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The first half of the Critique of Pure Reason argues that we can only obtain substantive knowledge of the world via sensibility and understanding. Very roughly, our capacities of sense experience and concept formation cooperate so that we can form empirical judgments. The next large section—the “Transcendental Dialectic”—demolishes reason’s pretensions to offer knowledge of a “transcendent” world, that is, a world beyond that revealed by the senses. “Dialectic,” says Kant, is “a logic of illusion” (A293): so in his vocabulary, a dialectical idea is empty or false.
However, the Critique of Pure Reason should not be read as a demolition of reason’s cognitive role. Kant certainly wants to delimit the bounds of reason, but this is not the same as arguing that it has no role in our knowledge. Three points are crucial: (§1.1) the relation of reason to empirical truth; (§1.2) reason’s role in scientific enquiry; and (§1.3) the positive gains that come from appreciating reason’s limits. In addition, sound philosophical reasoning requires that reason gain knowledge of itself—a task that the first Critique begins, but does not complete (§1.4).
The first thing to note is Kant’s bold claim that reason is the arbiter of truth in all judgments—empirical as well as metaphysical. Unfortunately, he barely develops this thought, and the issue has attracted surprisingly little attention in the literature. (But cf. Walker 1989: Ch. 4; Guyer and Walker 1990; Kant’s theory of judgment, §§1.3, 1.4). However, some basic points are clear from the text. We form judgments about the world around us all the time, without a second thought. We see a hand in front of us and judge it to exist; after a dream, we judge ourselves to have been dreaming and the dream’s contents to be illusory; we see the sun rise and assume that it orbits the earth. Kant devotes great philosophical efforts to show that all these judgments rely on categories, such as cause and effect, that must order our sensory impressions. A belief that conforms to these conditions meets the “formal” conditions of truth. However, unless we are fundamentally confused about something, all our beliefs meet these conditions.) So there is a further question: which of our beliefs are “materially” true, and which erroneous?
How does reason enter the matter? In the famous “Refutation of Idealism,” Kant says the following: “Whether this or that putative experience is not mere imagination [or dream or delusion, etc.] must be ascertained according to its particular determinations and through its coherence with the criteria of all actual experience” (B279). To see what Kant means, consider a simple example. Suppose that our dreamer believes she has won a lottery, but then starts to examine this belief. To decide its truth, she must ask how far it connects up with her other judgments, and those of other people. If it fails to connect up (she checks the winning numbers, say, and sees no match with her actual ticket), she must conclude that the belief was false. Otherwise, she would contradict a fundamental law of possible experience, that it be capable of being unified. As Kant summarizes his position: “ the law of reason to seek unity is necessary, since without it we would have no reason, and without that, no coherent use of the understanding, and, lacking that, no sufficient mark of empirical truth…” (A651/B679).
In sum, what separates material error from true cognition for Kant is that true cognitions must find a definite place within a single, unified experience of the world. Since reason is an important source of the unifying structure of experience, it proves essential as an arbiter of empirical truth.
The same principle of reasoned unity also applies to judgments that are not readily decided by everyday experience. Why are we sure that the sun does not orbit the earth, despite all appearances? To answer such questions, we need to consider reason’s role in scientific knowledge. Kant claims that reason is “the origin of certain concepts and principles” (A299/B355) independent from those of sensibility and understanding. Kant refers to these as “transcendental ideas” (A311/B368) or “ideas of [pure] reason” (A669/B697). And he now defines reason as a “faculty of principles” (A299/B356) or the “faculty of the unity of the rules of understanding under principles” (A303/B358). The problem is how to justify these concepts and principles. This problem is acute because Kant also argues that they often lead us into error and contradiction.
For Kant, the “constitutive” use of our faculties helps to constitute the objects of knowledge, by providing their form as objects of possible experience. Constitutive principles thereby have a strong objective standing—the paradigm case being the categories of the understanding. Regulative principles, by contrast, govern our theoretical activities but offer no (constitutive) guarantees about the objects under investigation. As Kant puts it, activities must have goals if they are not to degenerate into merely random groping (cf. Bvii, A834/B862). Science aims to discover the greatest possible completeness and systematicity (cf. Guyer 1989 & 2006, Abela 2006, Mudd 2017), subsuming objects and events under the most all-encompassing laws. When Kant speaks of the “unity of reason” in the first Critique, he means that reason gives “unity a priori through concepts to the understanding’s manifold cognitions” (A302/B359; cf. A665/B693, A680/B780).
As indicated, this unity must be a priori since it cannot be given through any set of experiences. Nor can we know in advance how far science will succeed, or that nature is wholly law-like. So the principle of seeking unity forms (what Kant calls) a “maxim” or regulative principle of reason (A666/B694; see Mudd 2016 for recent discussion of this principle and its practical nature). By contrast, the claim that such unity does exist would represent a “constitutive principle,” the sort of “cosmological” knowledge claim that we cannot justify.
Our judgment that the earth orbits the sun (and not vice versa) provides a simple illustration. The opposite claim seems more compelling to common sense, and consistency in observations is generally sufficient to confirm everyday knowledge. But scientific knowledge aspires to law-like completeness. It is not just that Galileo’s observations with the telescope suggest that everyday appearances are misleading. For Kant, more important is how reason unifies these observations through laws of gravity, momentum and so forth. By compassing the motion of all heavenly objects, and not only the movement of the sun relative to the earth, Newton provides the proper confirmation of Copernicus’ heliocentric hypothesis (Bxxii n; cf. §1.4 below). The extension and modification of Newton’s laws by general relativity gives one indication of the open-ended, “regulative” nature of this quest; so too does the as-yet unsatisfied ambition to integrate general relativity with quantum mechanics.
Kant’s account of science, and especially the role of “teleological” or purposive judgment, is further developed in the Critique of Judgment. See Guyer 1990, Freudiger 1996, and Nuzzo 2005, as well as Kant’s aesthetics and teleology, §3. On Kant’s account of science more generally, see Wartenberg 1992, Buchdahl 1992, Friedman 1992b & 2013, and Breitenbach [forthcoming]. On reason and science, see Neiman 1994: Ch. 2 and Grier 2001: Ch. 8. The entry on Kant’s philosophy of science considers Kant’s view of the natural sciences, especially physics.
The third point is the most well-known, and is considered in detail in the entry on Kant’s critique of metaphysics. Kant demolishes a series of supposed proofs of the existence of God (“The Ideal of Pure Reason”) and the soul (“The Paralogisms”). He also demonstrates that it is equally possible to prove some judgments about “world wholes” as it is to prove their opposites, such as the claims that space must be unbounded and that it must be bounded (“The Antinomies,” including the idea of an absolutely first cause: the problem of freedom as it is posed in the famous “Third Antinomy”). These sections have always been regarded as among the most convincing parts of the first Critique. Mendelssohn spoke for many of Kant’s contemporaries in calling him the “all destroyer,” for devastating reason’s pretenses to transcendent insight.
At the beginning of the Doctrine of Method (the last, least-read part of the first Critique) Kant alludes to the biblical story of Babel. God punished the attempt to build “a tower that would reach the heavens” (A707/B735) with a confusion of languages, leaving people unable to understand one another and unable to cooperate in such hubristic ventures. Again and again, reason returns to some very simple ideas with towering implications—the immortal soul, God, freedom; what is more, it cooks up more or less convincing proofs of these. Without the acid test of experience of a common world, people are bound to come up with conflicting versions of these ideas (unless, perhaps, they emptily ape one another’s words without real understanding). Then they will either talk past one another, or fall into conflict—or, one of Kant’s abiding fears, be forced to submit to an unreasoned authority. In metaphysics, Kant refers to “the ridiculous despotism of the schools” (Bxxxv). When we turn to the practical sphere, however, despotism is far from ridiculous: it is the last, brutal resort for securing coexistence among people who insist on conflicting doctrines. Thus Kant often alludes to Hobbes, on whose theory order is only possible if an unaccountable sovereign overawes all the members of society. Saner 1967, O’Neill 1989, and Neiman 1994 all offer interpretations that see Kantian reason as securing intersubjective order and overcoming threats of Babel-like hubris, conflict and despotism.
One of the most famous lines of the first Critique occurs in the second edition’s Preface, where Kant says, “I had to deny knowledge in order to make room for faith” (Bxxx). Knowledge of the world as a whole, or of entities that transcend this world (the immortal soul or God) is not humanly possible: it is not possible via experience, and reason has no power to supply knowledge in its place. However, as indicated in §1.2, Kant argues that science is entitled to rely on certain principles that regulate its project, without being known as objects. In the final section of the Critique, Kant argues that knowledge is not the only or even the primary end of reason: in its practical use, reason addresses our role within the world. So Kant describes his conception of philosophy as ‘cosmic’ (ein Weltbegriff, literally “world concept”) rather than ‘scholastic’ (A838/B866; cf. Ypi 2013 and Ferrarin 2015).
In line with this conception, Kant proposes three questions that answer “all the interest of my reason”: “What can I know?” “What must I do?” and “What may I hope?” (A805/B833). We have seen his answer to the first question: I can know this world as revealed through the senses, but I cannot know the total sum of all that exists, nor a world beyond this one (a supersensible world). Kant does not answer the second question until the Groundwork of the Metaphysics of Morals, four years later. (Arguably, he sees no need to answer the question in this form, since he is confident that people have long known what their duties consist in.) But the first Critique does include some observations on hope—that is, faith in God and a future world. We certainly fall into error if we think reason can know a world beyond the senses. Indeed, Kant insists that such knowledge would corrupt practical reasoning, by imposing an external incentive for moral action—fear of eternal punishment and hope of heavenly reward, what he will later call “heteronomy.” Nonetheless, human reason still has an inescapable interest in belief in God, immortality and freedom. Kant develops this claim more systematically in the second Critique, as discussed below (§2.3).
Kant’s idea that reason has “interests,” or even “needs,” may seem strange, and is discussed by Kleingeld 1998a and Ferrarin 2015: Ch. 1. For finite beings, reason is not transparently or infallibly given to consciousness (as some rationalist philosophers seemed to think), just as it cannot deliver transcendent truths. Thus reason “needs to present itself to itself in the process of gaining clarity about its own workings” (Kleingeld 1998a: 97)—above all, the principles that it must give to itself. As the next section discusses, this means that Kant views reason as essentially self-reflexive.
The first Critique argues that there has hitherto been no real progress in metaphysics. In the second edition Preface, Kant proudly proclaims that his book has finally put metaphysics on “the sure path of a science” (Bvii; cf. Axiii). What, then, is the relation of metaphysics—or philosophical reasoning more generally—to those areas of human enquiry that do seem to generate certainty (geometry and mathematics) and the expansion of knowledge (science in general)?
Kant had long insisted that mathematics could provide no model for philosophizing. “Mathematics gives the most resplendent example of pure reason happily expanding itself without assistance from experience” (A712/B740). But metaphysics cannot follow its course. This is not simply a rhetorical point, since many of Kant’s predecessors had tried to do exactly this—Spinoza’s Ethics is one example, Christian Wolff’s philosophy another (see Gava [forthcoming]). Kant’s basic argument against such efforts is that mathematicians are justified in constructing objects or axioms a priori, because they can work with pure intuitions (albeit very abstract ones: a line or the form of a triangle, say), rather than being restricted to analysis of concepts alone. (See the entry on Kant’s philosophy of mathematics.) This sort of procedure is not available to philosophers, who have no right to assume any a priori intuitions or axioms about metaphysical entities. Attempts that rely on such claims have only produced “so many houses of cards” (A727/B755).
But if mathematics does not provide a model for a genuinely scientific metaphysics, the relation between metaphysics and the empirical sciences is also unpromising. In the first place, Kant has argued that experience cannot reveal metaphysical entities. We could never know, for instance, that we are free: like everything else we can know, human conduct is in principle open to fully determinate causal explanation. Second, experience cannot generate the sort of necessity Kant associates with metaphysical conclusions. (This is a long-standing bone of contention between Humean and Kantian accounts of knowledge—for instance, as regards causation. See the entry on Kant and Hume on causality.) That is, our investigation of the world, no matter how systematic or scientific, only reveals contingent facts: it cannot show that such-and-such must be the case. To hold that scientific laws have the quality of necessity—so that they really are laws, and not mere generalizations or rules of thumb—is a metaphysical rather than an empirical claim.
In other words, reason, as “[self-]appointed judge,” does not sit by and merely observe whatever comes along. It actively proposes principled accounts of the phenomenon it investigates—that is, law-like hypotheses. Then it devises experiments to confirm or disprove these.
As a characterization of philosophical reasoning, this prompts Kant to optimism, but it may puzzle his readers. Kant is optimistic, because what philosophy has to investigate is not the infinite scope of the empirical world, but rather “what reason brings forth entirely out of itself… as soon as [its] common principle has been discovered” (Axx). (One application of this idea is found in the Transcendental Dialectic of the first Critique, where Kant insists that there are only three transcendental ideas—the thinking subject, the world as a whole, and a being of all beings—so that it is possible to catalogue exhaustively the illusions to which reason is subject.) But there is also much room for puzzlement. Kant is suggesting that reason conduct an experiment upon itself—an idea that comes close to paradox. He also suggests that reason has a “common principle”—but nowhere in the first Critique does he explain what this consists in.
How Kant’s “experiment” functions with regard to our everyday knowledge is well-known. His Copernican hypothesis (Bxvi f) is that experience is relative to the standpoint and capacities of the observer. Only on this basis, Kant contends, can we find an explanation for the a priori structure of that experience (for example, its temporality or causal connectedness). The alternative, that we take a “single standpoint” and do not distinguish between objects of experience and those that are “merely thought… beyond the bounds of experience” (i.e., things-in-themselves), fails because it results in “an unavoidable conflict of reason with itself” (Bxviii n)—for example, in the Antimonies mentioned above (§1.3), Kant argues that the failure to separate appearances (everyday items of experience) from things-in-themselves (metaphysical entities that lie beyond experience) leads us to dramatically contradictory conclusions.
However, this still leaves awkward questions about philosophical knowledge, and reasoning more generally. Kant’s philosophical task is not just a matter of “compelling” sensibility and understanding to act as “witnesses”: reason stands before its own tribunal, too, and must give account of itself. (This metaphor is investigated by Stoddard 1988; Kant’s juridical and political metaphors are given a central philosophical role by Saner 1967 and O’Neill 1989.) When reason decides to act as judge and jury in its own case, how can we expect the results to stand up to scrutiny?
Section 3 discusses the most thorough reply to this question in the literature, that offered by Onora O’Neill. To anticipate briefly: The general problem hinted at by Kant’s metaphors—of reason’s experiment upon itself, or compelling itself to give testimony—is that of reason’s self-knowledge (cf. Axi f). Kant assumes that we have a capacity of reason; but “reason grants [respect] only to that which has been able to withstand its free and public examination” (Axi n). We cannot, therefore, dogmatically assert the authority of this capacity: “reason… has no dictatorial authority” (A738/B766). This point is especially compelling given how fallible reason has proven in metaphysics: “how little cause have we to place trust in our reason if in one of the most important parts of our desire for knowledge it does not merely forsake us but even entices us with delusions and in the end betrays us!” (Bxv). Kant’s question, then, is how we might defend reason from various doubts and worries and how we might discipline it without begging questions— for instance, by invoking claims or premises that themselves are open to doubt (cf. O’Neill 1989: Ch. 1, 1992, 2004 & 2015). This is then the central task of critique (cf. Bxxxv): reason’s self-reflexive examination of itself, which establishes its limits and its “common principle,” and vindicates its authority.
In the first Critique there are only hints as to the form Kant’s moral theory would take. The account of practical reason in the Groundwork of the Metaphysics of Morals (1785) and Critique of Practical Reason (1788) is radically new. Kant now claims to have discovered the supreme principle of practical reason, which he calls the Categorical Imperative. (More precisely, this principle is an imperative for finite beings like us, who have needs and inclinations and are not perfectly rational.) Notoriously, Kant offers several different formulations of this principle, the first of which runs as follows: “act only in accordance with that maxim through which you can at the same time will that it become a universal law” (4:421). (On the different versions of the Imperative, which Kant claims are equivalent, see Kant’s moral philosophy, §§5-9.) Kant holds this principle to be implicit in common human reason: when we make moral judgments, we rely on this criterion, although invariably we do not articulate it as such.
The Categorical Imperative is not the only principle of practical reason that Kant endorses. At nearly the same level of generality is the principle underlying all “hypothetical imperatives.” (See Kant’s moral philosophy, §4.) Imperatives of skill and prudence rely on the principle: “Whoever wills the end also wills (insofar as reason has decisive influence on his actions) the indispensable necessary means to it that are within his power” (4:417f; cf. 5:19f). Following Hume, many philosophers hold that practical reasoning is essentially instrumental. They therefore see all practical demands as ultimately hypothetical, that is, conditional upon our having particular ends or inclinations (cf. Kant and Hume on morality, §6). Kant, however, sees the principle of hypothetical imperatives as subordinate to the Categorical Imperative (cf. Korsgaard 1997). Reason can also be the source of unconditional demands, that is, demands that do not presuppose any particular ends or inclinations. Kant’s claim can be put even more strongly: reason is the only source of unconditional demands that human beings can ever have access to.
Alongside the derivation of his supreme moral principle, the most difficult questions about Kant’s view of practical reason center on its relation to freedom. Although the broad outlines are consistent, Kant’s views on this topic seem to shift more than usual across his critical writings. (See Kant’s moral philosophy, §10, for a brief sketch, and Allison 1990 for a masterful, though not uncontroversial, account.) This and the following sub-section focus on Kant’s central, radical claim that “freedom and unconditional practical law [that is, for any finite being, the Categorical Imperative] reciprocally imply each other” (5:29f). On the one hand, freedom implies that practical reason can be pure (non-instrumental, unconditional), and hence that we are subject to the demands of the Categorical Imperative. On the other, our subjection to morality implies that we must be free.
Although Kant sometimes writes as if it were difficult to see what practical reason requires (for instance, in his comments about practical wisdom: §3.2 below), he usually assumes that everyone readily grasps the fundamental principles which all can follow. That is, he is remarkably optimistic about people’s capacity for independent moral insight. In the recent literature there is some consensus that Kant failed to recognize the complexity and difficulty of moral reasoning (cf. Herman 1993: Ch. 4 & 2007; O’Neill 1996). But judging what the Categorical Imperative requires only poses serious difficulties if Kant has adequately justified it. In particular, his equation of mere law-likeness with principles that all can follow may seem much too quick. To see what Kant means, it helps to consider some other principles that may sound stable or law-like but go beyond the “mere form of law”—and thereby fail to be justifiable.
There is a common difficulty underlying all the untenable alternatives Kant considers. They look for substantive guidance from outside of reason itself—just as hypothetical imperatives only guide action if some end is taken for granted. Kant calls this heteronomy—that is, reasoning directed from the outside, by an authority that is merely assumed or imposed. The problem is to find ways of acting and thinking that are authoritative—that is, are entitled to guide everyone’s acting and thinking. To gain this entitlement, they must be autonomous—that is, not dependent on an authority that itself refuses justification.
Kant’s injunction to look to the mere form of law at first appears to provide no guidance at all, and has often been reproached on this basis. Defenders of Kant’s ethics argue that it represents a substantial constraint: to avoid all those ways of thinking and acting that cannot be followed by all. (For discussion see inter alia O’Neill 1989: Ch. 5; Herman 1993; Allison 1990: Ch. 10 §II.) If so, the autonomy of reason can point to the positive sense of freedom at the heart of Kant’s practical philosophy (cf. Brandom 1979). This is the possibility of acting in ways that do not rely on “contingent, subjective conditions that distinguish one rational being from another” (5:21), and hence do not fall foul of others’ demands for justification.
As Kant also says, “the moral law, and with it practical reason, [have] come in and forced this concept [freedom] upon us” (5:30). In the next section, Kant introduces this idea in notorious terms, as a “fact of reason”: “Consciousness of this fundamental law may be called a fact of reason because one cannot reason it out from antecedent data of reason” (5:31; cf. 5: 6, 42f, 47, 55, 91, 104).
This “fact” has caused considerable controversy among commentators. This is partly because Kant is not altogether clear about what he takes this fact to demonstrate. It is also because he has repeatedly argued that morality cannot be based on facts about human beings, and must be revealed a priori, independently of experience. (In this regard it is significant that Kant also uses the Latin word factum, meaning deed. In other words, we are dealing with an act of reason and its result, rather than a merely given fact. See Kleingeld 2010.) Moreover, Kant speaks of “cognizing the moral law,” when he is well aware that no author before him has formulated this law as he has. A final source of difficulty is that this “fact, as it were” does not feature in his earlier treatise, the Groundwork, and does not appear again.
There are serious difficulties at issue in this scholarly dispute. But Kant’s line of thought in the long passage just quoted is relatively clear: We all (most of us) recognize that there are situations where we ought to do something, even though it will cost us something that is very dear to us (i.e., we feel ourselves subject to an unconditional moral imperative). So far as we really acknowledge this “ought,” we commit ourselves to believing that it will be possible for us to do this (i.e., that we are free). This reveals something that we could hardly be certain of except on the basis of this encounter with our own activity of moral reasoning (cf. Kleingeld 2010).
Clearly, this line of thought is not immune to criticism. Our feeling of moral constraint might be explained in terms of a Freudian super-ego, for instance. But it enables us to see why Kant thought that moral awareness—unlike any other sort of experience—gives us a practical certainty of our freedom, being “a fact in which pure reason in us proves itself actually practical” (5:42). (“Practical certainty” because it is not knowledge of the same sort as empirical and scientific knowledge.) At the same time, if Kant is right that only the Categorical Imperative reveals those ways of acting that we can justify to others, then we can see why he claims, “freedom and unconditional practical law reciprocally imply each other” (5:29f).
Kant does not give a complete account of the relation of practical reason to theoretical reason in the Groundwork or any later works. However, the second Critique does include an important section that bears on this question: “On the primacy of pure practical reason in its connection with speculative reason” (5:119–121). (See Gardner 2006 and Willaschek 2010.) At the most general level, Kant’s notion of autonomy already implied some sort of primacy for pure practical reason. Against various stripes of rationalism, Kant denies that theoretical reason can have any insight into the supersensible. So reason has no possible access to a transcendent authority that could issue commands for thought or action. Against Hume, Kant denies normative authority to the inclinations. These points rule out the only ways that theoretical or instrumental reasoning could supply authoritative reasons to act. If there are such reasons—as the “fact of reason” pronounces —then only pure practical reason is left to supply them. Now, however, Kant argues that pure practical reason has “primacy” even on the home turf of theoretical reason. That is, pure practical reason should guide some of our beliefs, as well as our actions.
In the original Preface to the first Critique, Kant had raised the idea of reason’s “common principle”: “Nothing here can escape us, because what reason brings forth entirely out of itself cannot be hidden, but is brought to light by reason itself as soon as reason’s common principle (gemeinschaftliches Prinzip) has been discovered” (Axx). Unfortunately, neither edition of the Critique considers what this principle might be.
The parallel with the first formulation of the Categorical Imperative—“act only in accordance with that maxim through which you can at the same time will that it become a universal law” (4:421)—hardly needs spelling out. Kant now says: think only in accordance with that maxim that could be a universal law. Differently put: thinking is an activity, and if the Categorical Imperative is indeed “categorical” then it applies to all our activities.
Kant sets out three “maxims of common human understanding” [= reason] which are closely related to the Categorical Imperative. They appear twice in his published writings, in relation to both acting and thinking. The maxims are discussed by O’Neill 1989: Ch. 2 & 1992, and by Neiman 1994: Ch. 5.
To think for oneself Kant describes as the maxim of unprejudiced thought; its opposite is passivity or heteronomy in thought, leading to prejudice and superstition. To think in the place of everyone else is the maxim of enlarged or broad-minded thought. And always to think in accord with oneself is the maxim of consistent thought (5:294). Although the last maxim sounds more straightforward, Kant is careful to emphasize its difficulty: it “can only be achieved through the combination of the first two and after frequent observance of them has made them automatic” (5:295). Consistency does not just involve getting rid of obvious contradictions in our explicit beliefs. It also requires consistency with regard to all the implications of our beliefs—and these are often not apparent to us. To achieve this sort of law-likeness in thought depends both on the genuine attempt to judge for oneself and the determination to expose one’s judgments to the scrutiny of others. In other words, it involves regarding oneself, first, as the genuine author of one’s judgments, and second, as accountable to others. As we might also say, it represents a determination to take responsibility for one’s judgments.
The maxims support the thesis that theoretical and practical reasoning have a unified structure, and flesh out the implications of the Categorical Imperative. As a matter of thought, to reason is to discipline one’s judgments so that others can follow them. As a matter of practical wisdom, it is to discipline one’s actions so that others can adopt the same principles.
Kant’s famous essay, “What is Enlightenment?” (1784), has been of particular importance to commentators concerned with Kantian reason and politics. (See O’Neill 1989: Ch. 2, 1990 & 2015: Ch. 3; Velkley 1989; Deligiorgi 2005; Patrone 2008.) Kant’s second maxim, “to think into the place of others,” shows that he regards communication as essential to making valid judgments and to acting wisely and well. Thus Kant writes: “…how much and how correctly would we think if we did not communicate with others to whom we communicate our thoughts, and who communicate theirs with us!” Kant also describes the first maxim as the way to achieve “liberation from superstition,” which he equates with “enlightenment” (5:294).
Here, Kant is not primarily concerned with enlightenment as the activity or condition of an individual—rather, as something that human beings must work towards together. For this, he says, “nothing is required but… the least harmful… freedom: namely, freedom to make public use of one’s reason in all matters” (8:36). This is not the freedom to act politically. Instead, it is what we now call freedom of the pen—in Kant’s words, the use of reason “as a scholar before the entire public of the world of readers” (8:37).
Kant’s contrast is with the reasoning someone undertakes as an employee: as a civil servant or military officer or cleric of an established church. In each case, the employee is bound to fulfill the dictates of a given leader or organization, at least so long as he can “in conscience hold his office” (8:38). In doing so, he uses his reason to decide the best way of achieving ends that have been laid down by others. (There is a loose parallel with instrumental reasoning, which decides the best means to achieve ends laid down by inclination. In Kant’s terms, both are heteronomous—“directed by another.”) Although this sort of reasoning is often undertaken in what we now call “public offices”—as a state employee, for instance—Kant describes such uses of reason as private—that is, deprived of freedom and accountable only to a particular authority. By contrast, the public use of reason is not bound to any given ends and is accountable to all: a person speaks as a member of “the society of citizens of the world” (8:37). Outside of his post, in a capacity he shares with all other human beings, the civil servant or cleric may reason freely, offering critical scrutiny of government policies or religious teachings. Nonetheless, he must continue to enact these in his employment, as a “passive member” (8:37) of the commonwealth.
In the term used by several contemporary Kantians (Herman 2007: Ch. 10, Korsgaard 2008, Reath 2013), this procedure constitutes reason. It makes reason the only unconditional (that is, non-heteronomous) form of authority for our thinking and acting.
Kant’s discussions of theoretical reason are not obviously connected to his account of practical reason. His accounts of truth, scientific method and the limited insights of theoretical reason are complex, as is his view of practical reason and morality. No one doubts that knowledge and scientific enquiry, no less than action, are subject to demands of rationality. However, if Kant’s account of reason is based—as O’Neill above all has argued—in avoiding principles of enquiry and of action that others cannot also adopt, it would be possible to see the underlying unity of these demands. We would understand, for example, why Kant so strenuously resists claims to transcendent insight. To give authority to such claims—those of revelation and religious authority, for example—would be irrational insofar as they rest on principles of belief that cannot be adopted by all.
Underlying the difficulty of synthesizing and interpreting Kant’s account of reason is, of course, the enormous question of what reason is. Many philosophers—both contemporary and historical figures—proceed as if this were already clear. However, once this question is raised—the question of reason’s self-knowledge, as Kant puts it—it is difficult to see grounds for such confidence. While the secondary literature discussing her proposal remains limited, O’Neill’s interpretation of Kant represents an ambitious and distinctive answer to this question.
O’Neill (2000) situates the Kantian account of reason against three alternatives, which she labels the instrumental, the communitarian, and the perfectionist. The first remains very widespread: with Hume, it regards instrumental reasoning as fundamental (cf. practical reason, §4; reasons for action: internal vs. external). The second sees reason as embedded within complex traditions: rationality is what a given tradition or community takes it to consist in (cf. MacIntyre 1988; communitarianism). A third option, akin to the forms of rationalism that Kant opposed, is to see reason as an individual capacity to discern or intuit normative truths (cf. moral non-naturalism, §3). Arguably, all three accounts fail in providing reasoned justification to some audiences. The instrumental reasoner is accountable to no-one—in fact, to nothing apart from whatever desires or ends he happens to have. Someone who takes her particular tradition to define what beliefs and practices count as reasonable can have little to say to those who stand outside it. And the person who believes he can intuit what is good or true will be mute—or worse—in the face of those with different intuitions.
On the interpretations advanced by Saner, O’Neill, Neiman and others, Kant was aware of all these options and rejected each. We saw above (§1.4) that Kant characterizes reason in terms of a self-reflexive procedure. Reason is autonomous and submits to no external authority; it gains authority from submitting itself to critique; and critique involves rejecting any mode of thinking or acting that cannot be adopted by all. In less abstract terms, the self-scrutiny of reason is scrutiny by all those who demand justification for any particular mode of thought or action. Such a view does not assume that we are necessarily bound to our interests and inclinations (as the instrumental account does). It does not ask us to rely on what others do accept (as the communitarian account does). It does not involve the fantasy that we already know or intuit what everyone should accept (as the perfectionist account does). It proposes, instead, a vision of human beings who are able to step back from their particular inclinations, habits and intuitions, and who are willing to use this ability to seek terms that all can accept—to construct an intersubjective order of co-existence, communication and cooperation on terms that all can accept.
Such an account depends on a particular interpretation of Kant’s texts, and is both ambitious and highly complex in its ramifications. Nonetheless, if successful, it captures two powerful attractions of Kant’s philosophizing: a universalism that transcends self-centeredness and community boundaries, and a modesty that respects the limits of human insight.
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For comments on this entry, my thanks to Graham Bird, Tatiana Patrone, Alison Stone, Lea Ypi, and the referees for this Encyclopedia, Paul Guyer and R. Lanier Anderson. Further thanks to the latter with regard to the 2013 revised version, and thanks to an anonymous referee with regard to the 2017 revisions. For additional assistance my thanks to Alix Cohen, Sebastian Gardner, Onora O’Neill and Jens Timmermann. My grateful thanks, too, to Nick Bunnin, for organizing the Chinese philosophy summer school which gave me the opportunity to lecture on this topic.
and that "by degrees afterward, ideas come into their minds." Book I of the Essay is devoted to an attack on nativism or the doctrine of innate ideas. Locke allowed that some ideas are in the mind from an early age, but argued that such ideas are furnished by the senses starting in the womb: for instance, differences between colours or tastes. If we have a universal understanding of a concept like sweetness, it is not because this is an innate idea, but because we are all exposed to sweet tastes at an early age.
One of Locke's fundamental arguments against innate ideas is the very fact that there is no truth to which all people attest. He took the time to argue against a number of propositions that rationalists offer as universally accepted truth, for instance the principle of identity, pointing out that at the very least children and idiots are often unaware of these propositions.
Thus there is a distinction between what an individual might claim to "know", as part of a system of knowledge, and whether or not that claimed knowledge is actual. For example, Locke writes at the beginning of Chap. IV (Of the Reality of Knowledge): "I doubt not my Reader by this Time may be apt to think that I have been all this while only building a Castle in the Air; and be ready to say to me, To what purpose all of this stir? Knowledge, say you, is only the Perception of the Agreement or Disagreement of our own Ideas: but who knows what those Ideas may be? ... But of what use is all this fine Knowledge of Man's own Imaginations, to a Man that enquires after the reality of things? It matters now that Mens Fancies are, 'tis the Knowledge of Things that is only to be priz'd; 'tis this alone gives a Value to our Reasonings, and Preference to one Man's Knowledge over another's, that is of Things as they really are, and of Dreams and Fancies."

References: §1
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