# Principles of Philosophy

Principles of Philosophy (Latin: Principia philosophiae) is a book by René Descartes. It is basically a synthesis of the Discourse on the Method and Meditations on First Philosophy. The book sets forth the principles of nature—the Laws of Physics—as Descartes viewed them. It set forth the principle that in the absence of external forces, an object's motion will be uniform and in a straight line. Newton borrowed this principle from Descartes and included it in his own Philosophiæ Naturalis Principia Mathematica; it is still generally referred to as Newton's First Law of Motion. The book was primarily intended to replace the Aristotelian curriculum then used in French and British universities. The work provides a systematic statement of his metaphysics and natural philosophy, and represents the first truly comprehensive, mechanistic account of the universe.

## Quotes

    (1850) Translation by John Veitch unless otherwise indicated. A Source.

### Letter of the Author

    to the French Translator of the Principles of Philosophy, Serving for a Preface
* [T]he word philosophy signifies the study of wisdom... [i.e.,] not merely prudence in the management of affairs, but a perfect knowledge of all... man can know... for... conduct... health and... discovery of all... arts, and that knowledge to subserve these ends must... be deduced from first causes...[i.e.,] Principles.
* God is... the only being... absolutely wise... who possesses a perfect knowledge of all things; but... men are more or less wise as their knowledge of the most important truths is greater or less.
* [T]o live without philosophising is... the same as keeping the eyes closed...
* [M]en, of whom the chief part is the mind, ought to make the search after wisdom their principal care, for wisdom is the true nourishment... [M]any... would not fail in the search, if they would but hope for success... and knew the degree of their capabilities...
* There is no mind... which remains so firmly bound up in the objects of the senses, as not sometime... to turn... away from them in the aspiration after... higher good, although not knowing... wherein that good consists.
* The greatest favourites of fortune—those who have health, honours, and riches in abundance—are not more exempt from [these] aspirations... nay... [T]hese are the persons who sigh... most deeply after another good greater and more perfect... But the supreme good, considered by natural reason without the light of faith, is... knowledge of truth through its first causes... [i.e.,] the wisdom of which philosophy is the study. ...[A]ll that is required to gain assent to their truth is that they be well stated.
* [T]hey who make pretensions to philosophy are often less wise and reasonable than others who never applied themselves...
* The first degree contains only notions so clear... that they can be acquired without meditation; the second comprehends all that the experience of the senses dictates; the third, that which the conversation of other men teaches us; ...the fourth, the reading, ...especially of such ...written by persons capable of ...proper instruction ...[A]ll the wisdom we in ordinary possess is acquired... in these... ways; for I do not class divine revelation among them, because it does not conduct us by degrees, but elevates us at once to an infallible faith.
* There have been... in all ages great minds who endeavoured to find a fifth road... more sure and elevated... The path they essayed was the search of first causes and true principles, from which might be deduced the reasons of all that can be known... [T]o them the appellation of philosophers has been... accorded.
* I am not aware that... any one of them... has succeeded.... The first and chief... are Plato and Aristotle, between whom there was no difference, except that the former, following... his master, Socrates, ingenuously confessed that he had never yet been able to find anything certain... contented to write what seemed... probable, imagining... principles by which he endeavoured to account for the other things.
* Aristotle... characterised by less candour, although... twenty years the disciple of Plato, and with no principles beyond... his master, completely reversed his mode... and proposed as true and certain what... he... never esteemed as such. But these two... acquired much judgment and wisdom by the four preceding means... which raised their authority... high, so much... that those who succeeded them were willing... to acquiesce in their opinions, [rather] than to seek... for themselves. The chief question among their disciples... whether we ought... doubt... all things or hold some as certain,—a dispute which led... into extravagant errors; for a part of those... for doubt, extended it... to the neglect of... ordinary rules... [of] conduct; those... who maintained the doctrine of certainty, supposing that it must depend upon the senses, trusted entirely to them. To such an extent was this carried by Epicurus, that... he... affirm[ed], contrary to all the reasonings of the astronomers, that the sun is no larger than it appears.
* [W]hile we only possess... knowledge... acquired in the... four grades of wisdom, we ought not to doubt... things that appear... true in... the conduct of life, nor esteem them as so certain that we cannot change our opinions.., even though constrained by... reason.
* From ignorance of this truth, or... from neglect of it, the majority of those who... aspired to be philosophers, blindly followed Aristotle, so... frequently corrupted... his writings, and attributed... opinions... he would not recognise... and those who did not follow him, among whom are... many of the greatest minds, did yet not escape... his opinions in their youth... the staple... in the schools; and thus... they could not rise to the knowledge of true principles.
* [T]he philosophers... all laid down as a principle what they did not perfectly know. ...I know none of them who did not suppose that there was gravity in terrestrial bodies; but although experience shows us... [heavy] bodies... descend towards the centre of the earth, we do not... know the nature of gravity... [i.e.,] the cause or principle in virtue of which bodies descend, and we must derive our knowledge of it from some other source.
* The same may be said of a vacuum and atoms, of heat and cold, of dryness and humidity, and of salt, sulphur, and mercury, and the other things... some... adopted as... principles.
** See also: Aristotelian physics, Paracelsus
* [N]o conclusion deduced from a principle which is not clear can be evident, even although the deduction be formally valid; and hence... no reasonings based on such principles could lead... to the certain knowledge... nor... advance them... in the search after wisdom.
* And if they did discover any truth, this was due to one or other of the four means above...
* [J]ust as in travelling, when we turn our back upon the place to which we were going, we recede the farther from it in proportion as we proceed in the new direction for a greater length of time and with greater speed, so that, though we may be afterwards brought back to the right way, we cannot nevertheless arrive at the destined place as soon as if we had not moved backwards at all...
* [S]o in philosophy, when we make use of false principles, we depart the farther from the knowledge of truth and wisdom... in proportion to the care with which we cultivate them, and apply ourselves to the deduction of diverse consequences from them, thinking that we are philosophising well, while we are only departing the farther from the truth; from which it must be inferred that they who have learned the least of all that has been hitherto distinguished by the name of philosophy are the most fitted for the apprehension of truth.
* [T]he grounds for holding that the true principles by which we may reach that highest... wisdom wherein consists the sovereign good of human life, are those I have proposed in this work...
* [T]wo considerations... are sufficient... first... that these principles are... clear, and... second, that we can deduce all other truths from them; for it is only these two conditions that are required in true principles.
* But I easily prove that they are very clear; firstly, by a reference to the manner in which I found them, namely, by rejecting all propositions that were... doubtful, for it is certain that such as could not be rejected... are the most evident and clear which the human mind can know. Thus by considering that he who strives to doubt of all is unable nevertheless to doubt that he is while he doubts, and that what reasons thus, in not being able to doubt of itself and doubting nevertheless of everything else, is not that which we call our body, but... our mind or thought.
* I have taken... for the first principle, from which I... deduced the following truths... that there is a God who is the author of all... and... being the source of all truth, cannot have created our understanding... as to be deceived in... judgments... of the things of which it possesses... clear and distinct perception.
* Those are all the principles of which I avail myself touching immaterial or metaphysical objects, from which I most clearly deduce these other principles of physical or corporeal things, namely, that there are bodies extended in length, breadth, and depth, which are of diverse figures and are moved in a variety of ways.
* Such are in sum the principles from which I deduce all other truths.
* The second circumstance that proves the clearness of these principles is, that they have been known in all ages, and even received as true and indubitable by all men, with the exception only of the existence of God, which has been doubted by some, because they attributed too much to the perceptions of the senses, and God can neither be seen nor touched.
* But... there has been no one up to the present, who... has adopted them as principles of philosophy... [i.e.,] as such that we can deduce from them the knowledge of whatever else is in the world.
* [T]hough I have not treated in it of all matters—that being impossible—I think I have so explained... that they who read... attentively will have ground... that it is unnecessary to seek for any other principles... to arrive at the most exalted knowledge of which the mind of man is capable...
* [T]hose imbued with my doctrines have much less difficulty in comprehending the writings of others, and estimating their true value... and this is... the opposite of... such as commenced with the ancient philosophy, namely, that the more they have studied it the less fit are they for rightly apprehending the truth.
* [T]here are hardly any so dull or slow of understanding as to be incapable of apprehending good opinions, or even of acquiring all the highest sciences, if... conducted along the right road.
* [T]he principles are clear, and... no one is so devoid of intelligence as to be unable to comprehend the conclusions that flow from them.
* [B]esides the entanglement of prejudices, from which no one is entirely exempt... the most ardent students of the false sciences... receive the greatest detriment... [G]enerally... people of ordinary capacity neglect... study from a conviction that they want ability, and... others... more ardent, press on too rapidly: whence it comes to pass that they frequently admit principles far from evident, and draw doubtful inferences...
* [T]here is nothing in my writings which they may not entirely understand, if they only take the trouble to examine them...
* I... warn those of an opposite tendency that even the most superior minds will have need of much time and attention to remark all I designed to embrace therein.

### Part I

    On the Principles of Human Knowledge
* I. [I]n order to seek truth, it is necessary... to doubt, as far as possible... all things.
* II. [C]onsider as false all that is doubtful.
* III. [W]e ought not... use... doubt in the conduct of life.
* IV. [W]e may doubt of sensible things. ...[W]e will doubt... whether of all the things... under our senses, or... imagined... really exist...
* V. [W]e may...doubt... mathematical demonstrations... and of their principles... hitherto deemed self-evident... because... sometimes... men fall into error... and admit as absolutely certain and self-evident what to us appeared false...
* VI. [W]e possess a free will, by which we can withhold... assent from what is doubtful, and... avoid error.
* VII. [W]e cannot doubt of our existence while we doubt... [T]his is the first knowledge we acquire when we philosophize...
* Cogito, ergo sum.
** I think, therefore I am.
** Variant: I think therefore I exist.
* VIII. [W]e... discover the distinction between the mind and the body, or between a thinking and corporeal thing.
* IX. What thought (cogitatio) is. ... [A]ll... we... are... conscious of... to understand (intelligere , entendre), to will (velle), to imagine (imaginari)... to perceive (sentire , sentir), are here the same as to think (cogitare, penser).
* X. [N]otions [but as born with us] which are simplest and self-evident., are obscured by logical definitions; and... are not to be reckoned among the cognitions acquired by study... [P]hilosophers erred in attempting to explain, by logical definitions, such truths as are most simple and self-evident; for they... render them more obscure. ...[W]hen I said... the proposition, I think, therefore I am, is... the first and most certain... I did not... deny that it was necessary to know what thought, existence, and certitude are, and... that, in order to think it is necessary to be... [etc.,] but, because these are the most simple notions, and... of themselves afford the knowledge of nothing existing, I did not judge it proper there to enumerate them.
* XI. [W]e can know our mind more clearly than our body.
* XII. [E]veryone does not come equally to know this.
* XIII. [K]nowledge of other things depends upon the knowledge of God.
* XIV. [W]e may validly infer the existence of God from necessary existence being comprised in the concept we have of him.
* XV. [N]ecessary existence is not in the same way comprised in the notions which we have of other things, but merely contingent existence.
* XVI. [P]rejudices hinder many from clearly knowing the necessity of the existence of God.
* XVII. [T]he greater objective (representative) perfection there is in our idea of a thing, the greater also must be the perfection of its cause.
* XVIII. [E]xistence of God may be again inferred from the above.
* XIX. [A]lthough we may not comprehend the nature of God, there is... nothing... we know so clearly as his perfections.
* XX. [W]e are not the cause of ourselves, but that... is God, and consequently... there is a God.
* XXI. [D]uration... of our life is sufficient to demonstrate the existence of God.
* XXII. [I]n knowing the existence of God... we... know all his attributes, as far as they can be known by the natural light alone.
* XXIII. God is not corporeal, and does not perceive by... senses as we do, or will the evil of sin.
* XXIV. [I]n passing from the knowledge of God to the knowledge of the creatures, it is necessary to remember that our understanding is finite, and the power of God infinite.
* XXV. [W]e must believe all that God has revealed, although it may surpass... our faculties.
* XXVI. [I]t is not needful to enter into disputes regarding the infinite, but merely to hold all that in which we can find no limits as indefinite, such as the extension of the world, the divisibility of the parts of matter, the number of the stars, etc. ...We will ...never embarrass ourselves by disputes about the infinite ...[I]t would be absurd for us who are finite to undertake to determine anything regarding it, and thus ...to limit it by endeavoring to comprehend it.
* XXVII. [D]ifference... between the indefinite and... infinite. ...[W]e will call ...things indefinite ...reserving to God ...the appellation of infinite...
* XXVIII. [W]e must examine, not the final, but the efficent, causes of created things. ...[W]e must only confide in this natural light so long as nothing contrary to its dictates is revealed by God...
* XXIX. God is not the cause of our errors. ...[T]o deceive seems ...some mark of subtlety of mind among men, yet ...the will to deceive only proceeds from malice or ...fear and weakness, and consequently cannot be attributed to God.
* XXX. [C]onsequently all which we clearly perceive is true, and... we are... delivered from the doubts above proposed.
* XXXI. [O]ur errors are, in respect of God, merely negations, but, in respect of ourselves, privations. ...[W]e frequently fall into error, although God is no deceiver ...[T]o inquire into the origin and cause of our errors, with a view to guard against them ...observe that they depend less on our understanding than on our will, and ...they have no need of the ...concourse of God, in ...their production ...
* XXXII. [T]here are only two modes of thinking in us, viz... perception of the understanding and... action of the will.
* XXXIII. [W]e never err unless when we judge of something which we do not sufficiently apprehend.
* XXXIV. [T]he will as well as the understanding is required for judging.
* XXXV. [T]he will is of greater extension than the understanding, and is thus the source of our errors.
* XXXVI. [O]ur errors cannot be imputed to God. ...[A]lthough God has not given us... omniscient understanding, he is not on this account... the author of our errors, for it is of the nature of created intellect to be finite, and of finite intellect not to embrace all things.
* XXXVII. [T]he chief perfection of man is... being able to act freely or by will, and that... this which renders him worthy of praise or blame.
* XXVIII. [E]rror is a defect in our mode of acting, not in our nature... [T]he faults of their subjects may be frequently attributed to other masters, but never to God.
* XXXIX. [T]he liberty of our will is self-evident.
* XL. God has foreordained all things. ...[W]e should soon be embarrassed in ...difficulties if we undertook to harmonize the pre-ordination of God with ...freedom of our will, and endeavored to comprehend both truths at once.
* XLI. [T]he freedom of our will may be reconciled with the Divine pre-ordination. ...[W]e possess sufficient intelligence to know ...this power is in God, but not enough to comprehend how he leaves the free actions of men indeterminate ...
* XLII. [A]lthough we never will to err, it is... by our will that we... err.
* XLIII. [W]e shall never err if we give our assent only to what we clearly and distinctly perceive.
* XLIV. [W]e uniformly judge improperly when we assent to what we do not clearly perceive, although our judgment may chance to be true; and... frequently our memory ...deceives us by leading us to believe ...certain things were ...sufficiently understood...
* XLV. What constitutes clear and distinct perception. ...I call that clear which is present and manifest to the mind giving attention ...just as we are said clearly to see objects ...but the distinct is that which is so precise and different from all other objects as to comprehend in itself only what is clear.
* XLVI. [F]rom the example of pain... a perception may be clear without being distinct, but... it cannot be distinct unless it is clear.
* XLVII. [T]o correct the prejudices of our early years, we must consider what is clear in each of our simple notions. ...[I]n our early years, the mind was so immersed in the body, that, although it perceived many things with... clearness, it... knew nothing distinctly; and since... at that time we exercised our judgment... numerous prejudices were thus contracted... the majority... never... laid aside. ...[T]o get rid of these, I will ...enumerate ...the simple notions of which our thoughts are composed, and distinguish ...[the] clear from what is obscure, or fitted to lead into error.
* XLVIII. [A]ll the objects of our knowledge are... either (1) ...things or the affections of things; or (2) ...eternal truths; with the enumeration of things.
* XLIX. [T]he eternal truths cannot be ...enumerated, but ...this is not necessary. ...When we apprehend that it is impossible a thing can arise from nothing, this proposition ex nihilo nihil fit, is not considered as something existing, or as the mode of a thing, but as an eternal truth having its seat in our mind, and is called a common notion or axiom. Of this class are the following: It is impossible the same thing can at once be and not be; what is done cannot be undone; he who thinks must exist while he thinks; and innumerable others, the whole of which it is... difficult to enumerate, but... if blinded by no prejudices, we cannot fail to know them when the occasion of thinking them occurs.
* Ex nihilo nihil fit.
** Nothing comes out of nothing.
* L. [T]hese truths are clearly perceived, but not equally by all men, on account of prejudices.
* LI. What substance is, and that the term is not applicable to God and the creatures in the same sense. ...By substance we can conceive nothing else than a thing which exists... such... as to stand in need of nothing beyond itself... [for] its existence. ...[T]here can be conceived but one substance ...absolutely independent, and that is God. ...[A]ll other things can exist only by help of the concourse of God. ...[A]ccordingly, the term substance does not apply to God and the creatures univocally ...[i.e.,] no signification of this word can be distinctly understood ...common to God and them.
* LII. That the term is applicable univocally to the mind and the body, and how substance itself is known. Created substances, however, whether corporeal or thinking, may be conceived under this common concept... things which, in order to their existence, stand in need of nothing but the concourse of God. ...[O]f nothing there are no attributes, properties, or qualities; for, from perceiving ...some attribute ...we infer ...some existing thing or substance to which it may be attributed is ...present.
* LIII. [O]f every substance there is one principal attribute, as thinking of the mind, extension of the body. ...[E]xtension in length, breadth, and depth, constitutes the nature of corporeal substance; and thought the nature of thinking substance. ...[E]very other thing that can be attributed to body, presupposes extension, and is ...some mode of an extended thing; as all the properties ...in the mind are only diverse modes of thinking.
* LIV. [W]e may have clear and distinct notions of the substance which thinks, of that which is corporeal, and of God.
* LV. [D]uration, order, and number may be... distinctly conceived.
* LVI. What are modes, qualities, attributes. ... [S]ince God must be conceived as superior to change, it is not proper to say that there are modes or qualities in him, but simply attributes... in created things that which is found in them always in the same mode, as existence and duration... ought to be called attribute, and not mode or quality.
* LVII. [S]ome attributes exist in the things to which they are attributed, and others only in our thought... [W]hat duration and time are. Of these attributes or modes... some which exist in the things... and others... have only an existence in our thought; thus... time, which we distinguish from duration... and call the measure of motion, is only a certain mode under which we think duration itself, for we do not... conceive... duration of things... moved... different from ... duration of... [those] not moved...
* [I]f two bodies are in motion for an hour, the one moving quickly and the other slowly, we do not reckon more time in... one... although there may be much more motion in... one... But... we may comprehend... duration of all things under a common measure, we compare their duration with that of the greatest and most regular motions that give rise to years and days, and which we call time; hence what is so designated is nothing superadded to duration, taken in its generality, but a mode of thinking.
* LVIII. [N]umber and all universals are only modes of thought. ...[N]umber, when ...not considered as in created things, but merely in the abstract or ...general, is only a mode of thinking ...[T]he same is true of all ...general ideas we call universals.
* LIX. How universals are formed; and ...the five common, viz., genus, species, difference, property, and accident. Universals arise... from our... use of... the same idea in thinking of all individual objects... [of] a certain likeness; and when we comprehend all the objects represented by this idea under one name, this term likewise becomes universal.
* LX. Of distinctions; and first of the real. ...[A]lthough we suppose ...God united a body to a soul so closely that it was impossible to form a more intimate union ...thus made a composite whole, the two substances ...remain ...distinct, notwithstanding this union: for with whatever tie God connected them, he was not able to rid himself of the power ...separating them, or of conserving the one apart from the other, and the things which God can separate or conserve separately are ...distinct.
* LXI. [M]odal distinction.
* LXII. [D]istinction of reason (logical distinction).
* LXIII. [T]hought and extension may be distinctly known, as constituting, the one the nature of mind, the other that of body.
* LXIV. [T]hese may... be distinctly conceived as modes of substance.
* LXV. [W]e may... know their modes.
* LXVI. [O]ur sensations, affections, and appetites may be clearly known, although... frequently wrong... regarding them.
* LXVII. [W]e are frequently deceived... regarding pain...
* LXVIII. [W]hat we clearly conceive is... distinguished from that in which we may be deceived.
* LXIX. [M]agnitude, figure, etc., are known far differently from color, pain, etc.
* LXX. [W]e may judge of sensible things in two ways, by the one... we avoid error, by the other fall into it.
* LXXI. [T]he chief cause of our errors is... in... prejudices of... childhood. ...In early life the mind was so closely bound to the body that it attended to nothing beyond... objects that made impression on the body; nor... refer these thoughts to anything... beyond itself, but simply felt pain... or pleasure... or... experienced... tastes, smells, sounds, heat, cold, light, colors... [etc.,] which... are representative of nothing existing out of our mind, and... vary according to... parts and modes... [of] the body... affected. The mind... also perceived magnitudes, figures, motions... [etc.,] which were not presented... as sensations but as... modes of things existing, or... capable of existing out of thought, although it did not... observe this difference between these two kinds of perceptions... [I]t judged... greater or less reality... according as the impressions it caused on the body...
* Hence arose the belief that there was more substance or body in rocks and metals than in air or water... Moreover, the air was thought to be merely nothing so long as we experienced no agitation... by... wind, or... hot or cold. And because the stars gave hardly more light than... candles, we supposed... each... but of this size. ...[S]ince the mind did not observe that the earth moved... or... was curved like... a globe, it was... ready to judge the earth immovable and its surface flat.
* [O]ur mind has been imbued from...r infancy with a thousand other prejudices... which afterward... we forgot we had accepted without sufficient examination, and admitted as possessed of the highest truth and clearness, as if... known by... our senses, or implanted... by nature.
* LXXII. [T]he second cause of our errors... we cannot forget these prejudices.
* LXXIII. The third cause... we become fatigued by attending to those objects which are not present to the senses; and... accustomed to judge... not from present perception but from preconceived opinion.
* LXXIV. The fourth source of... errors... we attach our thoughts to words which do not express them with accuracy.
* LXXV. Summary of what must be observed... to philosophize correctly. ...[L]ay aside our prejudices... [i.e.,] scrupulously... withhold our assent from the opinions we... formerly admitted, until upon new examination we discover... they are true. ...[R]eview... notions... and hold as true... only those... we... clearly and distinctly apprehend. In this way we will observe... that we exist in so far as it is our nature to think, and... that there is a God upon whom we depend; and after considering his attributes we will be able to investigate the truth of all other things, since God is the cause of them.
* [W]e will likewise find that we possess the knowledge of many propositions... eternally true... [e.g.,] that nothing cannot be the cause of anything, etc.
* We will... discover in our minds... knowledge of a corporeal or extended nature that may be moved, divided, etc., and also of... sensations... as... pain, colors, tastes, etc., although we do not yet know the cause of our being... affected...
* [W]e will acquire the habit of forming clear and distinct conceptions of... objects we are capable of knowing.
* In these few precepts... [are] the most general and important principles of human kowledge.
* LXXVI. [P]refer the Divine authority to our perception: but... apart from things revealed... assent to nothing... we do not clearly apprehend.
* [T]he infallible rule... what God has revealed is incomparably more certain... and... we ought to submit our belief to the Divine authority rather than to our own judgment, even although... the light of reason should, with the greatest clearness and evidence, appear to suggest... something contrary...
* But in things regarding which there is no revelation, it is by no means consistent with the character of a philosopher to accept as true what he has not ascertained to be such, and to trust more to the senses... [i.e.,] to the inconsiderate judgments of childhood than to the dictates of mature reason.

### Part II

    Of the Principles of Material Things
* I. [G]rounds on which the existence of material things may be known with certainty. ...[E]very perception we have comes... from some object different from our mind.
* II. How we... know that the human body is closely connected with the mind.
* III. [P]erceptions of the senses do not teach us what is is in reality in things, but what is beneficial or hurtful to the composite whole of mind and body.
* IV. [T]he nature of a body does not consist in weight, hardness, color, or other similar properties; but in extension alone. ...[W]e ...perceive that the nature of matter... does not consist in... affects of the senses... but only in the fact that it is a thing possessing extension in length, breadth, and depth.
* V. That the truth regarding the nature of body is obscured by the opinions respecting rarefaction and a vacuum with which we are pre-occupied. ...[W]here we conceive only extension in length, breadth, and depth, we are not in the habit of saying that body is there, but only space and further void space, which the generality believe to be mere negation.
* VI. In what way rarefaction takes place. ...[W]e ought not to attribute ...the extension of the pores or distances which its parts do not occupy when it is rarefied, but to the other bodies that fill these interstices ...that the body is diffused over a larger space.
* X. What space or internal place is. Space or internal place, and the corporeal substance which is comprised in it, are not different... merely in the mode in which... conceived by us. ...[I]n body we consider extension as particular, and... to change with the body; whereas in space we attribute to extension a generic unity, so that after taking... the body which occupied it, we do not suppose that we... removed the extension of the space, because it appears... the same extension remains... in respect to... bodies around it, by means of which we determine this space.
* XI. How space is not in reality different from corporeal substance. ...[I]t is the same extension which constitutes the nature of body as of space ...[N]othing remains in the idea of body, except that it is something extended in length, breadth, and depth; and this something is comprised in our idea of space, not only of that which is full of body, but... of what is called void space.
* XIV. Wherein place and space differ. ...[P]lace more expressly designates situation than magnitude or figure ...that it is situated in a determinate way in respect of certain other objects; and when we add that it occupies such a space... we understand besides that it is of such determinate magnitude and figure as exactly to fill this space.
* XVI. That a vacuum or space in which there is absolutely no body is repugnant to reason. ...[I]n the philosophical sense ...a space in which there is no substance ...does not exist ...[T]hat a body has extension in length, breadth, and depth, we ...conclude ...it is a substance, it being absolutely contradictory that nothing should possess extension ...[S]ince there is extension in it there is necessarily also substance.
* XX. That from this the non-existence of atoms may likewise be demonstrated. ...[T]here cannot exist any atoms or parts of matter that are... indivisible. For however small we suppose... yet because... extended, we... in thought... smaller parts, and... admit their divisibility. ...[S]uppose... God... reduced any particle... to a smallness... that... did not admit... being... divided... [H]e could not... deprive himself of the ability to do so... to lessen his own omnipotence... Wherefore... the smallest extended particle is always divisible...
* XXI. [T]he extension of the world is indefinite. ...[T]his world or the whole (universitas) of corporeal substance, is extended without limit, for wherever we fix a limit, we... not only imagine beyond it spaces indefinitely... but perceive these... truly imaginable... [i.e.,] in reality... so... they contain... corporeal substance indefinitely extended...
* XXII. [T]he matter of the heavens and earth is the same, and... there cannot be a plurality of worlds. ...[T]he earth and heavens are made of the same matter; and... [if] there were an infinity of worlds, they would... be composed of this matter... [I]t follows that a plurality of worlds is impossible, because... the matter... occupies all the imaginable spaces... and we cannot find... the idea of any other matter.
* XXIII. [A]ll... matter, or diversity of its forms, depends on motion.
* There is... one kind of matter... and this we know only by its being extended. All the properties... [we assign] to it are reducible to its capacity of being divided and moved according to its parts...
* [T]he partition of matter in thought makes no change in it; but all variation of it, or diversity of form, depends on motion.
* The philosophers... universally... observed this, for they said... nature was the principle of motion and rest, and by nature they understood that by which all corporeal things become... as... found in experience.
* XXIV. What motion is... in its common use.
* But motion (viz., local, for I can conceive no other... and... do not think... there is any other in nature), in the ordinary sense... is... the ACTION BY WHICH A BODY PASSES FROM ONE PLACE TO ANOTHER.
* And just as... the same thing may be said to change and not to change place at the same time, so... the same thing is at the same time moved and not moved.
* Thus, for example, a person seated in a vessel which is setting sail, thinks he is in motion if he look to the shore that he has left, and consider it as fixed; but not if he regard the ship itself, among the parts of which he preserves always the same situation. ...[B]ecause we... suppose... no motion without action, and that in rest there is... cessation of action... he is not conscious of being in action.
* XXV. What motion is properly so called... is THE TRANSPORING OF ONE PART OF MATTER OR OF ONE BODY FROM THE VICINITY OF THOSE BODIES THAT ARE IN IMMEDIATE CONTACT WITH IT, OR WHICH WE REGARD AT REST, TO THE VICINITY OF OTHER BODIES.
* By a body as a part of matter, I understand all that which is transferred together, although... perhaps composed of... parts, which in themselves have other motions; and... it is the transporting and not the force or action which transports... motion is always in the movable thing... [I]t is a mode of the movable thing, and not a substance, just as figure is a property of the thing figured, and repose of that which is at rest.

### Part III

    Of the Visible World
* I. [W]e cannot think too highly of the works of God.
* II. [W]e ought to beware lest, in our presumption, we imagine that the ends which God proposed to himself in the creation of the world are understood by us.
* III. In what sense... all things were created for the sake of man.

### Part IV

    Of the Earth
* CLXXXVIII. [W]hat is to be borrowed from disquisitions on animals and man to advance the knowledge of material objects.
* [B]ecause I have not ...acquired sufficient knowledge of all the matters ...to treat in these two last parts, and do not know whether I ever shall ...finish them, I ...here subjoin a few things regarding the objects of our senses.
* I have ...described this earth, and ...the whole visible world, as if ...a machine in which there was nothing ...consider except ...figures and motions of its parts, whereas our senses present ...many other things ...[e.g.,] colors, smells, sounds ...[etc.,] of which, if I did not speak ...it would be thought I ... omitted ...the majority of ...nature.
* CLXXXIX. [P]erception (sensus)... and how we perceive.
* [M]ovements... excited in the brain by the nerves... affect the soul or mind, which is intimately conjoined with the brain, according to the diversity of the motions... [T]he diverse affections of the mind or thoughts that... arise from these motions, are... perceptions of the senses (sensuum perceptiones), or... commonly... sensations (sensus).
* CXC. Distinction of the senses; and... of the internal... [i.e.,] affections of the mind (passions), and the natural appetites. ...We can distinguish ...seven principal classes of nerves, ...two belong to the internal, and ...five to the external senses. ...[N]rves ... to the stomach ...oesophagus ...fauces, and the other internal parts ...subservient to ...natural wants, constitute ...the natural appetite (appetitus naturalis) The other internal sense ...embraces all the emotions (commotiones) of the mind or passions, and affections, as joy, sadness, love, hate... [etc.] depends upon ...nerves ...to the heart ...
* [B]lood ...pure and well tempered ...dilates in the heart more readily and strongly ...so enlarges and moves the small nerves scattered around the orifices ...thence a corresponding movement in the brain ...affects the mind with a ...feeling of joy; and ...these ..nerves ... moved in the same way ...by other causes ...excite ...the same feeling (sensus, sentiment).
* [W]hen the blood is so thick that it flows ...sparingly into the ventricles of the heart ...not ...sufficiently dilated, it excites in the same nerves a motion ...which, communicated to the brain, gives to the mind the sensation of sadness ...And all the other causes which move these nerves in the same way may also give ...the same sensation. ...[O]ther movements of the same nerves produce other effects, as ...love, hate, fear, anger, etc... [A]s far as they are confused thoughts which the mind has not from itself ...but from ...the body ...[T]here is the widest difference between these passions and the distinct thoughts which we have of what ought to be loved, or chosen, or shunned, etc.
* CXCI. Of the external senses... first of touch.
* CXCII. Of taste. ...[N]erves scattered over the tongue and ...parts ...are diversely moved by the particles ...and ...cause sensations ...according to the diversity of figure in these particles.
* CXCIII. Of smell. [N]erves ...appendages of the brain ...are moved by the particles of terrestrial bodies, separated and flying in the air ...From the different motions of these particles arise the sensations of the different smells.
* CXCIV. Of hearing. ...[N]erves within the ears, so attached to three small bones ...the first ...rests on the small membrane ...the tympanum ...[D]iverse vibrations which the surrounding air communicates to this membrane, are transmitted to the mind by these nerves, and those vibrations give rise ...to the sensations of the different sounds.
* CXCV. Of sight. ...[E]xtremities of the optic nerves, composing the ...retina, are ...moved ...only by the globules of the second element, whence we have the sense of light and colors: as I ...explained in the Dioptrics and treatise of Meteors.
* CXCVI. [T]he soul perceives only in so far as it is in the brain. ...[T]he soul does not perceive in so far as it is in each [body] member ...but only in so far as it is in the brain, where the nerves ...convey to it ...actions of ...external objects ...[T]here are various maladies, which ...affect the brain alone, yet bring disorder upon, or deprive ...use of, our senses ...
* CXCVII. [T]he nature of the mind is such that from the motion alone of the body various sensations can be excited in it.
* CXCVIII. [B]y our senses we know nothing of external objects beyond their figure [or situation], magnitude, and motion.
* CXCIX. [T]here is no phenomenon of nature whose explanation has been omitted in this treatise.
* CC. [T]his treatise contains no principles which are not universally received; and... this philosophy is not new, but... the most ancient and common.
* CCI. [S]ensible bodies are composed of insensible particles. ...I allow many particles in each body... perceived by none of our senses... [W]ho ever observed by the senses those small bodies that are in one day added to a tree while growing? ...[I]n ...division ...parts may become so small as to be ...imperceptible.
* CCII. [T]he philosophy of Democritus is not less different from ours than from the common. ...[I]t may be said that Democritus ...supposed ...corpuscles ...of various figures, sizes, and motions, from the heaping together and mutual concourse of which all sensible bodies arose; and, nevertheless, his mode of philosophizing is commonly rejected by all. ...I reply that the philosophy of Democritus was ...rejected ...because he supposed ...corpuscles ...indivisible, on which ground I also reject it ...second ...because he imagined ...a vacuum about them, which I show to be impossible; thirdly, because he attributed gravity to these bodies, of which I deny ...in so far as a body is considered by itself, because it is a quality that depends on the relations of situation and motion which several bodies bear to each other; and, finally, because he has not explained ...how all things arose from the concourse of corpuscles alone, or, if he gave this explanation with regard to a few of them, his whole reasoning was far from being coherent (or such as would warrant ...extending the same explanation to the whole of nature).
* CCIII. [W]e may arrive at the knowledge of the figures (magnitude), and motions of the insensible particles of bodies. ...[S]ince I assign determinate figures, magnitudes, and motions to the insensible particles... how... have [I] come by my knowledge of them. ...I ...considered in general all the clear and distinct notions of material things ...in our understanding, and that, finding no others except ...figures, magnitudes, and motions, and ...rules according to which these three ...can be diversified by each other, which rules are ...of geometry and mechanics ...I judged that all the knowledge ...of nature must ...be drawn from this ...I considered the chief differences ...between the magnitudes, and figures, and situations of bodies insensible ... and what sensible effects could be produced by their ...modes of ...contact ...[W]hen I found like effects in the bodies that we perceive ...I judged ...they could have been thus produced ...[A]ll the rules of mechanics belong also to physics, of which it is a part ...for it is not less natural for a clock ...to mark the hours, than for a tree ...to produce ...fruit ...[S]o from considering the sensible effects and parts of natural bodies, I have essayed to determine the character of their causes and insensible parts.
* CCIV. [T]hings which our senses do not perceive... it is sufficient to explain how they can be (and... this is all... Aristotle has essayed).
* CCV. [T]here is a moral certainty that all the things of this world are such as has been here shown they may be. ...I ...distinguish two kinds of certitude. ...[F]irst is ...moral ...[i.e.,] a certainty sufficient for the conduct of life, though, if we look to ...God, what is morally certain may be false. ...[T]hey who observe how many things regarding the magnet, fire, and the fabric of the whole world, are here deduced from a very small number of principles, though they deemed ...I had taken them up at random and without grounds, will ...acknowledge that it could hardly happen that so many things should cohere if these principles were false.
* CCVI. [W]e possess even more than a moral certainty of it.
* CCVII. [H]owever, I submit all my opinions to the authority of the church. ... I affirm nothing, but submit all these my opinions to the authority [and judgment] of the church... I desire no one to believe anything I may have said, unless he is constrained to admit it by the force and evidence of reason.

## See also

* Ageless Wisdom Teachings
* Aphorisms
* Maxims
* Philosophy
* Proverbs
* Wisdom

## External links

* Selections from The Principles of Philosophy of Descartes, The Method, Meditations and Philosophy of Descartes (1901) Tr. John Veitch @Archive.org
* Principles of Philosophy (2017) Tr. John Cottingham @EarlyModernTexts.com
* Selections from the Principles of Philosophy by René Descartes @Project Gutenberg
* Renati Des-Cartes Principia philosophiae (1644) @Library of Congress