought is as follows. Think of gravity as

How is pure natural science possible?

•the amount of gravitational force received by an object of a given size
will vary with the proportion of its sphere-surface that it occupies, whichmeans that it will vary inversely with the square of its distance from thegravitational source.] The simplicity of the sources of this law,which rest merely on the relation of spherical surfaces ofdifferent radii, is matched by what follows from it, namely
such a splendid variety and harmony of consequences that
not only are all possible orbits of the celestial bodies conic
sections, but these orbits are inter-related in such a waythat no law of attraction other than the inverse-square one
can be imagined as appropriate for a cosmic system.
So here’s a nature that rests on laws that the understanding knows a priori, and chiefly from the universal principlesof the geometry of space. Now I ask: •Do the laws of naturelie in space, and does our understanding learn them merely
by trying to discover the great wealth of meaning that liesin space; or •do they inhere in the understanding and in its
way of configuring space . . . .?
Because it is so uniform and so indeterminate in its
particular properties, one wouldn’t look to space for laws ofnature. ·In contrast with that, there’s no threat of uniformity
in the understanding!· What imposes circles, cones andspheres on space is the understanding, in its role as providerof the basis for of the constructions of those figures.
So the mere universal form of intuition that we call ‘space’is the underlay of all intuitions of particular objects. There’s
no denying that •space makes the •intuitions possible inall their variety; but the unity of the •objects—·or rather
the unity among the intuitions that lets them qualify as
intuitions of objects·—comes ·not from space but· from theunderstanding, in accordance with conditions that lie in its
own nature. And so the understanding is the origin of theuniversal order of nature, in that it brings all appearances

radiating out from a point, exerting the same total force evenly acrossthe surface of each imaginary sphere with that point as centre. The•surface-areas of the spheres differ with the squares of their radii, i.e.
their distance from the central point; that’s simple geometry. Then

41

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Immanuel Kant

under its own laws, and thereby constructs the formal
aspects of experience a priori, so that nothing can be knownby experience except what conforms to the understanding’s
laws. The nature of •things in themselves is independentof the conditions of our sensibility and our understanding;but our concern is not with that but rather with •nature
considered as an object of possible experience; and here
the understanding, by making experience possible, brings it
about that the world of the senses either is nature (·in mysense, as given in section 14 above·) or is not an object ofexperience at all.

How is pure natural science possible?

search out in a language the general rules of theactual use of words, and thus collect elements for agrammar.
In fact the two researches are very closely related. Though ·adifference between them arises from the following fact·: wecan’t give a reason why each language has just this and noother grammatical structure, let alone why its formal rules
are just these, neither more nor less.
Aristotle collected ten pure elementary concepts under
the name of ‘categories’ and also ‘predicaments’. He thenfound that he had to add to his list five ‘post-predicaments’(though some of them were already contained in the firstten); but this random collection should be applauded moreas a hint for future enquirers than as an idea developed
according to a rule; which is why in philosophy’s present
more advanced state it has been rejected as quite useless.
In my research into the elements of human knowledge
that are pure (contain nothing empirical), my first success—achieved after long thought—was to distinguish and separate
the pure elementary concepts of sensibility (space and time)from those of the understanding. Thus Aristotle’s categories
of time, space, and place had to be excluded ·because they
pertain to sensibility, not understanding, and so are notcategories·. And the others on his list were useless to me,
because ·associated with them· there was no principle on
the basis of which the understanding could be surveyed
in its entirety, making possible a complete and precise
account of all the things it can do from which arise its pureconcepts—·its categories·.
Wanting to discover such a principle, I looked about for•an act of the understanding that contains all its other acts.
With the help of that •one kind of act I could bring all thevariety of representations into a unified theory of thinking
in general. The desired •act of the understanding turned

39: Appendix to the pure science of nature: the system of
the Categories

Nothing can be more desirable for a philosopher than totake the multitude of concepts or principles that he has
found himself applying in particular cases, and to derivethem a priori from a single principle, thus uniting them
all into a single cognition. Before that, all he had was thebelief that he had gathered together all ·the concepts or
principles· that remained after a certain abstraction andseemed to resemble one another enough to constitute a
specific kind of knowledge; but what he had gathered was
only an aggregate—·a disorderly heap·. Now, ·after his
derivation from a single principle·, •he knows that this kindof knowledge involves just these ·concepts or principles·,neither more nor less, •he understands that his classificationof them is necessary, and, at last, •he has a system.
You don’t need harder thought or more insight tosearch out in our daily knowledge the concepts which,though they don’t rest on any particular experience,occur in all experiential knowledge of which they are(as it were) the mere form of connection,
than you do to42

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Immanuel Kant

out to be: judging. Then I availed myself of the work of
the logicians, imperfect though it was. With its help I wasable to present a complete list of the pure functions of theunderstanding [= ‘basic kinds of thing the understanding can do’],considered ·at first· without any reference to any object towhich they might be applied. The last step was to relate thesefunctions of ·the understanding—i.e. these ways of·—judging,to the conditions that settle whether a given judgment isobjectively valid. And so there arose the pure concepts of theunderstanding, concerning which I could make certain thatjust exactly these ·on my list·—neither more nor less—settle
what knowledge of things we can have on the basis of pureunderstanding. It was all right for me to call them by theirold ·Aristotelian· name, categories. . . .
What distinguishes this system of categories from the old
unprincipled random collection of concepts, and what alone
entitles it to be considered as philosophy, is this essential
fact about it: By means of it the true significance of the pureconcepts of the understanding, and the condition of their use,
could be precisely determined. For here it became obviousthat in themselves •the categories are nothing but •logical
functions, ·corresponding to •logical kinds of judgment, suchas conditional, negative, universal and so on·; which meansthat they don’t by themselves yield the slightest concept ofan object. For that they need some sensory intuition as abasis. So their only role is to shape up empirical judgments. . . . enabling them to become judgments of experience.
Such an insight into the nature of the categories, which
limits them to merely experiential use, never occurred to theirfirst author [Aristotle] or to any of his successors; but without
this insight they are quite useless and only a wretched listof names, with no explanation and no rule for their use. If
the ancients had ever conceived such a notion, doubtless
the whole study of pure rational knowledge, which under

How is pure natural science possible?

the name ‘metaphysics’ has through the centuries spoiled somany sound minds, would have reached us in quite anothershape, and would have enlightened the human understanding instead of—as has actually happened—exhausting it in
obscure and pointless speculations, making it useless fortrue science.
This system of categories exhausts all the possible actionsof the understanding, and so every other concept must fallunder them. That puts all treatment of any object of purereason on a systematic basis, and provides an absolutely
reliable pointer or clue to how and through what points ofenquiry every metaphysical endeavour must proceed if it
is to be complete; for it exhausts all the workings of theunderstanding, under which every other concept must be
brought. Similarly with the table of principles: we can knowthat it is complete only through relation to the system of thecategories. And even in the classification of the concepts,
if it is to get beyond ·the sort of classification that might
be based on findings in· the empirical psychology of theunderstanding, it is always the very same guiding thread,which, as it must always be settled a priori by the same
fixed points of the human understanding, forms a closed
circle every time, leaving no doubt that if we want a complete
philosophical and a priori knowledge of the object of a pureconception either of the understanding or of reason, this is
the way to get it. So I couldn’t neglect this clue with regard
to one of the most abstract ontological divisions, namely all
the differences that fall under the concepts of something andof nothing, and to construct accordingly a rule-governed andnecessary table.
And this system, like any true one based on a universal
principle, shows its inestimable value in this, that it •keeps
out all foreign concepts that might otherwise slink in among
the pure concepts of the understanding, and •assigns to43Prolegomena

Immanuel Kant

every item of knowledge its proper place. ·Here’s an example·.
The concepts that I arranged in a table according to the clueof the categories, under the name ‘concepts of reflection’,turn up among the pure concepts of the understanding in
ontology, without having any permission or right to be there.
The pure concepts of the understanding are concepts ofconnection, and thereby of the objects themselves, whereas
the concepts of reflection have to do only with the merecomparison of concepts already given; so the nature anduses of the two kinds of concept are quite different; andmy systematic classification of the concepts of reflection
keeps them out of company where they don’t belong. But

How is pure natural science possible?

the value of putting the categories in a special table of theirown will be still more obvious when we do—as I shortly
shall—distinguish those concepts of the understanding fromthe transcendental concepts of reason. The latter are quite
different in nature and in origin from the former, so theymust have quite another form. This separation, necessary as
it is, has never yet been made in any system of metaphysics,
which is why the concepts of reason have been jumbled
together with the concepts of the understanding, as though
they were siblings. This mix-up was inevitable in the absenceof a separate system of categories.

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Immanuel Kant

How is metaphysics possible?

Main transcendental problem 3:
How is metaphysics possible in general?
of those •concepts without help of any kind from experience.6
Reason will never be satisfied until it has solved thisproblem—·i.e. answered the question ‘How is metaphysics
possible·?’. Reason won’t let pure understanding be used outside the domain of experience; but reason itself is destined to
go beyond those confines. Every particular experience is only
a part of the whole domain of experience; but the absolute
whole of all possible experience is not itself an experience,
yet it is something that reason has to think about, as aproblem. For reason to present this problem to itself, it needs
concepts quite different from those of the understanding.
The latter are applied only to items given in experience;but the concepts of reason have a use that is transcendent:
it transcends all actual and possible experience, because
it involves thinking about the completeness of all possible
experience, i.e. thinking about
the-totality-of-possible-experience
considered as a single unified item. Such an item couldn’t
itself be given in experience.
Just as •the understanding supplies categories, which areneeded for experience, so •reason supplies Ideas, by which I
mean concepts that one must have though their objects can’t
be given in any possible experience. •Ideas are as inherent
in the nature of •reason as •categories are in the nature ofthe •understanding. Ideas carry with them an illusion that
could easily mislead; this illusion is unavoidable, although it
can be prevented from actually leading us astray.

40
I have now provided an explanatory justification for •pure
mathematics and for •pure natural science. Neither of themneeded this for the sake of its own security and certainty;
for pure mathematics is supported by the self-evident truthof its propositions; and pure natural science, although its
ultimate sources lie in the understanding, is thoroughly
supported and confirmed by experience. (Certain as purenatural science is, it can never equal mathematics in thatregard, which is why it can’t entirely do without the testimony
of experience.) Both these sciences therefore needed myenquiry not for themselves but for another science, namely•metaphysics.
Metaphysics is concerned not only with concepts ofnature (which always find their application in experience)but also with pure concepts of reason, which never findapplication in any possible experience. No experience cantell us what’s true and what’s false involving concepts ofreason, or even whether these concepts are objectively realor mere fictions. Yet the part of metaphysics that involves
them is what the rest of metaphysics is for—and that’s why
this science ·unlike the other two· needs an explanatory
justification for its own sake. The third question now before
us concerns the heart of metaphysics, its special feature,
namely reason’s preoccupation with itself, and its assumption that by brooding over its own •concepts it can come toknow about •objects that it supposes to arise immediately out
6

If we can say that a science is actual, at least in the thinking of all men (·subjectively actual·), as soon as we have established that the problems
leading to it are ones that are set before everybody by the nature of human reason,. . . . then we are bound to say that metaphysics is subjectively
actual (and necessarily so), which leads us to the legitimate question: How is it (objectively) possible?

45

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Immanuel Kant

How is metaphysics possible?

42

All illusion consists in taking the subjective ground of
judgment to be objective, as though reason in its use of the
Ideas were acquiring a special kind of knowledge. Reason
falls victim to this, and is guilty of error, when it takes
something that merely concerns •reason’s own nature andmode of operation and tries to make it refer to •some object
in itself. The only safeguard against this temptation is forreason to know itself—to understand what’s going on when
it uses Ideas in a transcendent, extravagant manner that
goes beyond all possible experience.

All instances of pure knowledge by the understanding havethis in common: ·although they aren’t derived from experience·, they involve concepts that can have application in experience, and principles that can be confirmed by experience.
Transcendent knowledge by reason contrasts with this. The
Ideas that it involves can’t be applied in experience, nor canits propositions ever be confirmed or refuted by experience.
If any errors creep into the employment of reason, they will
have to be discovered by pure reason itself ·because neither
sensibility nor understanding can have anything to do with
them·. For reason thus to stand guard over itself is very
difficult, because the reason that is standing guard is thevery faculty that is necessarily prone to intellectual illusions,and we have no firm objectively grounded procedure foravoiding them—only a subjective enquiry into reason itself
as a source of Ideas.

41
We must distinguish Ideas, which are pure concepts ofreason, from the categories or pure concepts of the understanding; the two correspond to two sorts of knowledge thatare quite different from one another •in their natures, •inwhere they come from, and •in how they are used. In thefundamentals of a science that purports to cover all a priori
knowledge, the distinction between Ideas and categories is
crucial. If we don’t respect it, metaphysics will be absolutely
impossible—or at best a random, bungling attempt to build ahouse of cards in ignorance of the materials one is using andof what they are good for. If my Critique of Pure Reason had
done nothing but make this distinction plain for the first time,
it would have contributed more to our grasp of metaphysics—
what it is, and how it should be conducted—than all thefruitless efforts to do justice to the transcendent problems ofpure reason that had ever before been undertaken. Before
the Critique, no-one had even suspected that reason wasquite different from understanding; so everybody ran the
two together, mentioning concepts of the understanding andconcepts of reason in the same breath, as though they wereof the same kind.

43
My chief aim in the Critique was not only to distinguishcarefully the various sorts of knowledge but also to derivefrom their common source the concepts belonging to eachof them. I did this so that by knowing the origins of theseconcepts I could settle how they might safely be used; andit also gave me the priceless though unexpected advantage
of knowing, a priori and in a principled way, that •my list ofconcepts, and •my classification and •descriptions of them,
are complete. Without this, everything in metaphysics is amere jumble, in which you never know whether you haveenough ·for your purpose·, or whether and where somethingis still lacking. This advantage is the very essence of purephilosophy, and isn’t to be had anywhere else.
I have derived the twelve categories—the four trios ofpure concepts of the understanding - from a classification of46

Prolegomena

Immanuel Kant

this Idea is theological.7 [For an explanation of how theology

kinds of judgment that can be made. The concepts of reason
are three in number, and they derive from a classification
not of judgments but of logical arguments—specifically, thethree kinds of inferences of reason. For these pure conceptsof reason (the transcendental Ideas) are given—we simply
do have them—and if one doesn’t want to regard them assomething like innate, the only source that can be found forthem is the activity of reason. That activity in its concern
with logical form constitutes the logical element of the inferences of reason; but it also involves recognizing judgmentsof the understanding as involving this or that a priori form of
judgment, and in this role it yields transcendental conceptsof pure reason.

comes into this, see section 55 on page 59. It’s not very helpful,
but it’s all Kant gives us in this work.]

All three give rise to dialectics—·i.e. to characteristic dangers
of intellectual illusion, insoluble problems, lurking contradictions, and the like·. But their ways of doing so are different,
and so we have—·corresponding to the trio
•categorical, •conditional, and •disjunctive—a· three-part division of the dialectics intoits •Paralogism, its •Antinomy, and its •Ideal.
Through this way of coming at things we can feel assured
that all the claims of pure reason are completely represented,
nothing missed, because we have completely surveyed thefaculty of reason itself, from which they all take their origin.

The basic sorts of argument are: categorical, conditional,
and disjunctive. [A categorical argument has a first premise of the44

form ‘(Subject) is (Predicate)’; a conditional one has a premise of the form

It should be borne in mind that the Ideas of reason, unlike
the categories, don’t help us to bring the understanding
to bear on experience. In the knowledge of nature by theunderstanding, the Ideas of reason are entirely dispensable;
indeed they are a positive obstacle to what is going on. (Theyhave, however, their own good use, which we’ll come to later.)•The psychological Idea of reason brings up the question ‘Isthe soul a simple substance or not?’ The answer to thatis of no interest when we are doing empirical psychology.
No possible experience could be evidence for either answer
to the question. So far as the description and explanationof our mental histories is concerned, the concept of simple
substance is quite empty. •As for the questions raised by
the cosmological Idea—Did the world begin? Will it end?—
answers to these can have no role whatever in explaining‘If P, then Q’; a disjunctive one has one of the form ‘Either P or Q’.] ·Each

Idea involves the thought of a kind of completeness·. So the
Ideas—the concepts of pure reason—are as follows.
•Categorical: the Idea of a complete subject (the Ideaof what is substantial); ·this is the Idea of an ultimate ‘thing which. . . ’, like Locke’s idea of substancein general·; this Idea is psychological ·because thenatural home ground of this thought is in application
to oneself: I am a thing which·. . . ’.
•Conditional: the Idea of a complete series ofconditions—·e.g. the thought of all the causes of thepresent state of the world·; this Idea is cosmological.
•Disjunctive: the Idea of a complete reality that somehow encompasses the entire range of what’s possible;
7

How is metaphysics possible?

In disjunctive judgments we consider the whole range of what is possible as divided in respect to some particular concept. The ontological principlethat every object falls under one or the other out of each contradictory pair of predicates, which is also the principle of all disjunctive judgments,
essentially relies on this thought of the sum of all possibility—which goes with the thought that every possible object is completely determinate,·because it falls under just one out of each contradictory pair of predicates·. . . .

47

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Immanuel Kant

any event in the world. •And as for the theological Idea:
there’s a correct maxim of natural science that says that weshouldn’t try to explain how nature is by appealing to thewill of a highest being, because such an explanation would
no longer be natural science, but rather an admission that
we have reached the end of it. So the proper use of •Ideas ofreason must be quite different from the use of the •categories,
i.e. the •pure concepts of the understanding through which
experience becomes possible.
Now, reason and understanding are related with one
another in a certain manner, which brings in some parts of
my laborious analytic of the understanding [i.e. the part of the
Critique of Pure Reason labelled ‘Transcendental Analytic’]. How? Well,
it can’t have to do with the business of getting knowledge ofnature through experience: the part reason plays in that—in
mathematics and natural science—can be perfectly well
played without all this subtle examination of the natureand functions of the understanding. So my analytic of theunderstanding must link with the Ideas of pure reason fora purpose that lies beyond the empirical use of the understanding. ·So now we have a dilemma. On the one hand·, I
have said that we can’t use the understanding outside therealm of experience, as that would be a meaningless activity,with no subject-matter. On the other hand, the nature of
reason must conform with the activities of the understanding,contributing to their perfection and not disturbing them.
Here’s the solution—the truth about what reason has
to do with understanding. What pure reason does is todemand that understanding, when it is brought to bear on
the complex of experience, shall achieve completeness in
its operations. This, however, is only a completeness ofprinciples, not of intuitions and objects. ·To put the point
in simpler terms: The demand for completeness says ‘Aslong as there’s something you don’t yet understand, keep

How is metaphysics possible?

working on it’; it doesn’t say ‘Aim to grasp the whole story
all at once: •survey the mind in such a way that you haveall its properties on one side and the ultimate subject thatbears those properties on the other; •arrive at results about
the world’s entire past and entire future; •think in a concrete
way about God as the explanation of the entire world’·. The
illusion—which brings the risk of error—comes from the factthat reason, wanting to make its demand for completenessas sharp and graspable as possible, slips into treating it asthough it were a demand for knowledge of something—·theultimate subject of mental states, the world’s whole past,etc·.

45: Prefatory remark to the dialectic of pure reason
I have shown in sections 33 and 34 that the freedom of
the categories from any input from the senses may mislead
reason into extending their use, quite beyond all experience,
to things ·as they are· in themselves ·as distinct from things
as they appear to us·; though ·no such use is legitimate,
for the following reason·. Because the categories lack any
sensory element that can give them meaning or sense inparticular cases, they can represent anything in their role as
mere logical functions; but there’s nothing about which theycan, unaided, give specific information. The fancy objects·that reason wrongly tries to bring under the categories· areknown as ‘noumena’, or pure beings of the understanding(or better, beings of thought). Examples include
substance—but conceived without permanence intime,cause—but not conceived as acting in time,and so on. ·In thinking or talking like this· one attaches tothese ·supposed· objects predicates whose only ·legitimate·use is to enable experience to conform to laws; and yet ·byleaving time out of it· one deprives them of all the conditions48

Prolegomena

Immanuel Kantof intuition that have to be satisfied for experience to bepossible, and so these concepts lose all meaning again.
There’s no risk that the understanding, when left to itself
and not given orders from the outside, will so wantonly roamout of its own proper territory into that of mere creatures ofthought. But the empirical use of the rules of the understanding is conditioned, and reason can’t be fully satisfied with
that; so it demands a completion of this chain of conditions.

How is metaphysics possible?

escaping from the conditions of experience, and making its
hold complete. So there they are—the transcendental Ideas.
·•They are in themselves virtuous, though •they can maketrouble; but •the trouble can be averted·. •They don’t try
to produce concepts that are in themselves excessive or
extravagant; all they aim for—in conformity with the true
but hidden goal to which our reason is naturally drawn—isa limitless extension of the empirical use of the categories.
•But through an unavoidable ·intellectual· illusion they may
seduce the understanding into using the categories in atranscendent manner, ·i.e. in a manner that isn’t related toexperience·. Deceitful as this misuse is, ·it is hard to avoid·.
To keep yourself from it and confine the categories within
the bounds of experience, it won’t do merely to resolve inadvance to be on your guard against doing so. •What you
need is scientific instruction ·on how to avoid the trouble·,and even then it takes hard work.

[Until this point in the Preliminaries the concept of condition has beenused almost entirely in saying things about (a) the conditions that our
understanding must satisfy, the conditions that our sensibility mustsatisfy—e.g. in saying that spatiality and temporality are formal conditions of our sensibility. We now, for almost the first time, encounter adifferent use of the concept: the topic now is not(a) the general conditions that have to be satisfied if we are tohave certain kinds of engagement with the world,
but rather(b) the conditions that various items within the world have tosatisfy if they are to exist.
Examples of (b): Any event in the experienced world occurs only because

I. The psychological Ideas

something causes it to do so; any region of space exists only because
there is a larger region in which it is embedded; any period of time46

exists only because there was an earlier period of time leading up to

People have long since remarked that in all substances theproper subject—
namely, the substantial as such, i.e. what remains
after all the qualities (as predicates) are set aside—is unknown, and this limit on our knowledge has been the
topic of various complaints. But if our understanding is
at fault in this matter, it is not for its inability to know—to
determine by itself—the substance of things, but rather for
its wanting to know the substance of things, thereby treatinga mere Idea as though it were a given object ·into whose
nature one might enquire·. Pure reason demands that forevery predicate of a thing we seek its proper subject; but•this subject can’t be anything but a ·further· predicate, so

it; any state exists only if there is something that it is the state of ; andso on. Kant holds that (b) these in-the-experienced-world conditioning
relationships can be depended on because they are put there by (a) thegeneral conditions that our sensibility and understanding have to satisfy.
So all this is part of a single unified body of doctrine. Still, it is as well to
notice how, after so much talk about ‘conditions’, we are rather suddenlyon the presence of the notion of chains of conditions. It is just the shift
from (a) to (b).]This forces the understanding to leave its proper domain, sothat it can do two things: •represent objects of experiencein a series that stretches too far for any experience tocapture it, and •look completely outside itself for noumena
to which it can attach that chain, thus completing the series,
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reason tells us to find a subject for •it in its turn, and so on,indefinitely (or until we give up). So we are never to regard
anything that we arrive at as an ultimate subject: and ourunderstanding can never have the thought of the substantial
itself, however deeply it penetrates and even if all of nature isunveiled to us. That’s because the special characteristicof our understanding is that when it thinks something
it does so by representing it through concepts, and thus
through mere predicates; so it can never reach the absolutesubject—·the sheer thing, not understood as thing-that-is-F
for any predicate F·. Hence all the real properties throughwhich we know bodies are mere qualities of them; and that
includes impenetrability, which we can only represent toourselves as the effect of a power whose subject is unknown
to us.

principle, totally destroying all materialistic explanations of
the inner phenomena of the soul) leads through a wholly
natural misunderstanding to a highly plausible argument:from •this supposed knowledge of the substantial status of
our thinking being, the argument infers •conclusions about
the nature of the soul—the nature of it that lies right outside
the compass of experience. [See the explanation of ’regulative’ atthe end of section 56 on page 60.]47
We may call this thinking self (the soul) substance, as being
the ultimate subject of thinking that can’t be further represented as the predicate of something else; but the concept·of substance, in this use of it·, remains quite empty, withnothing following from it, if it can’t be shown to involve
permanence—which is what makes fruitful the concept of
substances that we encounter in experience.
But permanence can never be proved on the basis of theconcept of a substance considered as a thing in itself, but
only in relation to experience. This is adequately shownby the first Analogy of Experience ·in the Critique of Pure
Reason ·. If that proof doesn’t convince you, try for yourself
whether you can derive from the concept of a subject that
doesn’t exist as the predicate of another thing that its existence is thoroughly permanent and that it cannot—unaided
or through any natural cause—either come into existence or
be annihilated. Synthetic a priori propositions such as thatcan never be proved ·of things· in themselves, but only inapplication to things as objects of possible experience.

Now, it appears as if we do confront this ·absolute·subject in our consciousness of ourselves (of the thinking
subject), and indeed that we have this in an immediate
intuition; for all the predicates of inner sense refer to the
I as a subject, and I can’t conceive myself as the predicate
of some other subject. So it seems that we are given inexperience something that completes the process of relating
given concepts predicatively to a subject—given it not merely
as an idea but as an object, i.e. the absolute subject itself.
But this turns out to be a false hope. For the I isn’t aconcept,8 but only a designation of the object of inner sense
insofar as we know it by no further predicate. So it can’t
itself be a predicate of any other thing, any more than it
can be a determinate concept of an absolute subject; it’s
only a relating of inner phenomena to their unknown subject.
Yet this Idea (which does excellent service as a regulative
8

How is metaphysics possible?

If the representation of self-awareness, the I, were a concept through which something could be thought, it could be used as a predicate of otherthings or would contain such predicates in itself. But it is nothing more than the feeling of something existing, without the slightest concept of it; itis only the representation of that to which all thinking relates.

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Immanuel Kant

48

49
Here’s something else that can be proved ·as a requirement
for the intellectual management· of experience, but can’t be
shown to hold of things in themselves: Our outer experience
not only does but must correspond to something real outsideof ourselves. That tells us this much: there is something
empirical—thus, some phenomenon in space outside us—·the existence of· which can be satisfactorily proved. ·That’s
all it tells us·, for we have no dealings with objects otherthan those belonging to possible experience; because objects
that can’t be presented to us in any experience are nothing
to us. What is empirically outside me is what appears
in space. Now space, together with all the phenomena it
contains, belongs to the representations whose objective
truth is proved by how they are inter-connected according
to laws of experience, just as the actuality of my soul (as an
object of inner sense) is proved by how the phenomena of
inner sense are inter-connected. Accordingly, •it is by outer
experience that I am conscious of the actuality of bodies, as
external phenomena in space, just as •by inner experience
I am conscious of the existence of my soul in time; but I
know this soul only as an object of inner sense—knowing
it only through appearances that constitute my inner state;

So if we want to use the concept of •the soul as substanceas a basis on which to conclude that •the soul is permanent,
we can do this only in relation to possible experience; if wetake the soul to be a thing in itself, and look for a conclusionthat holds good beyond the bounds of all possible experience,permanence can’t be shown. But all our possible experience
requires us to be alive; so the only permanence-of-the-soulresult we can establish is that the soul is permanent throughout one’s life; for the death of man is the end of all experiencethat the soul could have of itself as an object—unless thecontrary is proved, but that ‘contrary’ is supposed to be theconclusion of the argument for the soul’s permanence, so it
can’t appear among the premises. The most we can show,
therefore, is that the soul is permanent throughout one’s
life—a result that nobody will disagree with! What we want
is to show that the soul lasts after death, and this we cannot
do, for the reason I have given: the necessary tie between
the concept of substance and the concept of permanence iscreated by the principles of possible experience, and so it
holds good only within the domain of possible experience.99

How is metaphysics possible?

It is indeed very remarkable that metaphysicians have always glided comfortably over the principle that substances are permanent without trying
to prove it. No doubt this is because as soon as they started on the concept of substance they found that every possible basis for a proof haddeserted them. Common sense, which strongly felt that perceptions couldn’t be unified in experience without this presupposition ·of the permanenceof substance· filled the gap by a postulate. (·It had to postulate permanence instead of proving it· because it could never derive the necessary
permanence of substance from experience itself, for two complementary reasons. (1) We have no way of tracking substances through all theiralterations and dissolutions and finding ·empirically· that their matter, their stuff, is never lessened. (2) The principle in question involves necessity,
which is a sure sign of its being an a priori principle ·and thus not knowable through or provable from experience·.) People then optimistically applied
this postulate ·about all substances· to the concept of soul as a substance, and inferred from this that a man’s soul must continue in existence
after his death (especially because this substance’s having no parts—which they inferred from the indivisibility of consciousness—guaranteed thatit couldn’t be destroyed by falling to pieces). If they had found the genuine source of this principle of the permanence of substance—a discovery
requiring deeper researches than they were ever inclined to make—they would have seen that the law of the permanence of substances holds good
only for the purposes of ·intellectually managing· experience; so it applies to things only so far as they are to be known and conjoined with others inexperience. It never applies independently of all possible experience, and consequently it cannot hold good of the soul after death.

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the nature of the soul in itself—the thing that has these
phenomena—is unknown to me. So all that Cartesianidealism achieves is to distinguish •outer experience from•dreaming; and to distinguish the •conformity to law that isa criterion of the truth of the former from the •irregularityand false illusion of the latter. In ·dealing with· both ·outer
experience and dreaming· Cartesian idealism presupposesspace and time as required for the existence of objects; its
only question is this:•Are the objects of the outer senses, which whenawake we put in space, actually to be found in space(as the object of inner sense, the soul, is actually to
be found in time)?
That amounts to this:•Does experience carry with it sure criteria to distinguish it from imagination?
Doubts about this are easy to dispose of. We dispose of themin ordinary life every time we investigate how appearancesin both space and time are connected according to universal
laws of experience: when the representation of outer things
agrees thoroughly with those laws, we can’t doubt that they
constitute truthful experience. So it is very easy to refutematerial idealism, which ·questions the existence of bodies,
although it doesn’t think of them as things in themselves,
but· considers appearances as appearances and takes account only of how they are connected in experience: it is justas sure an experience that bodies exist outside us (in space)as that I myself exist according to the representation of innersense (in time); ·I put it like that because· the concept ofoutside us means existing in space. Compare these two:•taking ‘body’ to refer not merely to outer intuition (inspace) but to the thing-in-itself that is the basis of
this appearance—·the thing that the appearance is anappearance of ·,How is metaphysics possible?

and•taking the ‘I’ in the proposition ‘I am’ to refer not

merely the object of inner intuition (in time) but tothe subject of consciousness—·the thing that has theconsciousness·.
The ‘body’ thought generates •the question of whether bodies(which are really phenomena of outer sense) exist as bodies
in nature apart from my thoughts—a question that can be
briskly answered in the negative. The ‘I’ thought generates•the question of whether I myself (an appearance of innersense, the soul that empirical psychology studies) exist apartfrom my faculty of representation in time; and this questionis on exactly the same footing as the other, and must likewise
be answered in the negative.
Everything is decided and certain in this way, once it
has been given its true meaning, Formal idealism (which I
have also called ‘transcendental idealism’) actually abolishes
material (or Cartesian) idealism. For if space is nothing but
a form of my sensibility, then it is—as a representation inme—just as actual as I myself am; and the only remaining
question concerns the empirical truth of the representationsin space. And if on the other hand space and the phenomena
in it are something existing outside us, then the actualityof these ·alleged· objects outside us can never be proved ·inthe way it would have to be proved, namely· by applying thecriteria of experience beyond the domain of our perception.
II. The cosmological Idea

50
The cosmological Idea is the most remarkable product ofpure reason in its transcendent use. It has more power thananything else to rouse philosophy from its dogmatic slumber
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Immanuel Kant

How is metaphysics possible?

·between the assertion and its contradictory· except the one

and to stimulate it to a hard task, namely making a critiqueof reason itself.
I term this Idea ‘cosmological’ because it never takes its
object from anywhere but the world of the senses, having nouse for anything that isn’t given to the senses. So in thatway it stays at home, does not become transcendent, and isto that extent not a mere Idea. (Whereas ·the psychological
Ideas don’t in that sense ‘stay at home’, because· merely
conceiving the soul as a simple substance involves conceiving something—the simple—that can’t be presented to thesenses.) Despite that, the cosmological Idea ·does in its own
way go outside the domain of the senses, because it· extends
the connection of the conditioned with its condition so farthat experience never can keep up with it. In this way, then,it is always an Idea, whose object can never be adequately
given in any experience.

that compels the philosopher to look into the first sourcesof pure reason itself. This Antinomy—·i.e. this conflict between dialectical assertions and their contradictories·—isn’t
something I thought up to amuse myself: it comes from thenature of human reason; so it can’t be avoided or brought
to an end. The Antinomy contains the following four theses
together with their antitheses:(1) Thesis: The world has a beginning in time and space(a limit).
Antithesis: The world is spatially and temporally
infinite.
(2) Thesis: Everything in the world consists of ·elementsthat are· simple.
Antithesis: There is nothing simple; everything iscomposite.
(3) Thesis: There are in the world causes through freedom.
Antithesis: There is no liberty; all is nature.
(4) Thesis: In the series of the world’s causes there issome necessary being.
Antithesis: There is nothing necessary in the world; inthat series everything is contingent.

51
Note first that in this territory ·of the cosmological Ideas· theusefulness of a system of categories is so obvious and unmistakable that this alone would show that it is indispensable inthe system of pure reason, even if there weren’t several otherproofs of that. There are exactly four such transcendent
Ideas, and exactly four classes of categories; but ·the Ideas
differ from the categories in that· they are concerned onlywith the absolute completeness of the series of the conditions
for a given conditioned item. Matching these cosmological
Ideas there are exactly four kinds of dialectical assertions ofpure reason. Their being dialectical shows in this:Against each of these assertions we can bring its
contradictory, on the strength of principles of purereason that are as plausible as those supporting theoriginal assertion.
No exercise of metaphysical art can fend off this conflict

52a
We have here the strangest phenomenon of human reason;no other use of reason produces anything like it. If we•think of the appearances of the world of the senses asthings in themselves (as we often do), and if we •take them
to be combined through principles that hold universally forthings in themselves rather than merely through principles
of experience (which we also often do—indeed without mycritique we can’t help it!), there arises a conflict that can’tbe removed dogmatically ·by proving one side and refuting
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Immanuel Kant

the other·. That’s because thesis and antithesis can bothbe shown by equally clear, evident, and compelling proofs (Iguarantee that all the proofs are correct), so reason is divided
against itself—which gladdens the heart of the sceptic but
must make the critical philosopher feel ill at ease.

the four natural Ideas of reason, from which arise four
assertions and four counter-assertions, each validly derived
from universally accepted principles, revealing the dialectical
illusion of pure reason in the use of these principles—anillusion that would otherwise have stayed hidden for ever.
So this is a decisive experiment, which must necessarily
reveal to us any error lying hidden in the presuppositionsof reason.10 Contradictory propositions can’t both be false
unless they both involve some self-contradictory concept.
·And then they can both be false·. For example A square
circle is round is false (it is false that the circle is round,because it is square), and A square circle is not round is
likewise false (it is false that the circle isn’t round, i.e. that it
has corners, because it is a circle.) The logical mark of theimpossibility of a concept consists precisely in this, that twocontradictory propositions involving it are both false, and as
no third proposition can be thought between them, nothingat all is thought through that concept.

52b
We can blunder in various ways in metaphysics without
any fear of being detected in falsehood. For as long as
we avoid self-contradiction, which we can always do when
we assert synthetic propositions (even if they are wholly
fictitious), ·our only way of being detected in falsehood isthrough experience. And· experience can’t refute us when
we assert propositions ·of the sort involved in metaphysics,
namely ones· in which the concepts that are involved aremere Ideas, instances of which can’t be presented to us inexperience. For how can we tell from experience whether theworld has lasted from eternity or had a beginning, whether
matter is infinitely divisible or consists of simple parts? Such
concepts can’t be instantiated in any experience, however
extensive, and consequently neither the positive nor thenegative proposition ·in the antinomy· can be empiricallydiscovered to be false. The only way in which reason could
unintentionally reveal its secret dialectic, which it falselyoffers as positive doctrine, would be for this to happen:
Reason bases an assertion on a universally admitted
principle, and infers the exactly opposite assertion,with the greatest correctness of argument, from another principle that is equally accepted.
That’s what actually does happen in our present case of
10

How is metaphysics possible?

52c
The first two antinomies, which I call ‘mathematical’ because
they are concerned with the addition or division of the homogeneous, are based on such a self-contradictory concept;
and that’s how it comes about that in each of them both thethesis and antithesis are false. [Re ‘homogeneous’: see the start of
the next section’s second paragraph.]

When I speak of objects in time and in space, I am speaking not about •things in themselves (of which I know nothing),
but about •things in appearance, i.e. about experience as aparticular way of knowing objects—the only way of knowing

I should therefore like the critical reader ·of the Critique of Pure Reason · to attend especially to this antinomy of pure reason, because nature itself
seems to have arranged it to pull reason up short in its bold claims, and to force it to look into itself. I take responsibility for every proof I have given
for the thesis as well as for the antithesis, and thereby promise to show the certainty of the inevitable antinomy of reason. If this curious phenomenon·of the ‘proofs’ of both P and not-P· leads you to go back to examine the presupposition on which it is based, you will feel yourself obliged to join me
in inquiring more deeply into the ultimate basis of all knowledge of pure reason.

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them that has been granted to mankind. When I think ofsomething as being in time or in space, I must not say ·or
think·:In itself it exists in space and in time, independentlyof these thoughts of mine;for if I did I would be contradicting myself. That’s because
space and time, together with appearances in them, are nothing existing in themselves and outside my representations,but are themselves only modes of representation [= ‘ways of
being represented-to’], and it is plainly contradictory to say thata mere mode of representation exists outside our representation. So objects of the senses exist only in experience; andto attribute to them a self-subsisting existence •apart fromexperience or •in advance of it amounts to telling ourselvesthat experience is real apart from experience or in advanceof it!
Now if I ask about the extent of the world in space andin time, my complete stock of concepts doesn’t enable me tocall it infinite or to call it finite. For neither state of affairs
can be contained in experience: there can’t be experienceof an infinite space, or of an infinite time elapsed; or of theworld’s being bounded by empty space or by empty time
before the world began—all these are mere Ideas. This finite
or infinite size of the world, ·not being cashable out in termsof experience·, would therefore have to belong to the world
itself apart from all experience. But this contradicts thenotion of •a world of the senses, which is merely a totality of
appearances that exist and are interconnected only in our
representation, that is, in experience, since •this world is
not an object in itself but a mere mode of representation.
From this it follows that the answer ‘Yes’ to the question ‘Is
the world finite in space or in time?’ is false, and so is theanswer ‘No’; because the concept of a world of the senses
existing for itself and in itself is self-contradictory.

How is metaphysics possible?

The same holds good for the second antinomy, concerning
the division of appearances. For these appearances are mere
representations, and their parts exist only in the representation, and consequently exist only in the dividing, i.e. in a
possible experience that presents them; and the dividing can
go only as far as this experience goes. If you assume thatan appearance, such as that of a body, contains—in itself,
in advance of all experience—all the parts that any possible
experience can ever reach, what you are doing amounts to
this:Attributing to a mere appearance, which can exist only
in experience, an existence preceding experience; orsaying that mere representations are there before we
encounter them through our faculty of representation.
This is self-contradictory, and consequently so is eachanswer to the misconceived question ·about divisibility·,
whether we answer that bodies in themselves consist of
infinitely many parts, or that they have a finite number ofsimple parts.

53
In the first (the mathematical) class of antinomies the falsehood of the assumed proposition consisted in •taking contradictory items (appearance, thing in itself) to be harmoniously
compatible within a single concept. In the second (dynamic)
group, on the other hand, the falsehood of the assumed
proposition consists in •taking a consistent pair of propositions to be mutually contradictory. Thus, in the first class ofantinomies the opposed assertions •were both false, while inthe second class the two propositions—which are treated as
opposed to one another through mere misunderstanding—•may both be true.
Connecting items mathematically through the conceptof ·spatial or temporal· size requires that the connected
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Immanuel Kant

items be of the same kind; but dynamic connections by nomeans require that kind of homogeneity. When it comes toextended magnitudes—·i.e. stretches of space or of time·—allthe parts must be homogeneous with one another and withthe whole; but in the connection of cause and effect, although•homogeneity may be found there too, it isn’t necessary.
Or at any rate the concept of causality doesn’t require it,
because cause-effect has to do with positing something
through something else •quite different from it.

which leads to the event according to a constant law. But thecause’s entering into this state that gives it its causal power
must likewise take place or happen; the cause must havebegun to act, for without that the effect’s following from itcannot conceived. ·Without such a beginning·, the effect, as
well as the effectiveness of the cause, would have to haveexisted always. This yields the result:•The state of the cause that makes it effective mustalso have started among appearances, being an event(just as the effect is), and so have been caused in its
turn, and so on ·backwards for ever·.
Which in turn yields the further result:•The condition that governs the coming-intoeffectiveness of causes must be natural necessity.
If on the other hand, certain causes of appearances have theproperty of being free, then freedom must be a capacity forstarting these appearances—these events—spontaneously;
there’s no such event as the cause’s starting to be effective,
and thus no need for anything outside the cause to prod
it into starting to be effective. But in that case the causemust have its effectiveness in a manner that doesn’t place
it in time; so it can’t be an appearance, and must be
regarded as a thing in itself, with only its effects being
appearances.11 If we can without contradiction think ofbeings of the understanding—·choices, decisions, etc.·—asIf the objects of the world of the senses were taken forthings in themselves, and the laws of nature discussed above
were taken to be laws of things in themselves, contradiction
would be unavoidable. Similarly, if the subject of freedom
were taken to be a mere appearance, like other objects,
contradiction would be equally unavoidable, for the same
predicate taken in the same sense would be at once affirmed
and denied of one and the same object. But if naturalnecessity is tied only to appearances, and freedom only tothings in themselves, there’s no contradiction in assumingor allowing both kinds of causality at once, however hardor impossible it may be to make the latter kind (freedom)
comprehensible.
In ·the realm of· appearance every effect is an event, something that happens in time; so according to the universal
law of nature it must be preceded by a cause, some state of11

How is metaphysics possible?

The only acceptable use of the Idea of freedom is in ·thinking about· the relation of the intellectual (as cause) to appearance (as effect)—·the relation
between what a person chooses and how his body moves·. So the incessant action through which ·any portion of· matter fills its space—·acting
so as to keep other matter out of that place·—though it takes place from an internal principle [here = ‘source’ or ‘driver’], can’t be an exercise of
freedom. Nor can we find a concept of freedom that is suitable for purely rational beings such as God. For his action, though independent of external
determining causes (because it is only his immanent or caused-from-within action that I am talking about), is determined in his eternal reason, thatis, in the divine nature ·which never changes·. It is only if something is to start by an action, so that the effect occurs in the time-series or in theworld of our senses (e.g. the beginning of the world), that the question arises of whether •the effectiveness of the cause must in its turn have been
started, or whether instead •the cause can initiate an effect without its own effectiveness beginning. In the former case the concept of this causality
is a concept of natural necessity, in the latter, that of freedom. From this you will see that in explaining freedom as the faculty of starting an event
spontaneously I have exactly hit the notion which is the problem of metaphysics.

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exercising such an influence on appearances, then ·thatenables us to have the second part of the following two-partstory·:•Natural necessity is what links all causes to all effects·when both cause and effect belong· in the world of
the senses.
•Freedom is possessed by any cause that isn’t itself an
appearance though it underlies an appearance.
So nature and freedom can without contradiction be attributed to the very same thing, but in different relations—on
one side as an appearance, on the other as a thing in itself.
We have in us a faculty that is not merely•connected with its subjective determining grounds
that are the natural causes of its actions, and isin that way the faculty of a being that belongs toappearances,
but is also•connected to objective grounds (that are only Ideas),
being connected to them in that they can determine[here = ‘influence’] this faculty—a connection expressedby the word ought.
·To spell that out a little in more familiar terms: When aperson decides how to act on some occasion, the question‘Why did he make that decision?’—a request to know what
determined him to make it—can have answers of two entirely
different kinds. •One kind explains the decision in termsof his prior state of mind, and the psychological laws thatled from that to his decision. •The other kind explains thedecision in terms of his reasons for it, his beliefs about what
he ought to do. The former kind of answer invokes naturalnecessity; the second doesn’t, because it doesn’t explain
the decision as an event arising from prior events; it is thesecond that takes us out of the realm of appearance, andmakes room for freedom·. This faculty—·the one involved

How is metaphysics possible?

in the second kind of answer·—is called reason. To theextent that we consider a man exclusively in the light of
his reason, viewed as responding to objective judgments
about what he ought to do, to that extent we aren’t viewing
him as a being of sense—·an inhabitant of the world of oursenses·—because this freedom ·or responsiveness to the‘ought’ thought· is a property of a thing in itself. We can’t at
all grasp how it is possible—i.e. how the ought, which is not·an event, not· something that has happened, can determine
the man’s activity, becoming the cause of actions ·which arethings that happen, and· whose effect is an appearance inthe world of the senses. Still, ·although we don’t grasp how
this can be·, if reason did relate in this ‘influencing’ way toa person’s decisions, that would bring freedom into what
happens in the world of the senses to the extent that we
can see those happenings as influenced by objective grounds(which are themselves Ideas). For reason’s effectiveness asa cause wouldn’t depend on subjective conditions—·i.e. on
facts about the person’s prior state of mind·—and therefore
wouldn’t depend on events in time or on the law of naturethat controls such events. That’s because the grounds ofreason—·the basic ‘ought’ thoughts·—govern actions in auniversal way, according to ·universal· principles, without
influence from the circumstances of either time or place.
What I am presenting here is meant merely as an example
to make things intelligible. It doesn’t necessarily belong toour problem—·i.e. the question How is metaphysics in general possible? ·—which must be decided from mere concepts,
independently of the properties that we meet in the actual
world.
Now I can say without contradiction that •all the actionsof rational beings, so far as they are appearances, fall under•the necessity of nature; but •those same actions, consideredpurely in terms of the rational subject and its ability to act
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according to mere reason, are •free. For what is required forthe necessity of nature? Only that every event in the worldof the senses come about in accordance with constant laws,
thus being related to causes in the domain of appearance;
and in this process the underlying thing in itself remains outof sight, as does its causality. But I maintain this:The law of nature still holds, whether or not the rational being causes effects in the world of the senses
through reason and thus through freedom. If it
does, the action is performed according to maxims
whose effects in the realm of appearance are always
consistent with constant laws; if on the other hand it
doesn’t, the action is ·not merely consistent with but·subject to the empirical laws of the sensibility, andin this case as in the other the effects hang together
according to constant laws.
This conformity to laws is all we demand for natural necessity; indeed, it exhausts all that we know about naturalnecessity. But in •the former case ·where the action iscaused by reason·, reason is the cause of these laws ofnature ·rather than being subject to them·, and therefore
it is itself free; in •the latter case, where the effects follow
according to mere natural laws of sensibility with reason
having no input, it doesn’t follow that reason is in this casedetermined by the sensibility, which indeed it couldn’t be, so
reason is free in this case too. Freedom, therefore, doesn’t
get in the way of natural law in the domain of appearance,
any more than natural law brings about a breakdown in
the freedom of the practical use of reason, which relates tothings in themselves as determining grounds.
Thus practical freedom—the freedom in which reason has
causal force according to objectively determining grounds—isrescued, without doing the slightest harm to natural necessity in relation to the very same effects, as appearances.

How is metaphysics possible?

These remarks explain what I said earlier about transcendental freedom and its compatibility with natural necessity(with a single subject taken in two different ways). For
whenever a being acts from objective causes regarded
as determining grounds ·of reason·, the start of its
action is a first beginning,althoughthe same action is in the series of appearances only asubordinate beginning, which must be preceded anddetermined by a state of the cause, which in turn isdetermined by another immediately preceding it ·andso on backwards·.
In this way we can have the thought, for •rational beings andquite generally for •any beings whose causality is determinedin them as things in themselves, of a being’s ability to begin aseries of states from within itself, without falling into conflictwith the laws of nature. For the relation the action has toobjective grounds of reason isn’t a temporal one; in thiscase what determines the causality does not precede theaction in time, because determining grounds such as reason
provides don’t involve •objects of sense such as causes inthe domain of appearance, but rather •determining causes
as things in themselves, which don’t exist in time. And so
the action can without inconsistency be seen •(with regard
to the causality of reason) as a first beginning and as free,and •(with regard to the series of appearances) as a merely
subordinate beginning and as subject to natural necessity.
The fourth Antinomy is solved in the same way as isreason’s conflict with itself in the third. The propositions at
issue in the fourth—·that there is a necessary being, andthat there is not·—are perfectly reconcilable provided we
distinguish the cause IN the domain of appearance from thecause OF the domain of appearance (with the latter thoughtof as a thing in itself). Then one proposition says:
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How is metaphysics possible?

III. The theological Idea

Nowhere in the world of the senses is there a cause (according to similar laws of causality) whose existence
is absolutely necessary;the other says:
This world is nevertheless connected with a necessary
being as its cause (but of another kind and according
to another law).
The ‘incompatibility ’of these propositions entirely rests on
the mistake of extending what is valid merely of appearances
to things in themselves, and in general running the two
together in one concept.

55·I have discussed the •psychological Idea(s) and the•cosmological Idea(s). Now·:
The •third transcendental Idea is the ideal of pure reason. The use of reason for which it provides material is
the most important of all; though if it is pursued in amerely ·theory-building or· speculative manner, that makes
it transcendent—·theorizing outside the domain of possible
experience·—which in turn makes it dialectical. With thepsychological and cosmological Ideas, reason starts withexperience, and goes wrong by taking a grandiose viewof its grounds and trying to achieve, where it can, theabsolutely complete series of grounds. Not so with thethird, theological Idea. Here reason totally breaks withexperience; and—starting from mere concepts of what would
constitute the absolute completeness of a thing in general,and thus bringing in the Idea of •a most perfect primal
being—it works down from there to secure the possibility andtherefore the actuality of •all other beings. And so the mere
presupposition of a being that is conceived not in the series ofexperience but for the purposes of experience—for the sake ofcomprehending its connection, order, and unity—that is, theIdea, is distinguished from the concept of the understanding
more easily in this case than in the others. Hence we
can easily expose the dialectical illusion that arises fromour taking the subjective conditions of our thinking to be
objective conditions of objects themselves, and taking an
hypothesis necessary for the satisfaction of our reason to
be an objectively established truth. I have nothing more to
say here about the pretensions of transcendental theology,
because my remarks about them in the Critique are easily
grasped, clear, and decisive.

54
This then is how I present and how I solve the entire antinomy in which reason finds itself caught when it applies its
principles to the world of the senses. The mere presentation
would contribute a lot to the knowledge of human reason,even if the solution hadn’t yet fully satisfied you—which
it well might not, because you have to combat a natural
illusion that has only recently been exposed to you and thatyou have previously always regarded as genuine. For there’s
no escaping from this: so long as you take the objects of
the world of the senses to be things in themselves, andnot the mere appearances which is what they really are,you haven’t any chance of avoiding this conflict of reason
with itself; so you must re-examine my deduction of all oura priori knowledge and the tests that I put it through, inorder to come to a decision on the question. This is all
I require at present; for if in carrying this out you takeyour thoughts deeply enough into the nature of pure reason,
you will familiarize yourself with the concepts through whichalone the solution of the conflict of reason is possible. Unlessthat happens, I can’t expect complete assent even from themost attentive reader.
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56: General remark on the transcendental Ideas

is impossible, the Idea of a totality of knowledge according
to principles is needed if our knowledge is to have a special
kind of unity, the unity of a system. Without that, ourknowledge is nothing but piece-work, and can’t be used
for the highest end (which is always the establishment of ageneral system of all ends). I am talking here not only about
practical ·or moral· ends, but also about the highest end of
the speculative use of reason.
The transcendental Ideas thus express reason’s specialrole, namely as setting a standard for systematic unity inthe use of the understanding. But if the following happens:
We see this unity in our way of knowing as attachedto the object of knowledge; we take something that ismerely regulative to be constitutive; and we persuade
ourselves that by means of these Ideas we can extend
our knowledge far beyond all possible experience (andthus in a transcendent manner),
this is •a mere misunderstanding in our estimate of theproper role of our reason and of its principles, and it is •adialectic that confuses the empirical use of reason and also
sets reason against itself. What makes it a misunderstandingis the fact that really reason serves merely to bring experience
as near as possible to completeness within itself, i.e. to stopits progress from being limited by anything that can’t belong
to experience. [A regulative principle is one that guides, advises, or

The objects given to us by experience are in many respects
incomprehensible, and the law of nature leads us to manyquestions ·about them· which, when carried beyond a certain
point (though still in conformity with those laws), cannot
be answered. For example: why do material things attract
one another? But if we •go right outside nature, or •·staywithin it but· in thinking about how it is interconnected
go beyond all possible experience and so enter the realmof mere Ideas, then we can’t say that our subject-matter isincomprehensible and that the nature of things confronts us
with insoluble problems. For in this case we aren’t dealing
with nature—or, to put it more generally, we aren’t dealing
with •given objects—but with •concepts that have their origin
solely in our reason, and with mere creations of thought;
and all the problems that arise from our concepts of themmust be soluble, because of course reason can and must
give a full account of its own process.12 As the psychological,
cosmological, and theological Ideas are nothing but pureconcepts of reason that can’t be applied to anything foundin any experience, the questions that reason confronts us
with regarding them don’t come from objects but from mere
maxims that our reason lays down for its own satisfaction. It
must be possible for them all, as a group, to be satisfactorily
answered, which is done by showing that they are principles
for bringing our use of the understanding into thoroughharmony, completeness, and synthetic unity, so that they
do in that way hold good for experience—but for experience
as a whole. But although an absolute whole of experience12

How is metaphysics possible?

even commands—such as ‘Never accept that you have found an eventthat didn’t have a cause’.

A constitutive principle is one that gives

information, has factual content, purports to tell truths about reality.
Kant holds, for example, that ‘Every event has a cause’ is acceptable asregulative, but not as constitutive.]

Herr Platner in his Aphorisms acutely says: ‘If reason is a criterion, there can’t be a concept that human reason cannot comprehend. Incomprehensibility comes up only with what is actual. . . .’ So it only sounds paradoxical and is not really strange to say that although much in nature is beyond
our comprehension (e.g. biological reproduction), if we rise still higher and go right out beyond nature everything will be comprehensible again. For
then we leave behind the objects which can be given us, and occupy ourselves merely with Ideas; and here we can easily grasp the law that reason,
through them, prescribes to the understanding for its use in experience, because that law is reason’s own product.

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Conclusion

Conclusion
Determining the boundaries of pure reason57

ing to favour the experiential use of reason, it started out by
declaring that whatever transcends this use is worthless anddeceitful; but little by little, as the awareness sank in that thea priori principles used in experience lead (surreptitiously,
and seemingly just as legitimately) further than experienceextends, there came to be doubts even about experience.
There’s no danger in this ·error·, for healthy common sense
will doubtless always assert its rights ·regarding experience·.
But a certain confusion arose in science, which can’t workout how far reason is to be trusted—and why just this far andno further?—and the only way to clear up this confusion andforestall any future relapses is through a formal, principledfixing of the boundary of the use of our reason.

After the clearest arguments, which I have provided, it would
be absurd for us to hope to know more of any object thanbelongs to the possible experience of it, or lay claim to theslightest knowledge of anything not taken to be an objectof possible experience—knowledge that would tell us what
the thing is like in itself. For how could we learn such facts,
given that time, space, and the categories—and even more
all the concepts drawn from empirical intuition or perception
in the world of the senses—don’t and can’t have any use
other than to make experience possible, and that even thepure categories are meaningless if they are removed fromthis relation to perception?
But on the other hand it would be even more absurd
if we rejected things in themselves, or declared that •ourexperience is the only possible way of knowing things, •ourintuition of them in space and in time the only possible
intuition, •our concept-using understanding the pattern forevery possible understanding—all of which would amountto taking the principles of the possibility of experience to beuniversal conditions of things in themselves.
My principles, which limit the use of reason to possible
experience, could in that way become transcendent, and thelimits of our reason might pass themselves off as limits ofthe possibility of things in themselves (Hume’s Dialogues·Concerning Natural Religion · illustrate this process), if acareful critique didn’t both watch over the bounds of ourreason. . . .and set a limit to its pretensions. Scepticism originally arose from metaphysics and its lawless dialectic. Want-

It is true: we can’t rise above all possible experience andform a definite concept of what things in themselves may be.
Nevertheless, we aren’t free to stop enquiring into them; forexperience never satisfies reason fully; rather, in answering·our· questions it points further and further back, leaving us
still hungering for their complete solution. You can see thisin the dialectic of pure reason, the solid subjective groundfor which consists in just this hunger for completeness. As
regards the nature of our soul: having a clear awarenessof oneself as a subject, and having become convinced thatpsychological phenomena can’t be explained materialistically,•who can refrain from asking what the soul really is? And
if no concept of experience suffices for the purpose, •who
can refrain from accounting for it by a concept of reason(the concept of simple immaterial being), even though weare totally unable to show its objective reality? As regards

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all the cosmological questions about the duration and sizeof the world, and about freedom versus natural necessity,•who can be satisfied with mere empirical answers to these,
when every answer given on empirical principles gives rise
to a fresh question, which then requires an answer in its
turn, and in this way clearly shows that reason can’t besatisfied by explanations relying on facts about how things
go in the empirical world? Finally, •who doesn’t see, fromthe utter contingency and dependence of everything hethinks and assumes using mere principles of experience,
the impossibility of stopping with those principles? And•who doesn’t feel himself compelled, despite all the prohibitions against losing himself in transcendent Ideas, to seek
peace and contentment (beyond all the concepts that hecan vindicate by experience) in the concept of a being the
Idea of which can’t be seen to be possible, but which can’tbe refuted either because it relates to a mere being of theunderstanding, without which ·Idea· reason would remain
forever dissatisfied?
Where extended things are concerned, boundaries always
presuppose a space existing outside a certain definite place,and enclosing it; limits don’t require anything like that, but
are mere negations, indicating of some quantity that it isn’t
absolutely complete. But our reason sees around itself aspace for knowledge of things in themselves, so to speak,though it can never have definite concepts of them and islimited to appearances only.
As long as the knowledge of reason is all of one kind—·forexample, reasoning within number-theory, within geometry,within natural science, or the like·—definite boundaries toit are inconceivable. In mathematics and in natural science
human reason recognizes limits, that is, recognizes thatits inner progress will never be complete; but it doesn’t
recognize boundaries, i.e. doesn’t recognize that outside

Conclusion

it there’s something it can’t ever reach. In mathematics
there’s no end to the enlargement of our insight or to the new
discoveries that may be made; similarly in natural science,
there’s no end to the discovery of new properties of nature,of new forces and laws, through continued experience andunification of it by reason. ·So these sciences are nevercomplete, which means that at any time they have limits·.
But these limits should not be misunderstood—·i.e. should
not be thought of as boundaries·—for mathematics bears
only on appearances, and so it has no dealings with anythingthat can’t be an object of sensible intuition, such as theconcepts of metaphysics and of morals, ·which means thatit has no dealings with anything that could be a boundary
for it·. Mathematics can never lead to such things, andhas no need for them. So there is a continual progress andapproach towards ·completion in· these sciences, towards
the point or line, so to speak, of contact ·with completeness·.
The inwardness of things doesn’t show up in the domain of
appearance, though ·the Idea of· it can serve as the ultimate
ground of explanation of appearances; and natural science
will never reveal it to us. But it isn’t needed for scientificexplanations. Indeed, even if such ·ultimate grounds of
explanation· were to be offered from other sources (forinstance, if angels told us about them), natural science
still ought to reject them and not use them to advance •its
explanations. For •those must be based only on what canbelong to experience as an object of sense and be connectedwith our actual perceptions in accordance with empiricallaws.
But metaphysics, in the dialectical attempts of purereason (which we don’t undertake arbitrarily or wantonly,
being driven to them by the nature of reason itself), leads us
to boundaries. And the transcendental Ideas, just because
we can’t evade them and can never realize them ·in the sense62

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Immanuel Kantof encountering an instance of one of them·, serve not only to•tell us that the pure use of reason has boundaries, but alsoto •show us where they are. That’s the purpose and functionof this disposition of our reason, which has given birth tometaphysics as its favourite child; and this child, like every
other in the world, is a product not of blind chance but ofan original seed that is wisely organized for great purposes.
For metaphysics, perhaps more than any other science, hasin its main outlines been placed in us by nature itself, andcan’t be viewed as the outcome of an arbitrary choice or ofan accidental enlargement ·of our thoughts· in the progressof experience—from which indeed it is wholly separate.
Concepts and laws of the understanding suffice for theempirical use of reason, that is, for the use of it within
the world of the senses; but they don’t satisfy reason itself,
because it faces an infinite sequence of questions with nohope of ever completely answering them. The transcendental
Ideas, which have that completion as their aim, are suchproblems of reason. Now reason sees clearly that the world of
the senses can’t contain this completion, neither (therefore)
can all the concepts that serve only for understanding theworld of the senses—space and time, and the ones I have presented under the label ‘pure concepts of the understanding’.
The world of the senses is nothing but a chain of appearances
connected according to universal laws; so it •has no existence
for itself, •isn’t really the thing in itself, and consequently
must stand in a relation to ·something other than itself,
namely· to what contains the grounds of this experience—tobeings that can be known not merely as phenomena but asthings in themselves. It is only in the knowledge of thesethat reason can hope to satisfy its demand for completenessin the advance from the conditioned to its conditions.
In sections 33–4 above I indicated the limits of reason withregard to all knowledge of mere creations of thought. [TheConclusionNow,since the transcendental Ideas have made it necessary for us
to approach them, and thus have led us to the spot where•occupied space meets •the void, so to speak—i.e. where•experience touches •that of which we can know nothing,namely noumena—we can settle what the boundaries areof pure reason. For in all boundaries there’s something
positive:for example, a surface is the boundary of corporeal
space, and is itself a space; a line is a space that isthe boundary of a surface; a point is the boundary ofa line but yet is always a place in space,
whereas limits contain mere negations. The limits pointed
out in sections 33 and 34 are still not enough [meaning ‘notenough to satisfy us’?] once we have discovered that there is stillsomething beyond them (though we can never know what
it is in itself). For the question now arises: How does ourreason conduct itself in this connection of what we know
with what we don’t know and never shall? There is here an
actual connection of the known with something completely
unknown (which will always remain so); and even if theunknown isn’t going to become the least bit known (andthere’s no hope that it will), the concept of this connection
must still be capable of being identified and brought intoclarity.
So we ought to have the thought of an immaterial being,a world of understanding, and a supreme being (all mere
noumena), because •it is only in these items—as things inthemselves—that reason finds completion and satisfaction,
which it can never hope for in deriving appearances fromgrounds that are homogeneous with them ·and therefore
demand to be grounded in their turn·. Another reason
why we ought to ought to have those thoughts is that•·appearances· really do bring in something distinct fromword ‘limits’—Schranken—doesn’t occur in those two sections.]

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themselves (and totally unlike them), in that appearances
always presuppose an object in itself ·of which the appearance is an appearance·, and thus they suggest its existence
whether or not we can know more of it.
Now, we can never know these beings of understanding
as they are in themselves, i.e. determinately, but still wehave to assume them in •relation to the world of the senses
and connect them with that world by means of reason; so weshall at least be able to think •this connection by means ofsuch concepts as express their relation to the world of thesenses. ·This relational approach to noumena is the best
we can do·. For if we think a being of the understanding•through nothing but pure concepts of the understanding, wereally think nothing definite, and consequently our concept
has no significance; and if we think it •through properties
borrowed from the world of the senses, it is no longer a beingof understanding but is thought as one of the phenomena
and belongs to the world of the senses. I shall illustrate thiswith the notion of the supreme being.
The •deistic concept—·i.e. the •thin concept of a (notnecessarily personal) supreme being·—is a wholly pure concept of reason; but all it represents is a thing containing all
realities. It can’t pick out any one reality—·thereby saying
something in detail about the supreme being·—because to
do so it would have to use an example taken from the worldof the senses, and in that case I (·as a user of the concept inquestion·) would after all be dealing only with an object of
the senses, not something of a radically different sort thatcan never be an object of the senses. Here’s an example:
Suppose that I attribute understanding to thesupreme being. My only concept of understandingis one that fits understandings like mine—one thathas to get its intuitions ·passively· from the senses,
and that occupies itself ·actively· in bringing thoseConclusion

intuitions under rules of the unity of consciousness. If
I applied that concept to the supreme being, I would besaying that the raw materials of the supreme being’s
thought come from the realm of appearance; but it was
the inadequacy of appearances ·to meet the demandsof reason· that forced me to beyond them to the concept of a being that doesn’t •depend on appearances
and isn’t •identified or characterized through them.
·To credit the supreme being with understanding,therefore, I need a concept of understanding fromwhich the notion of getting-data-from-the-senses has
been purged·. But if I separate understanding fromsensibility to obtain a pure understanding ·that thesupreme being might have·, then nothing remains but
the mere form of thinking without intuitions; and form
alone doesn’t enable me to know anything definite,and so it doesn’t enable me to point my thought at thesupreme being as an object. ·On the one hand, then,I mustn’t suppose that the supreme being •thinks
about sensible intuitions; on the other, I mustn’t suppose that the supreme being •thinks without having
intuitions to think about. So·: for my purpose ·of attributing understanding to the supreme being· I would
have to conceive another kind of understanding, suchas would ·actively· intuit its objects ·itself, instead of
passively having intuitions of them brought to it bysensibility·. But I haven’t the least notion of suchan understanding, because human understanding isconceptual, its only way of knowing is through general
concepts, ·and it has no ability to present itself withintuitive data·.
I shall run into exactly the same trouble if I attribute a willto the supreme being; for I have this concept only by drawing
it from my internal experience; I experience will in myself as
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based on facts of the form I shall not be satisfied unless I get
object x, which means that my will is grounded in sensibility,·through which desired objects are presented to me·; andthat ·dependence on sensibility· is absolutely incompatible
with the pure concept of the supreme being.

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Conclusion

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Hume’s objections to deism are weak, and affect only
the arguments and not the thesis of deism itself. But asregards theism, which is supposed to come from adding
certain content to deism’s merely transcendent ·and thus
empty· concept of the supreme being, his objections are verystrong; indeed they are irrefutable as arguments against
certain forms of theism, including all the usual ones. Humealways insists that •by the mere concept of an original
being, to which we apply only ontological predicates (‘eternal,‘omnipresent’, ‘omnipotent’), we don’t think anything definite,
and that •other properties must be added if we are to havea concept of a definite, concrete thing. ·This isn’t a trivial
requirement, Hume holds. For example, he says that· it isn’t
enough to say It is a cause; but we must explain what kind of
causality it has—for example, whether it is exercised throughunderstanding and will—and that is the point at which his
attack begins on his real topic, theism; up to there he had
been attacking only the arguments for deism, which isn’t anotably dangerous thing to do. All his dangerous arguments
refer to anthropomorphism [from Greek meaning ‘man-shaped-ism’;in theology anthropomorphism is the view that God is like man]. Humeholds this to be inseparable from theism, and to make itinternally self-contradictory; and if anthropomorphism wereleft out ·of the theological story·, theism would drop out withit, and nothing would remain but deism. We can’t makeanything out of deism: it is worthless, and can’t serve as afoundation for religion or morals. If this anthropomorphism
were really unavoidable, no proofs whatever of the existenceof a supreme being, even if they were all granted, could give
us a detailed concept of this being without involving us incontradictions.

Conclusionof immanent (empirical) use, we become aware that thetwo commands can subsist together, but only right on theboundary of all permitted use of reason—for this boundary
belongs equally to the domain of experience and to that of thecreations of thought [= Ideas]. And through that awarenesswe also learn how these Ideas, remarkable as they are, serve
merely for marking the boundaries of human reason. On
the one hand they give warning •not to go on extending
our empirical knowledge with no thought of boundaries, as
though nothing but sheer world remained for us to know,
and yet on the other hand •not to overstep the bounds ofexperience and want to make judgments about things beyond
them, as things in themselves.
But we stop at this boundary if we limit our judgment
merely to how •the world may relate to •a being whose very
concept lies beyond the reach of any knowledge we arecapable of within the world. For we don’t then attribute to thesupreme being in itself any of the properties through which
we represent objects of experience, and so we avoid dogmatic
anthropomorphism; but we attribute those properties to thesupreme being’s relation to the world, thus allowing ourselvesa symbolic anthropomorphism, which in fact concerns only
language and not the object itself.
When I say that we are compelled to view the world as
if it were the work of a supreme understanding and will,
I actually say nothing more than that a watch, a ship, a
regiment, are related to the watchmaker, the shipbuilder, thecommanding officer in the same way that the sensible world(or everything that underlies this complex of appearances) isrelated to the unknown; and in saying this I don’t claim toknow the unknown as it is •in itself, but only as it is •for me
or •in relation to the world of which I am a part.

When we connect the command to avoid all transcendentjudgments of pure reason with the apparently conflicting
command to proceed to concepts that lie beyond the domain
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58

nothing but the ontological predicates of ‘substance’, of‘cause’, etc. ·Two comments on this·:

Such knowledge is knowledge by analogy. This doesn’t
involve (as the word ‘analogy’ is commonly thought to do)•an imperfect similarity of two things, but rather•a perfect similarity of relations between the membersof two quite dissimilar pairs of things.13
By means of this analogy we are left with a concept of thesupreme being that is detailed enough for us, though wehave omitted from it everything that could characterize it
absolutely or in itself ; for we characterize only its relation
to the world and thus to ourselves, and that is all we need.
Hume’s attacks on those who want to determine this concept
absolutely, taking the materials for doing so from themselves
and the world, don’t affect my position; he can’t object
against me that if we give up the objective anthropomorphismof the concept of the supreme being we have nothing left.
Hume in the person of Philo in his Dialogues grants to
Cleanthes as a necessary hypothesis the deistic concept of
the original being, in which this being is thought through13

Conclusion(1) We must think the original being in this way; because·there’s no other way to think it, and if we don’t have any
thought of the original being· reason can’t have any satisfaction in the world of the senses, where it is driven by mere
conditions that are in their turn conditioned, ·thus driving us
back along a never-starting sequence of causes·. If the use ofreason in relation to all possible experience is to be pushed
to the highest point while remaining in complete harmony
with itself, the only possible way to do this is to assume
a highest reason as a cause of all the connections in theworld. Such a principle must be thoroughly advantageous toreason, and can’t hurt it anywhere in its natural use.
(2) We can properly think the original being in this way,
because those predicates—·‘substance’, ‘cause’ etc.·—aremere categories, which yield a concept of the original beingthat isn’t determinate and for just that reason isn’t limited

Thus, there is an analogy between the legal relation of human acts and the mechanical relation of motive powers. I can never do something tosomeone else without giving him a right to do the same to me in the same circumstances; just as no body can act on another through its motive
power without thereby causing the other to react equally against it. Right and motive power are quite dissimilar things, yet in their relation there iscomplete similarity. By means of such an analogy I can give a relational concept of things that are absolutely unknown to me. For example, as
a = the promotion of the happiness of children
is related tob = parental love,soc = the happiness of the human species
is related tox = the unknown in God, which we call love.
Not because (x) God’s love has the least similarity to (b) any human inclination; but because we can suppose its relation to the world to be similar toa relation that some things in the world have to some others. But the relational concept in this case is a mere category, namely the concept of cause,
having nothing to do with sensibility.

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to any conditions of sensibility. In thinking of the original
being in this way we don’t fall into anthropomorphism, whichtransfers predicates from the world of the senses to a being
quite distinct from that world. We aren’t transferring reason
as a property to the original being in itself, but only to its
relation to the world of the senses; and so anthropomorphismis entirely avoided. For all we are considering here is thecause of •something that is perceived everywhere in theworld, namely •the world’s rational form; and insofar as
the supreme being contains the ground of this rational
form of the world, reason is to be attributed to it. But insaying that the supreme being has reason, we are speaking
analogically, expressing only the relation that the unknown
supreme cause has to the world, and we do this so as tosee everything in the world as being in the highest degree
reasonable. This procedure doesn’t involve us in treating
reason as an attribute through which we can conceive God;
what we do, rather, is to conceive the world in the way
that’s needed if we are to tackle it with the greatest possible
principled use of reason. In this way we admit that thesupreme being in itself is quite inscrutable and isn’t evenconceivable in any determinate way, and that keeps us from·two errors that we might otherwise make. Roughly andbriefly, they are the errors of trying to •explain God in termsof the world, and trying to •explain the world in terms of God.
A little more fully, •one of the errors is that of·losing ourselves in gross and extravagant notions;and ·the other error consists in·allowing our contemplation of the world to be flooded
with supernatural patterns of explanation, led by thetransfer to God of our notions of human reason—
thereby deflecting this contemplation from its proper
role, which is to study mere nature through ·human·reason, not rashly to derive nature’s appearances froma supreme reason.
The best way to put it, given our weak concepts, is this: weshould conceive the world AS IF its existence and its inner
nature came from a supreme reason.
In thinking of it in this way, we achieve two things. •We
recognize what the world itself is like, without wanting todetermine what its cause is like in itself. And •we see
the ground of what the world is like (the ground of theworld’s rational form) in the relation of the supreme cause to
the world, not finding the world sufficient by itself for that
purpose.14
Returning now to what Philo granted to Cleanthes: We are
perfectly free to predicate of this original being a causality
through reason in respect of the world, thus moving on totheism; and this doesn’t oblige us to attribute this kind of·causally powerful· reason to the original being itself, as aproperty attached to it.
Thus the difficulties that seem to stand in the way of
theism disappear. We achieve this by joining to Hume’s
principle:
Don’t push the use of reason dogmatically beyond thedomain of all possible experiencemaking a transcendent use of our concept of reason
as an efficient cause (by means of the will), trying
to describe the nature of God in terms of properties
that are only borrowings from human nature—thereby
14

Conclusion

I shall say: •the causality of the highest cause relates to •the world in the same way that •human reason relates to its artifacts. That leaves thenature of the supreme cause itself still unknown to me: I only compare •its effect (the order of the world) which I know, and the conformity of this toreason, with •the effects of human reason, which I also know; and hence I call the supreme cause ‘reason’, without thereby attributing to it what I
understand by ‘reason’ as applied to man, or assigning to it any property of anything else that I know.

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this other principle, which he quite overlooked:
Don’t consider the domain of experience as somethingwhich in the eyes of our reason sets its own boundaries.
The critique of reason here indicates the true middle way
between •the dogmatism against which Hume fought and•the scepticism that he wanted to introduce to oppose it. It
isn’t the usual kind of ‘middle way’, which one is advised topick out for oneself as it were mechanically (a little of the one,a little of the other), making nobody any the wiser. Rather, it
is a middle way that can be delineated exactly, according toprinciples.

Conclusion

domain is for us an empty space; and when we are dealing
with concepts whose instances we can identify and study,
we can’t move out of the domain of possible experience. But
a boundary is itself something positive, which belongs asmuch to •what lies inside it as to •the space lying outside thegiven totality; so reason partakes in real positive knowledge
when it stretches out to this boundary. Reason doesn’t try to
go beyond the boundary, because out there it is confronted
by an empty space in which it can conceive •forms of things
but can’t conceive •things themselves. Still, even when it
adopts this stance towards the boundary, just in setting
the boundary reason has knowledge. In this knowledge it
isn’t confined within the world of the senses, but it doesn’tstray outside it either; rather, as befits the knowledge ofa boundary, it focuses on the relation between what lies
outside the boundary and what’s contained inside it.
Natural theology is such a concept at the boundary ofhuman reason, ·because at that boundary· reason finds
itself compelled to look out further towards the Idea of a•supreme being (and, for moral purposes towards the Idea ofa world that can be thought but not experienced). It doesn’t
do this so as to find out anything about this •mere creationof the understanding lying outside the world of the senses;
its purpose is rather to employ principles of the greatest
possible (theoretical as well as practical) unity to guide its
conduct within the world of the senses—a purpose that isserved by relating these principles to an independent reason,
as the cause of all the connections ·found in the world of thesenses·. The aim isn’t to merely invent a being ·of reason.
Invention isn’t in question here·, because beyond the world of
the senses there must be something that can be thought onlyby the pure understanding. Reason’s aim is to characterize
this being, though of course only by analogy.
And so we are left with our original proposition, which is59
At the beginning of this note I used the metaphor of a boundary in order to fix the limits on the proper use of reason.
[This ‘note’ is this Conclusion. Its title includes the term ‘boundaries’,
but the concept isn’t discussed until pages 62–3.] The world of thesenses contains mere appearances, which aren’t things inthemselves; but the understanding must assume things inthemselves (noumena), because it recognizes the objectsof experience as appearances ·and understands that theymust be appearances of something·. Our reason covers both•appearances and •things in themselves, and the question
arises: How does reason go about setting boundaries tothe understanding with respect to both these domains?
Experience, which contains all that belongs to the worldof the senses, doesn’t set bounds for itself; it proceeds inevery case from some conditioned item to some other ·item
that is its condition, and is also a· conditioned object; ·andnothing in this procedure requires it ever to come to a halt·.
The boundary of experience must lie •wholly outside it, andthat is •the domain of pure beings of the understanding. But
when it comes to finding out what these beings are like, this69

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the upshot of the whole critique:
Reason, through all its a priori principles, neverteaches us about anything except objects of possible
experience, and about these it teaches us nothing
more than can be known in experience.
But this limitation on what reason can do doesn’t prevent itfrom leading us to the objective boundary of experience, i.e.
to the relation to something that is the ultimate ground of
all objects of experience without itself being one of them.
Still, reason doesn’t teach us anything about what this‘something’ is like in itself —only about how it relates toreason’s own complete and utterly high-minded use in thedomain of possible experience. But this is all the usefulness
we can reasonably want ·reason to have·, and we have cause
to be satisfied with it.

Conclusion

Such an enquiry is risky, and I admit that what I cansay about it is only conjecture, like every speculation about
nature’s original purposes. But this is permissible, just thisonce, because I am enquiring not into •the objective validityof metaphysical judgments but into •our natural tendency tomake such judgments, so that the enquiry belongs not to thesystem of metaphysics but rather to the study of mankind.
The transcendental Ideas, taken all together, form thereal problem of natural pure reason, a problem that compels
reason to quit the mere observation of nature, to go beyond
all possible experience, and in so doing to bring into existence
this thing (whether it is knowledge or sophistry!) called
metaphysics. When I consider all of these Ideas, I think I see
that
the aim of this natural tendency—this metaphysics—
is to free our thinking from the fetters of experienceand from the limits of the mere observation of nature,taking this freedom at least far enough to open up
to us a domain containing only objects for the pureunderstanding, which no sensibility can reach.
This is not so that we can speculatively occupy ourselveswith this domain (for we can find no ground to stand on while
we do that), but so that we can think of moral principles
as at least possible. ·The connection between their being
possible and the domain opened up by metaphysics is asfollows·. Reason absolutely requires that moral principles
be universal; and they can’t achieve universality unlessthey can fix their expectations and hopes on the domainof transcendental Ideas ·because in the domain of experience
strict universality is never to be found·.
Now I find that the •psychological Idea, little as it shows
me of the nature of the human soul ·thought of as something· elevated above all concepts of experience, does plainly
enough show the inadequacy of these concepts, and in that60
So I have fully exhibited metaphysics as something we can do,
showing it as an automatic upshot of the natural tendencyof human reason, and showing what our essential goal iswhen we do metaphysics. But we have found that ·things
can go wrong in this pursuit·: this wholly natural use ofsuch a tendency of our reason, if it isn’t reined in andgiven limits (which can come only from a scientific critique),entangles us in transcendent dialectical inferences ·leading
to conclusions· of which some are illusory and others areeven in conflict with one another; and this fallacious metaphysics is not a help but an obstacle to the advancementof our knowledge of nature. So it is worth our while toinvestigate the natural goals towards which we can steer thisliking that our reason has for transcendent concepts, ·and
this will counteract the mishaps mentioned above·, because
everything that is natural must be originally aimed at some
useful purpose.
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way steers me away from a materialist theory of mind—atheory that’s unfit to explain anything in nature, as wellas cramping the use of reason in moral thinking. The•cosmological Ideas serve similarly to keep us from naturalism, which asserts that nature is sufficient unto itself;they do this through ·bringing home to us· the obvious factthat even if we had all possible knowledge of nature, reason’s
legitimate demands wouldn’t be satisfied. Finally there is the•theological Idea, ·whose service to us is as follows·. All natural necessity in the sensible world is conditioned, because
it always involves something’s being necessitated by something else that is also conditioned; and thus unconditional
necessity is to be looked for only in a cause that is different
from the world of the senses. And the causality of thiscause can’t be yet another example of natural necessity, for
if it were it could never make comprehensible the existenceof the contingent (as its consequent). So the theologicalIdea, ·which is the Idea of a non-natural cause of everything
contingent·, is something that reason uses to free itself fromfatalism, and to arrive at the concept of a cause possessing
freedom, or of a highest intelligence. (This frees us fromboth versions of fatalism: (1) blind natural necessity inthe system of nature itself, without a first principle, and(2) blind causality of a first principle of nature.) Thus thetranscendental Ideas serve, if not to instruct us positively, atleast to put a stop to the impudent assertions of materialism,of naturalism, and of fatalism—assertions that restrict thedomain of reason—thereby making room for the moral Ideas
to operate outside the domain of speculation. This, I should
15

Conclusion

think, goes some way towards explaining reason’s naturaltendency ·to engage with Ideas·, which I mentioned earlier.
The facts about the •practical or moral usefulness thata purely •speculative science can have don’t lie within theprovince of the science itself; so they can be seen simply asa scholium [= ‘explanatory note, marginal comment’] which, like allscholia, is not a part of the science itself. Still, this material
surely lies within the boundaries of philosophy, especiallyof philosophy drawn from the well of pure reason—a part ofphilosophy in which reason’s •speculative use in metaphysics
must necessarily be all of a piece with its •practical use inmorals. Hence the unavoidable dialectic of pure reason,
considered as something occurring in metaphysics as anatural tendency, deserves to be explained not only as •anillusion that needs to be cleared away but also, if possible,
as •·an upshot of· something put in place by nature for a
purpose—though this task lies outside the job-description of
metaphysics proper, and so can’t rightly be assigned to it.
The solutions of the questions put forward in the Critiqueat A647–68 = B675–96 should be regarded as a second
scholium—this time one that’s more closely related to thecontent of metaphysics.15 For that part of the Critiquepresents certain principles of reason that characterize a priori•the order of nature or rather •the understanding which is
to seek nature’s laws through experience. They seem to havepropositional content and not merely to be rules for how theunderstanding should be employed, and to be law-giving withregard to experience, although they spring from mere reason,
which cannot like the understanding be considered as a

Throughout the Critique I stuck to my resolve not to neglect anything, however deeply hidden, that could enable the inquiry into the nature of purereason to be completed. Afterwards it is up to each person to decide how far to take his researches, once he has been shown what remains to be
done. This ·attitude to further research· can reasonably be expected from someone who has made it his business to survey the whole field, so as toleave it to others for future cultivation and for whatever subdividing of it they think fit. The scholia belong to this ·part of the total project·; becauseof their dryness they can hardly be recommended to amateurs, and hence they are presented only for experts.

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principle of possible experience. This agreement ·between
principles issued by reason and what is found in experience·
may rest on this:Nature isn’t attached to appearances (or to the sensibility through which appearances come) in themselves,
but is to be found only in the relation of sensibility
to the understanding. ·And just as nature takes us
upwards from sensibility to understanding, so thetheoretical use of the understanding takes us further
upwards to reason·. A thoroughgoing unity in the useof the understanding for the sake of a systematicallyunified possible experience can be had only if •theunderstanding is related to reason. And so, ·putting
the two together·, experience is indirectly subject toConclusion

the legislation of reason.
The question of whether this is where the agreement comesfrom may be pondered by those who want to trace the natureof reason even beyond its use in metaphysics, into the general principles for making general natural history systematic.
In the Critique I represented this task as important, but I
didn’t try there to carry it out.
And thus I conclude the analytic solution of my own
chief question: How is metaphysics in general possible? by
starting with the actual doing of metaphysics (or at least
with the consequences of that) and climbing from there to
the grounds of its possibility. [See the explanation of ‘analytic’, inthis sense of the word, on page 6.]72

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How metaphysics is possible as a science

Solution of the general question of the Prolegomena:
How is metaphysics possible as a science?
is possible as •how to get it under way, inducing good minds
to quit their mistaken and fruitless cultivation in favour of
one that won’t deceive, and •how such an alliance for thecommon end may best be directed.
This much is certain, that someone who has sampled
criticism will for ever after be disgusted with all the dogmatic
twaddle that he used to endure—he had to endure it because
his reason was in need of something and couldn’t findanything better ·than the twaddle· for its nourishment.
Criticism relates to ordinary academic metaphysics exactly as chemistry does to alchemy, or as astronomy does tothe astrology of the fortune-teller. I guarantee that when you
have thought through and grasped the principles of criticism,even if only in these preliminaries, you will never returnto that old and sophistical pseudo-science ·of dogmatic
academic metaphysics·; rather, you will look forward witha certain delight to a metaphysics that is now surely inyour power, that requires no more preparatory discoveries,
and, above all, that can provide reason with permanentsatisfaction. For here is an excellence that metaphysics canconfidently count on and that no other possible science can:it can be completed and put into a permanent state where
there are no more changes to be made, and no additions
through new discoveries. That’s because in metaphysics
reason has the sources of its knowledge in itself, not in
objects and the intuition of them (reason has nothing to learn
from intuition); and when it has presented the fundamental
laws of its own capacities completely, and so definitely as toprevent any misunderstanding, there’s nothing left for purereason to know a priori—indeed, there isn’t even any basis

Metaphysics as a natural tendency of reason is real, but byitself it is dialectical and deceitful (as the analytic solution of
the third principal question showed). If we set ourselves
to take principles from it, and in using them to follow
the natural (but nonetheless false) illusion, we can neverproduce science, but only a pointless dialectical art in which
one school may outdo another but none can ever get, and beentitled to, lasting approval.
For metaphysics as a science to be entitled to claim notmere fallacious plausibility but insight and conviction, a
critique of reason itself must exhibit•the whole stock of a priori concepts,•the classification of them according to their different
sources (sensibility, understanding, and reason),•a complete list of these concepts, and•the analysis of each of them together with all theconsequences of that analysis;
but above all the critique must show•the possibility of synthetic a priori knowledge (doing
this through a deduction of these concepts),•the principles governing the use of the a priori concepts, and finally•the boundaries of that use;
and all of this is to be presented in a complete system!
Thus criticism, and that alone, contains in itself the wholewell-tested and verified plan for achieving metaphysics asa science—the plan and indeed all the means for carrying
it out. By any other ways or means the task is impossible.
[Here and below, ‘criticism’ translates Kritik, which is usually renderedas ‘critique’.] So the question here isn’t so much •how the task

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left for it to ask any further questions. There’s something
especially attractive about the sure prospect of knowledge
that is so definite and so completed—even apart from all its
advantages (of which more later).
All false art, all empty ‘wisdom’, lasts its time out but
eventually destroys itself, and its cultural high-point comesat the moment of ·the onrush of· its decay. That this time
has come for metaphysics is shown by the condition into
which it has fallen in all the learned nations, in contrast
with all the zeal with which other sciences of every kindare pursued. The old organization of university studies stillpreserves its shadow; and now and then a solitary academyof science, by offering prizes, tempts someone or other tohave a shot at it; but it is no longer counted among the solid
sciences. You can judge for yourself how a gifted man would
take it if he were called ‘a fine metaphysician’! It might bemeant as a compliment, but hardly anyone would want tobe so labelled.
Yet, though •the time of the collapse of all dogmatic metaphysics has undoubtedly arrived, we are still far from being
able to say that •the time has come for its rebirth througha solid and complete critique of reason. When someone’s
inclinations shift from running one way to running in the
opposite direction, he passes through an intermediate stageof indifference ·in which he isn’t inclined in any direction.
And this fact about human desires and tendencies has
its analogue in shifts of intellectual direction among thesciences·. This moment ·of ‘indifference’, with an old scienceon the wane and no new one to take its place·, is •the mostdangerous for an author, but in my opinion it’s •the mostfavourable for the science. For when the total breaking offormer ties has extinguished the partisan spirit, minds arein the best state to take in, gradually, proposals for a newscheme of alliances.

How metaphysics is possible as a science

If I say:
I hope that these preliminaries may excite investigation in the domain of criticism, and provide something
new and promising to nourish the universal spirit ofphilosophy that seems ·except for moral philosophy·
to be under-nourished,
I can already imagine that everyone who is tired and crossfrom walking the thorny paths of my critique will ask me:
What’s your basis for hoping that? I answer: The basis of
the irresistible law of necessity.
Will the human mind ever give up metaphysical researches altogether? There’s no more chance of that thanthere is of our choosing to give up breathing altogether so as
to avoid inhaling impure air! So there will always be metaphysics in the world; what’s more every person—especiallyevery thinking person—will have metaphysical views, and in
the absence of a public standard he will tailor them to suithimself. What has been called ‘metaphysics’ up to now can’tsatisfy any demanding mind, but it’s quite impossible togive up metaphysics completely; so a critique of pure reason
itself must now be attempted; or if one exists it must be
investigated and comprehensively tested. There’s no otherway to meet this pressing need, which is something morethan mere thirst for knowledge.
Ever since I have come to know criticism, when I finish
reading a book with metaphysical content—one that hasentertained and enriched me by its precision of thought,
variety, orderliness, and easy style—I can’t help asking: Has
this author really advanced metaphysics a single step? I
hope they will forgive me—those learned men whose writings
have been useful to me in other respects and have always
helped me to develop my mental powers—for saying that I
have never been able to find that the science of metaphysics
has been advanced in the least by their works or by my own
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lesser ones (even when my egotism speaks in their favour!).
The reason for this is very obvious: it is that metaphysics
didn’t then exist as a science; and ·those other writers and I
couldn’t make small steps towards bringing it into existence,
because· it can’t be assembled bit by bit, but must haveits seed fully preformed in the critique. However, in orderto prevent any misunderstanding we should bear in mind
something I have already said: the understanding gains agreat deal from the analytic treatment of our concepts, but
the science (of metaphysics) isn’t in the least advanced byit, because these analyses of concepts are merely materials
out of which the science is to be assembled in the first place.
Let the concepts of substance and of accident be ever sowell analysed and fixed; that’s an excellent preparation forsome future use. But if I can’t prove that in everythingthat exists the substance endures and only the properties
change, our science isn’t the least advanced by all thisanalysis. Metaphysics has so far not been able to provea priori either •the above proposition, or •the principle ofsufficient reason, still less •any compound principle such as
belongs to psychology or cosmology, or indeed •any synthetic
proposition whatsoever. So all this analysis has achieved
nothing, created and advanced nothing; and despite all thisbustle and clatter the science is right back where it was inAristotle’s time; though the preparations for it would havebeen better advanced ·now· than they were ·back then·, if
only the guiding thread to synthetic knowledge had beenfound.
If anyone thinks himself wronged in this, he can easily
refute my charge by producing a single synthetic proposition
belonging to metaphysics that he offers to prove a priori
in the dogmatic manner. Until he has done this I shan’t
grant that he has really advanced the science; even if the
proposition ·that he claims to be able to prove· is sufficiently

How metaphysics is possible as a science

confirmed by common experience. No demand can be more
moderate or fairer than this, and if it isn’t fulfilled (as it quite
certainly won’t be), the fairest verdict we can give is this: Up
to now, metaphysics has never existed as a science.
In case my challenge is accepted, I must rule out just two
things.
(1) One is: playing around with probability and conjecture,
which are as little suited to metaphysics as they are togeometry. Nothing can be more absurd than to think ofgrounding our judgments on probability and conjecture in
metaphysics, which is a philosophy based on pure reason. Everything that is to be known a priori is for that very
reason announced as absolutely certain, and must therefore
be proved as such. We might as well think of basing geometryor arithmetic on conjectures! The calculus of probability,
which is part of arithmetic, contains no •merely probablejudgments. Rather, it consists of •completely certain judgments about the degree of possibility of certain upshots ingiven homogeneous conditions. What happens across thetotality of all possible cases must be in accordance withsuch rules ·or judgments·, though these aren’t determinate
enough to say what will happen in any particular case. Onlyin empirical natural science can conjectures be tolerated(they come in there through induction and analogy), andeven there it must at least be quite certain that what one isassuming could be true.
(2) The second thing I rule out is decision by means of
the divining rod of so-called sound common sense, which
doesn’t dip ·in the same place· for everyone and is guided by·the· personal qualities ·of the person holding it·. When weare dealing with concepts and principles not considered asvalid with regard to experience but considered as valid evenbeyond the conditions of experience, appealing to •common
sense is even more absurd ·than relying on probability·, if75Prolegomena

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How metaphysics is possible as a sciencefor our judgments. ·And these immediately certain
propositions are the ones we know to be true throughour common sense·.

that’s possible. For what is common sense? It is ordinary
understanding insofar as it judges correctly. But what is•ordinary understanding? It is the capacity for knowledge
and for using rules in application to particular cases, asdistinguished from •speculative understanding, which isthe capacity for knowledge of rules in the abstract. So
common sense can hardly understand the rule that every
event is determined by its cause, and can never take it in asa general proposition. It therefore demands an example fromexperience; and when it ·is given one, and· hears that thisrule means nothing but what it (·common sense·) always
thought when a window-pane was broken or an article offurniture went missing, then it understands the principle
and agrees to it. •Ordinary understanding is thus of use
only to the extent that it can see its rules confirmed byexperience (though actually the rules are in it a priori);consequently the job of having insight into these rules a priori
and independently of experience is assigned to •speculative
understanding, and lies quite outside the domain of visionof common sense. But metaphysics has to do only with
speculative understanding; and someone who appeals tocommon sense for support in metaphysics shows that hedoesn’t have much of it! For in this context common sense
has no judgment at all; and ·when it is invoked, there is akind of bad faith in that, because· it is looked down on with
contempt except when people are in difficulties and don’tknow where else to turn for advice or help.
These false friends of common sense (who occasionallyprize it highly, but usually despise it) customarily offer thisexcuse ·for sometimes appealing to it·:
There must in the end be some propositions that are
immediately certain, and for which there’s no need togive any proof, or even any account at all; because if
there were not, there would be no end to the grounds

But these people can never prove their right to say this bypointing to anything indubitable that they can immediately
ascribe to common sense—with two exceptions ·that are
irrelevant to our present concerns·. •One is the principleof contradiction, which ·we can set aside because it· isinadequate for showing the truth of synthetic judgments.
•The other is comprised of mathematical propositions, suchas that twice two make four, and that between two points
there is only one straight line, etc. But these judgments
are vastly different from those of metaphysics. For inmathematics when I conceptually represent something tomyself as possible I can also make it, construct it, in mythought: to one two I add the other two, one by one, and somyself make the number four; or from one point to anotherI draw in thought all kinds of lines, and can draw only one
in which every part is like every other part ·which meansthat the line is straight·. But ·no such construction has aplace in metaphysics, as I shall explain through the exampleof the concept of causation·: with all my power of thinking
I can’t extract from the concept of one thing the conceptof something else whose existence is necessarily connectedwith the first thing; rather, ·if I want a basis for connecting
something with something else· I must call in experience.
Now, my understanding provides me a priori (yet always only
in reference to possible experience) with the concept of such aconnection ·between different things·, namely causation. But
I can’t exhibit this concept a priori in intuition, thus showing
its possibility a priori, as I can the concepts of mathematics.
In metaphysics the concept of causation (together with theprinciples of its application) has to be valid a priori, andfor that there must be a justification and deduction of its
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Making metaphysics actual as a science

we find that all we can achieve is •reasonable belief —which suffices for our needs and may indeed be morewholesome for us than knowledge ·strictly so-called·.

possibility—for otherwise we can’t know what its range ofvalidity is, ·e.g.· whether it can be used only in experienceor also outside it. Such a •justification and deduction arenothing remotely like the •intuitive construction throughwhich we can show possibility in mathematics. [See the long

When we make that switch, the shape of the situation iscompletely altered. Metaphysics must be science, over-all
and in each part; otherwise it is nothing. That is because
metaphysics, as speculation of pure reason, has nothing
to hold it steady except universal insights. Beyond its
domain, however, probability and common sense can beused legitimately and to good effect, but following principlesof their own, the importance of which always depends ontheir reference to practical life.

note on page 9.]

In metaphysics as a speculative science of pure reason,
therefore, we can never appeal to common sense. We canmake such an appeal when
we are forced to •abandon pure reason and to renounce all purely speculative knowledge (which must
always be knowledge ·strictly so-called·, which involves renouncing metaphysics itself and its teachingon certain matters,·this ‘forcing’ coming about because·That’s what I consider myself entitled to require for the
possibility of metaphysics as a science.

Appendix:
On what can be done to make metaphysics actual as a science
As none of the paths that have so far been followed havereached the goal ·of metaphysics as a science·, and since
it never will be reached except through a preceding critique of pure reason, it seems reasonable to ask that thispresent attempt ·at such a critique· be examined carefully
and accurately—unless you think it better to give up all
pretensions to metaphysics, which is all right as long as you
stick to it.

precedes the investigation, (2) a judgment that comes after
the investigation. In our case (1) is what happens when thereader pronounces judgment on the Critique of Pure Reason
on the basis of his own metaphysics, though the possibilityof that is what the Critique aimed to investigate. In (2) ·in
our case· the reader is able to •set aside for a while theconsequences of the critical enquiries, which may clash
violently with the metaphysics that he used to accept, and•first examines the grounds from which those consequences
can be derived. If what ordinary metaphysics offers wereIf we take the course of things as it is, not as it ought
to be, there are two sorts of judgments: (1) a judgment that77

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demonstrably certain (like the theorems of geometry, forinstance), judgments of kind (1) would be legitimate; for if theconsequences of certain principles conflict with establishedtruths, the principles are false and can be rejected without
further enquiry. But if •metaphysics doesn’t have a stock ofindisputably certain (synthetic) propositions, and if •it is eventhe case that a number of the propositions of metaphysics—
though as plausible as the best of them—have consequencesthat conflict with one another, and if •metaphysics contains
absolutely no criterion for the truth of specifically metaphysical (synthetic) propositions, then the (1) kind of judging isn’t
admissible, and ·the (2) method should be followed, thatis· the investigation of the principles of the Critique must
precede all judgments as to its worth or unworth. ·In theremainder of this Appendix I shall present an actual exampleof (1), followed by a proposal for an example of (2)·.

which the author started, the author may dislike the severityof his judgment but the public doesn’t mind it because here
the public is the winner. And the author can be glad that an
opportunity to correct or explain his work has come throughthe examination of a competent judge. If he thinks he ismainly right, he can in this way remove any obstacles thatmight eventually hurt the success of his work.
It’s not like that with my reviewer. He seems to havemissed entirely the real point of my enquiry. Perhaps he
was impatient with thinking his way through a lengthy work;or angry at the threatened reform of a science in which hethought he had settled everything long ago; or—what I reluctantly believe is the case—narrowness of grasp stopped himfrom carrying his thoughts beyond his school metaphysics.
Anyway, he •impetuously whips through a long series of
propositions which no-one cd grasp without knowing theirpremises, and •scatters around his condemnations without
giving understandable reasons. So his review is no use to
the reader, and doesn’t do the slightest harm to me in the
judgment of experts. So I would have passed over this review
completely if it hadn’t given me an opportunity to provide
some clarifications that may save some readers of thesePreliminaries from misinterpretations.
Wanting to position himself so as to set the whole workin a light that is most unflattering to its author, doing thiseasily without putting any work into it, Reviewer begins andends by saying: ‘This work is a system of transcendental (or,as he translates it, of higher) idealism.’16

On a sample of a judgment of the Critique prior to its
examination
This judgment occurs in an ·anonymous review· in the
Göttingen Scholarly News for January 1782. . . . When an
author who knows the subject-matter of his work and has
worked hard to present his own thoughts in it falls into thehands of a reviewer who for his part •is sharp enough to see
the points on which depend the value (if any) of the book,who •doesn’t hang on the words but goes for the content, and•confines himself to sifting and testing the principles from
16

Making metaphysics actual as a science‘Higher’—no way! High towers, and metaphysically-great men that resemble them, are not for me—there is usually too much wind around them! My
place is the fertile bottom-land of experience; and the word ‘transcendental’—whose meaning was so often explained by me but not once grasped by
Reviewer (so carelessly has he looked at everything)—doesn’t signify something that •goes beyond all experience, but something that •does indeed
precede experience a priori, but whose role is simply •to make knowledge through experience possible. If these concepts step beyond experience,
their employment is termed transcendent, as distinct from their immanent use, that is, their use limited to experience. ·Don’t confuse ‘transcendent’with ‘transcendental’·. All misunderstandings of this kind have been adequately guarded against in the work itself, but it suited the reviewer’s turnto misunderstand me. [The use here of ‘Reviewer’, as though it were a proper name, is Kant’s. He will do this once more.]78

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A glance at this line showed me what sort of review
was in store for me. It was like someone who has neverseen or heard of geometry, finds a copy of Euclid and on
flipping through its pages sees various figures, is asked
his opinion of it, and replies: ‘The work is a text-book ofdrawing; the author uses a special language in which to give
dark, incomprehensible directions that in the upshot teachnothing more than what everyone can accomplish with agood natural eye, etc.’
Meanwhile, let us see what sort of idealism it is that runs
through my whole work, although it is far from constituting
the soul of the system.
The thesis of all genuine idealists from the Eleatic school
to Bishop Berkeley is contained in this formula:
All knowledge through the senses and experience isnothing but sheer illusion, and only in the ideas of
the understanding and reason is there truth.
The principle that governs and determines my idealism
throughout is on the contrary:All knowledge of things through unaided pure understanding or pure reason is nothing but sheer illusion,and only in experience is there truth.
This is precisely the opposite of the former, genuine idealism.
So how did I come to use this expression for a completely
opposite purpose, and how did my reviewer come to see
genuine idealism everywhere?
The solution of this difficulty rests on something that
could have been very easily understood—by anyone whowanted to!—from the over-all structure of the work. Space
17

Making metaphysics actual as a scienceand time, together with everything they contain, are notthings or qualities in themselves, but belong merely to theappearances of such things and qualities; up to this point I
am doctrinally at one with the ·genuine· idealists. But they,and especially Berkeley, regarded space ·itself· as a mere
empirical representation, and held that it together with allits properties is known to us only by means of experience orperception—just like the appearances in space. As against
this, I show in the first place, thatspace (and also time, which Berkeley ignored) and allits properties can be known by us a priori, because
space (as well as time) •is present in us before all perception or experience as a pure form of our sensibilityand •makes possible all sensible intuition and thus
all appearances.
It follows from this ·contrast between Berkeley and me· that•because truth rests on universal and necessary laws as
its criteria, experience for Berkeley can have no criteriaof truth, because its appearances (according to him) havenothing underlying them a priori, from which it follows inturn that they are nothing but sheer illusion; whereas forme space and time (in combination with the pure concepts of
the understanding) prescribe their law a priori to all possible
experience, and this at the same time yields the sure criterion
for distinguishing truth from illusion in experience.17
My so-called idealism (properly: critical idealism) is thusof a quite special kind, in thatit overthrows ordinary idealism; and through it alla priori knowledge, even that of geometry, first re-

Genuine idealism always has a visionary purpose; it is bound to. But my idealism is designed solely for grasping the possibility of our a priori
knowledge of objects of experience—a problem that has never been solved before, and never even been posed. In this way all visionary idealism
collapses. As was already to be seen in Plato, visionary idealism inferred from our ·having· a priori knowledge (even that of geometry) ·that there
is· another intuition different from that of the senses, namely an intellectual intuition. It never occurred to any of them that the senses themselves
might intuit a priori ·as I say they do, for example in geometry·.

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ceives objective reality; and even the most zealous·ordinary· realists couldn’t have claimed that, because
they lacked my demonstrated ideality of space andtime—·that is, my proof that space and time are formsof our sensibility·.

rebuke: ‘What’s the point, then, of this quarrel with accepted
language, what’s the point—and what’s the source—of theidealistic distinction?’ After a first judgment that all that is
special in my book is metaphysically heretical, now at theend it is said to be a mere change of language; which clearly
proves that my would-be judge hasn’t the slightest grasp ofit, and hasn’t even understood himself.18

In these circumstances I would have liked, so as avoid all
misunderstanding, to name this concept of mine differently;
but I can’t very well alter the name totally. So I may bepermitted in future to call it ‘formal idealism’ (·as I did on
page 52·) or, better, ‘critical idealism’, to distinguish it fromthe dogmatic idealism of Berkeley and the sceptical idealismof Descartes.

Reviewer speaks like a man who must be aware of having
important and excellent insights—but ones that he keeps
hidden, for I don’t know of anything recent relating tometaphysics that would justify his tone. It is wrong for
him to withhold his discoveries from the world, for there are
doubtless many others like me who haven’t been able to find,in all the fine things that have been written in this branchof philosophy, anything that has advanced the science ·of
metaphysics· by so much as a finger-breadth. What we dofind are •definitions sharpened, •lame proofs fitted out withnew crutches, •the crazy-quilt of metaphysics supplied withnew patches or with a change of pattern; but none of thisis what the world requires! The world has had enough ofmetaphysical assertions; what is wanted is ·to establish·•the possibility of this science, •the sources from which
certainty could be derived in it, and •sure criteria by whichto distinguish the dialectical illusion of pure reason fromtruth. The reviewer must have the key to all this; otherwise
he would never have spoken in such a high tone.

I find nothing else worthy of comment in this review
of my book. All the way through the reviewer presentsblanket judgments—a smart procedure for a reviewer touse, because it doesn’t reveal the state of his knowledge or
ignorance; ·whereas· a single criticism that was thoroughand detailed, if it concerned the main issue (as it oughtto), might have exposed error in my work, and might also
have revealed the reviewer’s level of skill in this sort ofenquiry. Another well-conceived device for removing earlyon the readers’ desire to read the book itself—readers who
usually form their conceptions of books merely from newspaper articles—is •to pour out all in one breath a numberof propositions, torn out of the context of their grounds
of proof and explanations, which are bound to strike thereader as nonsensical (especially considering that they arepoles apart from all ·ordinary· school-metaphysics), •to makethe reader disgusted with the demands on his patience adnauseam, and then, after presenting ·and attributing tome· the brilliant proposition that constant illusion is truth(which was news to me!), •to conclude with the firm, fatherly
18

Making metaphysics actual as a science

But ·joking aside· I am inclined to suspect that no suchrequirement for the science ·of metaphysics· has ever entered
his head. If it had,
he would have focussed on this matter in his review,and ·if he thought I had been wrong about it· evena failed attempt in such an important affair would[Kant has a footnote here, protesting at two of the reviewer’s misunderstandings.]

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have won his respect. If that is how things stand,we are good friends again. He can think his way as
deeply as he likes into his metaphysics; no-one willstop him; but he can’t make judgments about thesource of metaphysics in reason, for that lies outside
metaphysics.
·That’s if the requirements for a science of metaphysics hadentered his head·. But my suspicion ·that they didn’t · is
not unfounded, as is shown by the fact that he doesn’t
say a word about •the possibility of synthetic knowledge apriori, though this was •the real problem on the solution ofwhich the fate of metaphysics wholly rests, and to which my
Critique (along with the present Preliminaries) was entirely
directed. The idealism that he stumbled on ·in the Critique ·,and was pinned down by, was incorporated in the system
only because it was the sole means for solving •the above
problem (though it was later confirmed on other grounds);
so ·if he had understood what was going on· he would haveto have shown either that the problem isn’t as importantas I make it out to be in the Critique (and again now inthese Preliminaries), or that my appearance concept doesn’t
solve it at all or provides a solution that is inferior to someother. But I don’t find a word of this in the review. So thereviewer understood nothing of what I wrote, and perhaps
also nothing of the spirit and nature of metaphysics itself;
unless (and I would rather think this) a reviewer’s haste,
and annoyance at the difficulty of working through so many
obstacles, threw an unfavourable shadow over the work lying
before him, hiding from him its fundamental features.
In the domain of metaphysics as elsewhere, there is agood deal to be done before a learned journal—however
carefully recruited and well-chosen its contributors are—canmaintain its otherwise well-deserved reputation. Other
sciences and branches of knowledge have their standards.

Making metaphysics actual as a science

Mathematics has its standard within itself; history andtheology have it in secular or sacred books; natural science and medicine have it in mathematics and experience;jurisprudence has it in law books; and even matters of taste
have standards in the examples of the ancients. But for
judging the thing called metaphysics the standard has yet
to be found (I have made an attempt to settle what it isand how it should be used). Until it is worked out, what
is to be done when works of this kind are to be judged?
If the works are of the dogmatic kind, do what you likewith them; when someone plays the master over others ·inthis game·, it won’t be long before he runs into someone
else who pays him back in kind. But if the writings areof the critical sort—offering a critique not of other writings
but of reason itself—then the standard of judgment can’t
be •taken for granted but must first be •sought for. Whensuch writings are in question, it may still be all right to offer
objections and blame; but underlying them should be an
attitude of co-operation, because the need ·for standards,
and for metaphysics to become a science· is common to us
all, and the lack of the needed insight makes it inappropriate
for anyone to come across as a judge handing down verdicts.
But so as to connect this defence ·of my work· with
the interests of the philosophizing public, I propose a test
that will settle the question of how metaphysical enquiries
should be directed towards their common end. It is just whatmathematicians have done to show by competition which
methods are best:
I challenge my reviewer critic to prove in his way
any one really metaphysical principle that he accepts.
Being metaphysical it must be synthetic and knowna priori from concepts. It •could be one of the mostindispensable principles, as for instance the principleof the persistence of substance, or of the necessary81Prolegomena

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determination of events in the world by their causes;
but it •must (this is a fair demand) be proved on apriori grounds.
If he can’t do this (and silence is a confession), he must
admit thatas metaphysics is nothing at all without the absolutecertainty of propositions of this kind, ·and as hecan’t prove any of them in his dogmatic manner·,the first thing that’s needed—before anything else isdone—is to establish the possibility or impossibility of
metaphysics, in a critique of pure reason.
So he is obliged either to admit that my principles of criticism
are correct, or to prove that they are not. I can already foresee
that, although he has been carefree in his reliance on thecertainty of his principles, when it comes to a strict test he
won’t find a single one in the whole range of metaphysics
that he can boldly bring forward. So I shall grant him themost favourable terms that can ever be expected in such acompetition, namely: I shall take the onus of proof from him
and lay it on myself.
He finds in these Preliminaries [section 51] and in my Critique (B 454–89, the Antinomies chapter) eight propositions,in pairs whose members contradict each other, but eachof which necessarily belongs to metaphysics, which must
either accept or disprove it (although each has in its day
been accepted by some philosopher). Now the reviewer
is at liberty to select •any one he likes out of these eight
propositions, and to accept it without any proof (that’s agift from me), but •only one (for wasting time won’t doeither of us any good); and then to attack my proof of thecontrary proposition. If I can rescue the latter, thereby
showing that the opposite of the proposition he chose can bejust as clearly proved in accordance with principles thatevery dogmatic metaphysics must necessarily recognize,

Making metaphysics actual as a science

then this will settle that metaphysics has a hereditary fault
that can’t be explained—let alone removed—until we ascend
to the birth-place of metaphysics, pure reason itself. So
my critique—·which makes that ascent·—must either be
accepted or replaced by a better one; it must at least be
studied, which is all I am demanding now. If on the other
hand I can’t save my proof, then •a synthetic proposition apriori from dogmatic principles is firmly entrenched on my
opponent’s side, •my impeachment of ordinary metaphysics
is ·revealed as· unfair, and •I pledge myself to recognize his
censure of my critique as justified (though none of this will
happen!). But for this it would be necessary, it seems to me,that he should drop his anonymity. Otherwise I don’t see
how I could avoid, instead of having just one problem to deal
with, being honoured or assailed by many problems fromanonymous and indeed uninvited opponents.
Proposal for an investigation of the Critique, on which
a judgment can follow
I am obliged to the learned public for the silence with which
it has for a long time honoured my Critique; for this shows•a postponement of judgment, and thus •some supposition
that a work that leaves all the beaten paths and strikes out
on a new and initially difficult one may contain something
through which an important but currently withered branchof human knowledge might derive new life and fruitfulness;and thus it also shows •a concern not to break off anddestroy the still delicate graft through a hasty judgment.
A specimen of a judgment that was delayed for the above
reasons is now before my eyes in the Gotha Scholarly News.
Setting aside my own (suspect) praise for this review, any
reader can see for himself that it is a solid piece of work; thiscan be seen from its graspable and accurate presentation of
a portion of the basic principles of my work.
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Because an extensive structure can’t be judged as a wholefrom a hurried glance, I propose that it [the system of Critique ofPure Reason ] be tested piece by piece from its foundations,and that the present Preliminaries be used as a generaloutline with which the work itself could then sometimes
be compared. If this suggestion were based only on theimagined importance that vanity usually attributes to one’s
own output, it would be immodest and would deserve to
be indignantly rejected. But ·that isn’t how things stand;something very serious is at stake·; the affairs of speculative
philosophy are now on the brink of total extinction, althoughhuman reason hangs onto them with undying affection,
an affection that is now trying (and failing) to change into
indifference because it has been constantly disappointed.

Making metaphysics actual as a science

the attraction is greater than any other theoretical knowledge
has; one wouldn’t lightly trade this one for any of them—e.g.
forgoing metaphysics in order to take up chemistry.
However, for this investigation I am not offering thework itself, but rather these Preliminaries as plan and guide.
AlthoughI am even now well satisfied with the Critique as far
as its content, order, and manner of presentation
are concerned, and with how carefully I weighed andtested every sentence before writing it down (for it has
taken me years to be completely satisfied not only
over-all but ·also in detail·, sometimes labouring tobecome satisfied with the sources of one particular
proposition),
I am nevertheless not completely satisfied with my exposition
in some chapters of the Doctrine of Elements—for example
on the Deduction of the Concepts of the Understanding,or on the Paralogisms of Pure Reason—because a certain
long-windedness takes away from their clarity; and yourexamination ·of the work as a whole· could be based on whatthese Preliminaries say about those chapters rather than on
the chapters themselves.
The Germans are praised for taking things further thanpeople of other nations in matters where steady and continuous work is needed. If this opinion is well founded, thenan opportunity to confirm it presents itself here: a project inwhich all thinking men have an equal interest, and whose
successful outcome is hardly in doubt, though it has neversucceeded before. The prospects for completing the project
are good especially because the science in question is of sucha peculiar sort that it can be brought to completion all at
once, reaching a permanent state in which it can never be•taken the least bit further, •amplified by later discoveries,
or even •altered in any way (apart from improvements in

In our thinking age one might expect that many deserving
men would use any good opportunity to work together forthe common interest of an ever more enlightened reason, if
only there were some hope that in this way they would reach
their goal. Mathematics, natural science, law, arts, evenmorals etc., don’t completely fill the soul; there’s always aspace staked out for pure, speculative reason. The emptinessof this space prompts us to resort to grotesque masks andworthless glitter, or to mysticism, ostensibly in search of
employment and entertainment though really we are justdistracting ourselves so as to drown out the burdensome
voice of reason, which, true to its own nature, demands
something that can satisfy it, and not merely somethingthat started up so as to serve other ends or to satisfy our
inclinations. So a study that is concentrated on •this territoryof reason existing for itself must (or so I have reason to hope)
have a great attraction for anyone who has tried in this way tostretch his thought, because it is just precisely •here that allother kinds of knowledge—all other goals, even—must cometogether and unite into a whole. I would venture to say that83

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Making metaphysics actual as a science·that it had performed for reason· by encouraging conceit

clarity in some places, or improvements in how the scienceis used for all sorts of purposes). This advantage ·of being
made finally complete in a single operation· is one that noother science can have, because none of the others concernsa cognitive faculty that is as completely isolated and as
independent of other faculties ·as is the faculty of purereason·. The present moment seems to be favourable to my
expectations, because just now in Germany no-one seems
to know how to occupy himself outside the so-called usefulsciences, doing something that isn’t mere play, but a projectin which success will be permanent.
I must leave to others to work out how the efforts ofscholars might be united in the pursuit of this goal. I’m notlooking for a mere acceptance of my theses by anyone; I’m
not even flattering myself with the hope of that. But as longas the matter is investigated from the ground up, perhaps
with ·my system’s being subjected to· attacks, repetitions,
qualifications, or confirmation, completion, and extension,
the outcome is certain to be a system—maybe not mine—
which can become a possession for which future generations
will have reason to be grateful.
It would take too long for me to show here •what kindof metaphysics may be expected to ensue if we first get
right about the principles of criticism, and to show •how theresultant metaphysics would appear richly and respectably
outfitted, not cutting a poor, paltry, plucked figure justbecause its old false feathers had been pulled out! But
other great benefits that such a reform would bring withit are immediately obvious. The ordinary metaphysics had
good uses, in that it sought out the elementary concepts ofpure understanding in order to clarify them through analysis
and make them definite through explanation. In this way
it was a training for reason, in whatever direction it mightgo. But that’s all the good the ordinary metaphysics did;·and really it did less than that·. For it negated this service

through reckless assertions, sophistry through subtle escapes and excuses, and shallowness through the ease withwhich it settled the most difficult problems by means ofa little school-philosophy. This school-philosophy is all
the more seductive the more it has the option of taking
something from the language of science (on the one hand)and something from ordinary speech and thought (on theother), thus being all things to all men—but in reality nothingat all! In contrast, criticism gives to our judgment a standard
by which knowledge can with certainty be distinguishedfrom pseudo-knowledge; and, by being brought into fulloperation in metaphysics, criticism lays the basis for a wayof thinking that goes on to extend its healthy influence overevery other use of reason, for the first time inspiring thetrue philosophical spirit. But the service that criticism does
for theology also, by making it independent of the judgmentof dogmatic speculation and thus securing it completely
against the attacks of all dogmatic opponents, is certainly
not to be valued lightly. For ordinary metaphysics, although
it promised to give great help to theology, couldn’t keep thispromise; ·indeed it was worse than useless to theology·. By
calling speculative dogmatics to its assistance, ·which it
did under the influence of common metaphysics·, theology
succeeded only in arming enemies against itself. Mysticism,
which can prosper in a rationalistic age only when it hides
behind a system of school-metaphysics, under the protectionof which it may venture to rave rationally, so to speak, will be
driven by critical philosophy from this, its last hiding-place.
And concerning all of this nothing else can be as important
for a teacher of metaphysics as to be able to say—once andfor all, with universal agreement—that what he is expounding is at last science, and that it will bring real benefit to the
public.
84