Philosophical Works · Chapters
Chapters explained
Chapter companions for Philosophical Works by René Descartes.
Chapter 1
Discourse on the Method of Rightly Conducting One’s Reason and of Seeking Truth in the Sciences
Descartes introduces his method of systematic doubt and rational inquiry, rejecting uncertain beliefs to establish a foundation for knowledge. He recounts his education, his decision to doubt all that is not certain, and his discovery of the cogito as the first principle of philosophy. The chapter outlines his methodical approach to seeking truth and his provisional moral code, leading to proofs of God's existence and the distinction between mind and body.
Chapter 2
Prefatory Note by the Author
The author provides a brief outline of the six parts of the Discourse, summarizing the topics covered in each: the sciences, the method, morals, metaphysics (including proofs of God and the soul), physics and medicine, and the requirements for advancing natural investigation.
Chapter 3
Discourse on the Method
Descartes recounts his method of radical doubt, rejecting all beliefs that could be doubted, leading to the first certainty: 'I think, therefore I am.' He establishes the mind as distinct from the body, uses the idea of a perfect being to prove God's existence, and outlines his plan to explain the material world through laws established by God.
Chapter 4
Meditations on the First Philosophy: In Which the Existence of God and the Immortality of the Soul Are Demonstrated
The chapter outlines the structure of the six Meditations, beginning with a universal doubt to free the mind from prejudice and the senses. It establishes the cogito as the first certainty, distinguishes mind from body, proves God's existence and veracity, and concludes that material things exist and that mind and body are really distinct. The author emphasizes that true knowledge depends on knowledge of God, who guarantees clear and distinct perceptions, and that error arises from the finite nature of human will and understanding.
Chapter 5
Dedication
Descartes addresses Princess Elizabeth, praising her intellectual virtues and unique ability to comprehend both metaphysics and geometry. He distinguishes between apparent and true virtues, defining wisdom as the firm resolution to use reason and do what is best. He dedicates his work to her as a model of wisdom.
Chapter 6
Preface to the Reader
Descartes addresses readers, summarizing his earlier treatment of God and the soul in the Discourse on Method, and responds to two objections: that the mind's essence may not be solely thinking, and that the idea of a perfect being does not guarantee its existence. He defends his method, warns against superficial reading, and introduces his plan to present the Meditations along with replies to objections from learned critics.
Chapter 7
Synopsis of the Six Following Meditations
Descartes outlines the six meditations, explaining the method of universal doubt, the discovery of the self (cogito), the distinction between mind and body, proofs for the existence of God, the nature of error, and the real distinction between mind and body. He emphasizes that the mind is known more clearly than the body, that God guarantees truth, and that the soul is immortal.
Chapter 8
Meditations on the First Philosophy
In this chapter, Descartes establishes that clear and distinct knowledge depends on the knowledge of God, who guarantees truth. He explores the distinction between imagination and pure intellection, using the example of a piece of wax to demonstrate that the mind, not the senses, perceives the essence of material things. He also discusses the proper use of free will to avoid error, emphasizing that error arises from judging without clear and distinct understanding.
Chapter 9
Meditation I: Of the Things of Which We May Doubt
Descartes resolves to systematically doubt all his former opinions in order to establish a firm foundation for the sciences. He begins by questioning the reliability of the senses, noting that they sometimes deceive us. He then considers the dream argument, realizing that there are no certain marks to distinguish waking from sleeping. Even mathematical truths like 2+3=5 are called into doubt by the possibility of an all-powerful deceiver (God or an evil demon). Ultimately, he decides to withhold assent from all beliefs that are not absolutely certain, adopting a method of hyperbolic doubt.
Chapter 10
Meditation II: Of the Nature of the Human Mind; and That It Is More Easily Known Than the Body
Descartes, having resolved to doubt all things, discovers the indubitable truth of his own existence through the Cogito: 'I am, I exist.' He then examines the nature of the self, concluding that he is essentially a thinking thing—a mind that doubts, understands, affirms, denies, wills, imagines, and perceives. Using the example of a piece of wax, he demonstrates that bodies are known not through the senses or imagination but through the intellect alone, and that the mind is known more clearly and distinctly than the body.
Chapter 11
Meditation III: Of God: That He Exists
Descartes, having established his own existence as a thinking thing, now seeks to extend his knowledge. He formulates the general rule that whatever is clearly and distinctly perceived is true, but recognizes that this rule requires a guarantee against the possibility of a deceiving God. To resolve this, he classifies his thoughts into ideas, volitions, and judgments, and distinguishes ideas as innate, adventitious, or factitious. He then examines the idea of God as an infinite, perfect being, arguing that such an idea could not originate from his finite self and must therefore be caused by God himself. This leads to a proof of God's existence, which in turn serves as the foundation for the certainty of all clear and distinct perceptions.
Chapter 12
Meditation IV: Of Truth and Error
Descartes explores the nature of error and truth, arguing that error arises from the misuse of free will when the will extends beyond the clear and distinct perceptions of the understanding. He affirms that God, being supremely perfect, cannot deceive, and that by restraining judgment to what is clearly known, one can avoid error and attain truth.
Chapter 13
Meditation V: Of the Essence of Material Things; and, Again, of God; That He Exists
Descartes examines the essence of material things, focusing on the clear and distinct ideas of quantity, extension, and mathematical truths. He argues that these ideas possess true and immutable natures independent of his thought. He then presents an ontological argument for God's existence, claiming that existence is inseparable from God's essence, just as properties are inseparable from a triangle. He concludes that knowledge of God is the foundation of all certain science.
Chapter 14
Meditation VI: Of the Existence of Material Things, and of the Real Distinction Between the Mind and Body of Man
Descartes concludes that material things exist because God, who is no deceiver, would not allow him to be systematically mistaken about clear and distinct perceptions of corporeal nature. He distinguishes imagination from pure intellection, arguing that imagination depends on something distinct from the mind, suggesting a union with a body. He then examines sensory perceptions, noting that while they often contain error, the natural inclinations toward pain, hunger, and thirst reveal that the mind is not merely lodged in the body like a pilot in a vessel but is intimately conjoined with it, forming a unity. Through God's veracity, he affirms the existence of material objects and the real distinction between mind and body.
Chapter 15
Selections from the Principles of Philosophy
Descartes establishes the method of radical doubt as the foundation for seeking truth, arguing that we must doubt all that can be doubted—including sensory perceptions and mathematical demonstrations—to arrive at certain knowledge. He identifies the first indubitable principle as 'I think, therefore I am' (the Cogito), which reveals the nature of the mind as a thinking substance distinct from the body. Descartes further explores the clarity of self-knowledge, the role of free will in avoiding error, and the proper approach to understanding God and creation, emphasizing that we should rely on efficient rather than final causes and accept divine revelation even when it surpasses reason.
Chapter 16
Letter of the Author to the French Translator of the Principles of Philosophy Serving for a Preface
In this preface, Descartes critiques the philosophical tradition, especially the errors of those who doubted all or trusted the senses entirely, and those who blindly followed Aristotle. He argues that true principles must be clear and able to generate all other truths. He presents his own metaphysical principles—the cogito, the existence of God as guarantor of truth, and the nature of body as extension—as the only ones that meet these criteria. He also warns against misinterpretations of his work, citing a recent case where a follower published a flawed book, and encourages readers to seek truth through his method.
Chapter 17
Dedication
Descartes addresses Princess Elizabeth, praising her intellectual virtues and unique ability to comprehend both metaphysics and geometry. He distinguishes between apparent and true virtues, defining wisdom as the firm resolution to use reason and do what is best. He dedicates his work to her as a model of wisdom.
Chapter 18
Selections from the Principles of Philosophy
Descartes establishes the method of radical doubt as the foundation for seeking truth, arguing that we must doubt all that can be doubted—including sensory perceptions and mathematical demonstrations—to arrive at certain knowledge. He identifies the first indubitable principle as 'I think, therefore I am' (the Cogito), which reveals the nature of the mind as a thinking substance distinct from the body. Descartes further explores the clarity of self-knowledge, the role of free will in avoiding error, and the proper approach to understanding God and creation, emphasizing that we should rely on efficient rather than final causes and accept divine revelation even when it surpasses reason.
Chapter 19
Part I: Of the Principles of Human Knowledge
Descartes establishes the method of radical doubt as the foundation for seeking truth, arguing that all previously held beliefs—including those from the senses and even mathematical demonstrations—must be doubted at least once in life. He identifies the first indubitable certainty as 'I think, therefore I am,' which reveals the mind as better known than the body. He then introduces the idea of God as a perfect being, uses it to prove God's existence, and argues that God's veracity guarantees the truth of clear and distinct perceptions. He distinguishes mind from body, identifies the essence of body as extension, and explains that error arises from the misuse of free will rather than from God. He concludes by advising that we rely on efficient rather than final causes and that we accept divine revelation even when it surpasses reason.
Chapter 20
I
Descartes establishes the principles of material things derived from reason rather than sensory prejudice, and emphasizes the importance of recognizing the infinite power and goodness of God to avoid underestimating His works.
Chapter 21
II
This chapter warns against the presumption of understanding God's purposes in creation and against assuming that the universe was made solely for human benefit. It cautions against overestimating the power of human thought to comprehend divine ends without natural reason or revelation.
Chapter 22
III
Descartes argues that while it is morally edifying to believe all things were created for humanity's sake, this view is not physically plausible. Many things exist or have existed without ever being seen, known, or used by humans, so God must have had other ends in creation.
Chapter 23
IV
Descartes argues that the essence of body or matter is not found in sensible qualities like hardness, weight, or color, but solely in extension—being a substance extended in length, breadth, and depth. He illustrates that hardness is known only through resistance to touch, and if bodies receded from our hands, we would not perceive hardness, yet they would still be bodies. Similarly, all other sensible qualities can be removed without destroying the body's nature, proving that extension alone constitutes its essence.
Chapter 24
V
Descartes argues that the true nature of body consists solely in extension, and addresses two common prejudices that obscure this truth: the belief in rarefaction and condensation as changes in extension, and the mistaken notion that space or void can exist without body.
Chapter 25
In what way rarefaction takes place
Descartes explains rarefaction and condensation as mere changes in the figure of a body, where rare bodies have larger distances between parts filled by other bodies, and dense bodies have those distances reduced or eliminated. He emphasizes that extension should not be attributed to the pores or empty spaces but to the bodies that fill them, using the analogy of a sponge to illustrate that rarefaction does not increase the body's own extension.
Chapter 26
VII
Descartes argues that rarefaction can only be intelligibly explained by the addition of new extended substance (another body) to fill enlarged pores, rather than by an unintelligible augmentation of quantity. He dismisses the objection that we do not perceive this new body with our senses, asserting that not all bodies must be perceptible.
Chapter 27
VIII
Descartes explains that quantity and number are not really distinct from extended substance and the thing numbered, but are merely conceptual distinctions. He argues that one cannot conceive of a continuous quantity without some extended substance, and that any change in quantity necessarily entails a corresponding change in substance.
Chapter 28
IX
Descartes argues that when people distinguish corporeal substance from its quantity or extension, they either conceive of nothing or form a confused idea of incorporeal substance, mistakenly attributing it to body. He asserts that extension is not an accident but the true nature of corporeal substance, and that those who speak otherwise contradict their own thoughts.
Chapter 29
Of Space or Internal Place
This chapter explains that space and corporeal substance are not distinct in reality but only in our conception. Extension in length, breadth, and depth constitutes both space and body; the difference is that in body extension is considered particular and changeable, while in space it is attributed a generic unity that persists even after the body is removed, as long as the magnitude, figure, and situation relative to surrounding bodies remain the same.
Chapter 30
How space is not in reality different from corporeal substance
Descartes argues that space and corporeal substance are not distinct; the essence of body is extension in length, breadth, and depth, and all other qualities such as hardness, color, weight, and temperature are non-essential.
Chapter 31
How Space Differs from Body in Our Mode of Conceiving It
Descartes explains the conceptual distinction between space and body: while a particular body's extension is inseparable from it, the extension of a place can remain even when the body is removed, because we conceive of extension in general as common to all bodies and even to a vacuum.
Chapter 32
XIII
Descartes defines external place as not distinct from the body itself, but as the magnitude, figure, and situation of a body relative to others. He illustrates that place is relative, depending on which bodies are considered immovable, and concludes that nothing has a permanent place except as fixed by thought.
Chapter 33
Wherein Place and Space Differ
This chapter distinguishes between the concepts of place and space, emphasizing that place refers to situation relative to other objects, while space additionally implies determinate magnitude and figure.
Chapter 34
XV
This chapter distinguishes between internal and external place, defining external place as the surface of the surrounding body. It clarifies that space and extension are identical, and that place can be considered either as internal (identical to space) or external (the boundary between bodies). The chapter uses the example of a boat moving with equal and opposite forces to illustrate that a body can remain in the same place even if its surrounding surface changes.
Chapter 35
XVI
Descartes argues that a vacuum—space devoid of all body—is impossible, because extension is inseparable from substance; wherever there is extension, there must be a substance that is extended.
Chapter 36
XVII
Descartes clarifies the ordinary use of the term 'vacuum,' explaining that it does not mean a space devoid of all body, but rather a space lacking the objects we expect to be there. He warns against the error of assuming that a space called 'vacuum' contains no substance at all, using examples like a pitcher filled with air or a fishpond full of water.
Chapter 37
How the prejudice of an absolute vacuum is to be corrected
Descartes argues against the common prejudice that a vacuum can exist, explaining that extension is necessarily connected to substance, and that removing all body from a vessel would cause its sides to touch, as distance without extended substance is contradictory.
Chapter 38
XIX
This chapter confirms the earlier explanation of rarefaction by arguing that the nature of corporeal substance is simply extension, and that extension is identical to the space a body occupies regardless of its weight or hardness. Therefore, a vessel contains the same amount of matter whether filled with lead or air, because quantity depends solely on extension.
Chapter 39
XX
This chapter demonstrates the impossibility of atoms or indivisible particles of matter, arguing that any extended particle, no matter how small, is divisible in thought and therefore divisible in reality, and that even God cannot make a particle absolutely indivisible without contradicting His own omnipotence.
Chapter 40
Chapter XXI
This chapter demonstrates that the extension of the world is indefinite, arguing that wherever a limit is imagined, further extended spaces are perceived as truly imaginable and thus contain corporeal substance indefinitely extended, consistent with the idea of extension being identical to corporeal substance.
Chapter 41
Chapter XXII
Descartes argues that the matter of the heavens and earth is identical, consisting solely of extension, and therefore a plurality of worlds is impossible because extended matter already occupies all imaginable space.
Chapter 42
XXIII
This chapter argues that all diversity in matter arises from motion, that there is only one kind of matter in the universe known by its extension, and that all properties of matter are reducible to divisibility and motion of its parts.
Chapter 43
XXIV
Descartes defines local motion as the action by which a body passes from one place to another, noting that motion and rest are relative to the frame of reference, as illustrated by a person seated in a moving vessel.
Chapter 44
XXV
Descartes defines motion properly as the transporting of a body from the vicinity of those bodies in immediate contact with it to the vicinity of others, emphasizing that motion is a mode of the movable thing, not a substance, and distinguishing it from the force or action that transports.
Chapter 45
XXVI
Descartes advises against engaging in disputes about the infinite, arguing that finite beings cannot determine anything about it. He proposes to treat things without discoverable limits as indefinite rather than infinite, such as the extension of the world, the divisibility of matter, and the number of stars, to avoid absurd questions.
Chapter 46
Chapter XXVII: On the Indefinite and the Infinite
This chapter distinguishes between the indefinite and the infinite, reserving the term 'infinite' for God alone, who is positively conceived as having no limits, while other things are called indefinite because their limits, if any, cannot be discovered by us.
Chapter 47
XXVIII
This chapter argues that in natural philosophy we should not seek final causes (the ends God or nature intended) but rather efficient causes, because we cannot presume to know God's purposes. Instead, we should use the natural light God gave us to investigate the effects we perceive, trusting that light only as long as it is not contradicted by divine revelation.
Chapter 48
XXIX
This chapter argues that God is not the cause of human error, emphasizing God's absolute veracity and role as the source of all light. It asserts that deception stems from malice or weakness, attributes incompatible with God's nature.
Chapter 49
The Guarantee of Truth Through God's Veracity
This chapter establishes that because God is not a deceiver, the clear and distinct perceptions of the intellect are necessarily true. It resolves the earlier hyperbolic doubts by grounding certainty in the divine guarantee, and extends this assurance to mathematical truths and sensory knowledge, provided one distinguishes the clear from the obscure.
Chapter 50
XXXI
Descartes explains that errors arise not from God's deception but from the interplay of understanding and will. Errors are mere negations from God's perspective, but privations from our own, as they depend on our will rather than on God's concurrence.
Chapter 51
XXXII
Descartes distinguishes two fundamental modes of thinking: perception (understanding) and volition (will). All mental acts—sensing, imagining, conceiving—are forms of perception, while desiring, avoiding, affirming, denying, and doubting are forms of willing.
Chapter 52
That we never err unless when we judge of something which we do not sufficiently apprehend
This chapter explains that error arises not from apprehension itself but from judging without clear and distinct perception. If we refrain from judging or only assent to what we clearly and distinctly perceive, we avoid error.
Chapter 53
XXXIV
Descartes explains that judgment requires both the understanding (to apprehend something) and the will (to assent to it), and that a perfect apprehension is not necessary for forming a judgment.
Chapter 54
XXXV
Descartes explains that the will is far more extensive than the intellect, and that error arises when the will is applied to matters not clearly and distinctly perceived by the understanding.
Chapter 55
XXXVI
This chapter argues that God cannot be blamed for human errors, because it is inherent in created intellect to be finite and not all-encompassing.
Chapter 56
XXXVII
Descartes argues that the chief perfection of man is his ability to act freely or by will, which renders him worthy of praise or blame. He contrasts this with self-acting machines, which are not praised for their motions because they are necessary, but their maker is praised for acting freely. Similarly, humans merit greater esteem when they embrace truth freely rather than by necessity.
Chapter 57
XXXVIII
Descartes argues that error is a defect in our mode of acting, not in our nature, and that God is not responsible for human errors because His dominion is absolute and free, unlike human authority. He emphasizes gratitude for the goods God has given rather than complaint for what He has withheld.
Chapter 58
XXXIX
Descartes argues that the freedom of the will is self-evident and among the innate notions. He points out that even when he supposed a deceiving God, he was still conscious of his freedom to withhold assent from uncertain things, making this freedom as clear and self-evident as anything can be known.
Chapter 59
XL
The chapter affirms that God has foreordained all things, emphasizing the immense power of God and the sinfulness of thinking we can act outside His preordination. It acknowledges the difficulty of reconciling divine preordination with human free will, suggesting that we should not attempt to fully comprehend both truths simultaneously.
Chapter 60
XLI
Descartes reconciles human free will with divine preordination by emphasizing the limited nature of the human mind and the infinite power of God. He argues that while we cannot comprehend how God's foreknowledge and will coexist with human freedom, we have clear and distinct consciousness of our own liberty, which we should not doubt simply because we cannot grasp God's incomprehensible ways.
Chapter 61
XLII
Descartes explains that although no one wills to be deceived, error arises because we will to assent to opinions that contain hidden error, often driven by a misguided desire for truth without adequate knowledge.
Chapter 62
XLIII
Descartes asserts that error is avoided when assent is given only to clear and distinct perceptions, because God, being no deceiver, has given us infallible faculties of knowledge and will, provided we do not extend the will beyond what is clearly known. He adds that even without reasoning, nature impels us to spontaneously assent to clear perceptions and makes doubt impossible.
Chapter 63
XLIV
Descartes warns against assenting to what we do not clearly perceive, noting that even true judgments made without clear perception are unreliable. He identifies memory as a frequent source of error, leading us to believe we understand things we do not truly know.
Chapter 64
XLV
Descartes defines clear and distinct perception, explaining that for a judgment to be certain and indubitable, knowledge must be both clear (present and manifest to the attentive mind) and distinct (precise and different from all other objects, containing only what is clear).
Chapter 65
XLVI
This chapter uses the example of pain to illustrate that a perception can be clear without being distinct, but cannot be distinct without being clear. It explains that people often confuse the clear sensation of pain with an obscure judgment about its nature, mistakenly believing there is something in the suffering part similar to the sensation.
Chapter 66
XLVII
Descartes explains that to correct the prejudices of early youth, one must examine each simple notion to distinguish what is clear from what is obscure or error-prone, since the mind was once so immersed in the body that it perceived many things clearly but knew nothing distinctly.
Chapter 67
XLVIII
Descartes classifies all objects of knowledge into two categories: things (or their affections) and eternal truths. He identifies two highest kinds of things: intellectual (thinking substance) and material (extended substance), and notes that certain experiences arise from the union of mind and body, such as appetites, passions, and sensations.
Chapter 68
XLIX
The chapter discusses eternal truths, such as 'ex nihilo nihil fit' and 'he who thinks must exist while he thinks,' which are not things or modes but common notions or axioms seated in the mind. It notes that while these truths are numerous and difficult to enumerate, such enumeration is unnecessary because they are self-evident when considered without prejudice.
Chapter 69
Chapter L
Descartes discusses that common notions or truths are clearly and distinctly known, but not equally by all men due to prejudices. He argues that the faculty of knowledge is equal among men, but prejudices prevent some from readily embracing these truths, while others free from prejudice apprehend them with great clearness.
Chapter 70
LI
This chapter defines substance as that which requires nothing beyond itself to exist, asserting that only God is absolutely independent, while all other things depend on God's concourse. It clarifies that the term 'substance' is not applied univocally to God and creatures.
Chapter 71
LII
Descartes discusses the univocal application of the term 'substance' to both mind and body, explaining that substance is known through its attributes and that created substances require only God's concurrence to exist.
Chapter 72
LIII
Descartes explains that each substance has one principal attribute that constitutes its nature: thought for the mind and extension for the body. All other properties are merely modes of these primary attributes.
Chapter 73
LIV
This chapter explains how we can form clear and distinct notions of thinking substance, corporeal substance, and God by carefully distinguishing attributes of thought from those of extension, and by attending only to the characters that belong to the nature of an absolutely perfect Being without mixing in any fiction.
Chapter 74
LV
This chapter explains how duration, order, and number can be distinctly conceived by not mixing them with the concept of substance. Duration is a mode of conceiving a thing insofar as it continues to exist, while order and number are merely modes of considering things arranged or counted, not realities separate from the things themselves.
Chapter 75
LVI
This chapter defines and distinguishes the terms modes, qualities, and attributes in relation to substance. Modes are variations of substance that also determine its kind (qualities) or are considered simply as in the substance (attributes). It notes that in God, who is immutable, only attributes are proper, while in created things, constant features like existence and duration are called attributes rather than modes or qualities.
Chapter 76
LVII
This chapter distinguishes between attributes that exist in things themselves and those that exist only in thought, using time as an example. Time is described as a mode of thinking about duration, not a separate reality, and is measured by comparing durations with regular motions like years and days.
Chapter 77
LVIII
Descartes argues that number and universals, when considered abstractly rather than as inherent in created things, are merely modes of thought.
Chapter 78
LIX
This chapter explains how universals are formed through the mind's use of a single idea to represent multiple similar individual objects. It describes the five universals—genus, species, difference, property, and accident—using examples such as the number two, triangles, and right-angled triangles to illustrate how these categories arise from comparing and naming shared characteristics.
Chapter 79
Of distinctions; and first of the real
This chapter discusses the threefold distinction among substances: real, modal, and of reason. It argues that two substances are really distinct if one can be clearly and distinctly conceived without the other, and that God's power guarantees the possibility of such separation. The real distinction between mind and body is affirmed, even in the case of their intimate union.
Chapter 80
Of the Modal Distinction
This chapter distinguishes two kinds of modal distinctions: between a mode and its substance, and between two modes of the same substance. It clarifies that a mode cannot be conceived without its substance, whereas two modes of the same substance can be conceived separately. It also argues that modes belonging to different substances are better understood as real distinctions rather than modal ones.
Chapter 81
Of the Distinction of Reason (Logical Distinction)
This chapter explains the distinction of reason, which is a logical distinction between a substance and its inseparable attributes, or between two attributes of the same substance that cannot be conceived separately. Descartes illustrates this with the example of duration being distinct from substance only in thought, and clarifies that he previously classified this under modal distinction in his Reply to the First Objections, but only for general purposes.
Chapter 82
How Thought and Extension May Be Distinctly Known
This chapter explains that thought and extension constitute the natures of mind and body respectively, and that these attributes are known with the greatest clarity and distinctness when conceived as the substances themselves. It also notes that it is easier to conceive extended or thinking substance than substance abstracted from these attributes, and that a concept is more distinct by accurate distinction from other notions rather than by containing fewer properties.
Chapter 83
LXIV
This chapter explains how thought and extension can be distinctly conceived as modes of substance, not as substances themselves. It emphasizes that when considered as modes inherent in substance, they are clearly and distinctly understood, but if separated from substance, they are mistakenly regarded as self-subsisting things, leading to confusion between mode and substance.
Chapter 84
LXV
This chapter explains how we can best understand the diverse modes of thought (such as intellection, imagination, recollection, volition) and the modes of extension (such as figures, situation of parts, and motions) by considering them simply as modes of the things in which they inhere, and by treating motion merely as locomotion without investigating its producing force.
Chapter 85
LXVI
Descartes explains that sensations, affections, and appetites can be clearly known if we restrict our judgments to only what is immediately contained in our perception. He notes the difficulty, especially with sensations, because from youth we habitually judge that external things exist exactly as we perceive them, leading to errors.
Chapter 86
LXVII
Descartes argues that we are frequently deceived in our judgments about pain and other sensations, as we mistakenly locate sensations like pain in body parts (e.g., the foot) rather than recognizing them as purely mental perceptions. He attributes this error to childhood prejudices, drawing an analogy with light, which we wrongly believe to exist in the sun as we perceive it.
Chapter 87
LXVIII
Descartes distinguishes between clear and obscure knowledge in sensations, emphasizing that sensations like pain and color are clearly known only as thoughts, not as external realities.
Chapter 88
LXIX
Descartes distinguishes between primary qualities (magnitude, figure, motion, etc.) which are clearly perceived in bodies, and secondary qualities (color, pain, smell, taste) which are known only through the senses, arguing that primary qualities are known with far greater clarity.
Chapter 89
LXX
This chapter explains that we can judge sensible things in two ways: one that avoids error and one that leads to it. It argues that perceiving colors in objects is equivalent to perceiving an unknown something that produces a sensation, and that error arises when we assume the sensation resembles the object's property, whereas we only clearly know properties like size, figure, and number.
Chapter 90
LXXI
Descartes identifies the chief cause of human error as the prejudices acquired in childhood, when the mind was so closely tied to the body that it mistook sensory perceptions for objective truths, attributing to external objects qualities like tastes, smells, and colors that exist only in the mind, and later forming false beliefs about the size, motion, and substance of things based on bodily impressions.
Chapter 91
LXXII
Descartes explains that the second cause of error is our inability to forget prejudices formed in childhood. Even in adulthood, when the mind seeks truth independently of the body, these ingrained false judgments persist in memory and lead to errors, as illustrated by the difficulty of conceiving the true size of stars despite astronomical proof.
Chapter 92
LXXIII
Descartes explains that the mind's fatigue from attending to non-sensible objects leads to a habitual reliance on preconceived opinions, causing many to mistakenly believe that only corporeal, imaginable, and sensible substances exist, whereas true understanding requires reason exercised upon sensible objects.
Chapter 93
LXXIV
Descartes identifies the fourth source of error: attaching thoughts to imprecise words. He explains that because we associate concepts with words and recall words more easily than meanings, we often assent to terms without clear understanding. He postpones a full treatment until the nature and existence of body are established, but notes that enough has been said to distinguish clear and distinct conceptions from obscure and confused ones.
Chapter 94
LXXV
Descartes summarizes the essential steps for correct philosophizing: lay aside prejudices, withhold assent from former opinions until re-examined, and hold as true only what is clearly and distinctly apprehended. He outlines the order of knowledge, beginning with the existence of the thinking self, then God as the cause of all things, followed by eternal truths, corporeal nature, and sensations, leading to clear and distinct conceptions of all knowable objects.
Chapter 95
LXXVI
Descartes asserts that divine revelation is more certain than any human perception, and that in matters not revealed, one should only assent to what is clearly apprehended by reason, rejecting the unreliable judgments of childhood and the senses.
Chapter 96
Part II: Of the Principles of Material Things
Descartes establishes the certainty of material things through God's veracity, defines the essence of body as extension alone, and clarifies the nature of space, rarefaction, and the distinction between quantity and substance.
Chapter 97
I
Descartes establishes the principles of material things derived from reason rather than sensory prejudice, and emphasizes the importance of recognizing the infinite power and goodness of God to avoid underestimating His works.
Chapter 98
II
This chapter warns against the presumption of understanding God's purposes in creation and against assuming that the universe was made solely for human benefit. It cautions against overestimating the power of human thought to comprehend divine ends without natural reason or revelation.
Chapter 99
III
Descartes argues that while it is morally edifying to believe all things were created for humanity's sake, this view is not physically plausible. Many things exist or have existed without ever being seen, known, or used by humans, so God must have had other ends in creation.
Chapter 100
IV
Descartes argues that the essence of body or matter is not found in sensible qualities like hardness, weight, or color, but solely in extension—being a substance extended in length, breadth, and depth. He illustrates that hardness is known only through resistance to touch, and if bodies receded from our hands, we would not perceive hardness, yet they would still be bodies. Similarly, all other sensible qualities can be removed without destroying the body's nature, proving that extension alone constitutes its essence.
Chapter 101
V
Descartes argues that the true nature of body consists solely in extension, and addresses two common prejudices that obscure this truth: the belief in rarefaction and condensation as changes in extension, and the mistaken notion that space or void can exist without body.
Chapter 102
In what way rarefaction takes place
Descartes explains rarefaction and condensation as mere changes in the figure of a body, where rare bodies have larger distances between parts filled by other bodies, and dense bodies have those distances reduced or eliminated. He emphasizes that extension should not be attributed to the pores or empty spaces but to the bodies that fill them, using the analogy of a sponge to illustrate that rarefaction does not increase the body's own extension.
Chapter 103
VII
Descartes argues that rarefaction can only be intelligibly explained by the addition of new extended substance (another body) to fill enlarged pores, rather than by an unintelligible augmentation of quantity. He dismisses the objection that we do not perceive this new body with our senses, asserting that not all bodies must be perceptible.
Chapter 104
VIII
Descartes explains that quantity and number are not really distinct from extended substance and the thing numbered, but are merely conceptual distinctions. He argues that one cannot conceive of a continuous quantity without some extended substance, and that any change in quantity necessarily entails a corresponding change in substance.
Chapter 105
IX
Descartes argues that when people distinguish corporeal substance from its quantity or extension, they either conceive of nothing or form a confused idea of incorporeal substance, mistakenly attributing it to body. He asserts that extension is not an accident but the true nature of corporeal substance, and that those who speak otherwise contradict their own thoughts.
Chapter 106
Of Space or Internal Place
This chapter explains that space and corporeal substance are not distinct in reality but only in our conception. Extension in length, breadth, and depth constitutes both space and body; the difference is that in body extension is considered particular and changeable, while in space it is attributed a generic unity that persists even after the body is removed, as long as the magnitude, figure, and situation relative to surrounding bodies remain the same.
Chapter 107
How space is not in reality different from corporeal substance
Descartes argues that space and corporeal substance are not distinct; the essence of body is extension in length, breadth, and depth, and all other qualities such as hardness, color, weight, and temperature are non-essential.
Chapter 108
How Space Differs from Body in Our Mode of Conceiving It
Descartes explains the conceptual distinction between space and body: while a particular body's extension is inseparable from it, the extension of a place can remain even when the body is removed, because we conceive of extension in general as common to all bodies and even to a vacuum.
Chapter 109
XIII
Descartes defines external place as not distinct from the body itself, but as the magnitude, figure, and situation of a body relative to others. He illustrates that place is relative, depending on which bodies are considered immovable, and concludes that nothing has a permanent place except as fixed by thought.
Chapter 110
Wherein Place and Space Differ
This chapter distinguishes between the concepts of place and space, emphasizing that place refers to situation relative to other objects, while space additionally implies determinate magnitude and figure.
Chapter 111
XV
This chapter distinguishes between internal and external place, defining external place as the surface of the surrounding body. It clarifies that space and extension are identical, and that place can be considered either as internal (identical to space) or external (the boundary between bodies). The chapter uses the example of a boat moving with equal and opposite forces to illustrate that a body can remain in the same place even if its surrounding surface changes.
Chapter 112
XVI
Descartes argues that a vacuum—space devoid of all body—is impossible, because extension is inseparable from substance; wherever there is extension, there must be a substance that is extended.
Chapter 113
XVII
Descartes clarifies the ordinary use of the term 'vacuum,' explaining that it does not mean a space devoid of all body, but rather a space lacking the objects we expect to be there. He warns against the error of assuming that a space called 'vacuum' contains no substance at all, using examples like a pitcher filled with air or a fishpond full of water.
Chapter 114
How the prejudice of an absolute vacuum is to be corrected
Descartes argues against the common prejudice that a vacuum can exist, explaining that extension is necessarily connected to substance, and that removing all body from a vessel would cause its sides to touch, as distance without extended substance is contradictory.
Chapter 115
XIX
This chapter confirms the earlier explanation of rarefaction by arguing that the nature of corporeal substance is simply extension, and that extension is identical to the space a body occupies regardless of its weight or hardness. Therefore, a vessel contains the same amount of matter whether filled with lead or air, because quantity depends solely on extension.
Chapter 116
XX
This chapter demonstrates the impossibility of atoms or indivisible particles of matter, arguing that any extended particle, no matter how small, is divisible in thought and therefore divisible in reality, and that even God cannot make a particle absolutely indivisible without contradicting His own omnipotence.
Chapter 117
Chapter XXI
This chapter demonstrates that the extension of the world is indefinite, arguing that wherever a limit is imagined, further extended spaces are perceived as truly imaginable and thus contain corporeal substance indefinitely extended, consistent with the idea of extension being identical to corporeal substance.
Chapter 118
Chapter XXII
Descartes argues that the matter of the heavens and earth is identical, consisting solely of extension, and therefore a plurality of worlds is impossible because extended matter already occupies all imaginable space.
Chapter 119
XXIII
This chapter argues that all diversity in matter arises from motion, that there is only one kind of matter in the universe known by its extension, and that all properties of matter are reducible to divisibility and motion of its parts.
Chapter 120
XXIV
Descartes defines local motion as the action by which a body passes from one place to another, noting that motion and rest are relative to the frame of reference, as illustrated by a person seated in a moving vessel.
Chapter 121
XXV
Descartes defines motion properly as the transporting of a body from the vicinity of those bodies in immediate contact with it to the vicinity of others, emphasizing that motion is a mode of the movable thing, not a substance, and distinguishing it from the force or action that transports.
Chapter 122
Part III: Of the Visible World
Descartes outlines the principles for philosophizing about the visible world, emphasizing the infinity of God's power and goodness, warning against presuming to know God's ends, and clarifying the limited sense in which all things may be said to be created for man.
Chapter 123
I
Descartes establishes the principles of material things derived from reason rather than sensory prejudice, and emphasizes the importance of recognizing the infinite power and goodness of God to avoid underestimating His works.
Chapter 124
II
This chapter warns against the presumption of understanding God's purposes in creation and against assuming that the universe was made solely for human benefit. It cautions against overestimating the power of human thought to comprehend divine ends without natural reason or revelation.
Chapter 125
III
Descartes argues that while it is morally edifying to believe all things were created for humanity's sake, this view is not physically plausible. Many things exist or have existed without ever being seen, known, or used by humans, so God must have had other ends in creation.
Chapter 126
Part IV: Of the Earth
Descartes concludes the Principles of Philosophy by discussing the nature of perception, the distinction between internal and external senses, and the mechanical explanation of natural phenomena. He argues that all knowledge of material things must be derived from clear and distinct notions of figures, magnitudes, and motions, and that the rules of mechanics apply to physics. He introduces the concept of moral certainty versus absolute certainty, grounding the latter in God's veracity and the proper use of reason. He submits his opinions to the authority of the church.
Chapter 127
Of what is to be borrowed from disquisitions on animals and man to advance the knowledge of material objects
Descartes explains that he originally intended to write a Fifth and Sixth Part of the Principles of Philosophy on animals, plants, and man, but lacking sufficient knowledge and leisure, he instead adds a few remarks on sensory objects to avoid delaying publication. He notes that he has so far described the visible world as a mere machine of figures and motions, but must now address other sensory qualities like colors, smells, and sounds.
Chapter 128
CLXXXIX
This chapter explains what perception (sensus) is and how we perceive. It describes the human soul's principal seat in the brain, where it understands, imagines, and perceives through the nerves, which transmit motions from the body to the brain. These motions affect the soul or mind, giving rise to perceptions of the senses, or sensations.
Chapter 129
CXC
Descartes distinguishes between internal and external senses, focusing on the internal senses: natural appetite (e.g., hunger, thirst) and the passions or emotions (e.g., joy, sadness, love, hate). He explains how these sensations arise from the movement of nerves connected to the stomach and heart, and how the mind's intellectual judgments differ from the bodily passions that follow.
Chapter 130
Of the external senses; and first of touch
Descartes discusses the five external senses, focusing on touch. He explains how nerves in the skin are moved by various qualities of objects (hardness, gravity, heat, humidity) and how these motions produce diverse sensations in the mind. He also describes how moderate nerve motion yields titillation (pleasure), while stronger motion causing harm produces pain.
Chapter 131
Of Taste
This chapter explains how the nerves in the tongue and nearby parts are moved by particles of bodies separated in saliva, producing diverse taste sensations based on the figure of those particles.
Chapter 132
Of Smell
Descartes explains that the sense of smell is caused by subtle particles from terrestrial bodies entering the nostrils and moving the nerves or appendages of the brain, with different motions producing different smell sensations.
Chapter 133
CXCIV
This chapter explains the mechanism of hearing, describing how two nerves within the ears are attached to three small bones that transmit vibrations from the tympanum to the mind, producing sensations of different sounds.
Chapter 134
CXCV
This chapter discusses the sense of sight, explaining that the optic nerves' extremities (the retina) are moved only by globules of the second element, not by air or terrestrial objects, thereby producing the sensation of light and colors, as previously detailed in Descartes' Dioptrics and treatise on Meteors.
Chapter 135
CXCVI
This chapter argues that the soul perceives only insofar as it is in the brain, not in each member of the body. Three proofs are offered: maladies affecting only the brain disrupt sensation; obstruction of nerves anywhere between brain and member abolishes sensation in that member; and phantom limb pain demonstrates that sensations like pain are felt in the brain, not in the limb itself.
Chapter 136
CXCVII
Descartes argues that the mind is so constituted that mere bodily motions can excite all sorts of sensations and thoughts in it, without any resemblance between the motions and the ideas they produce. He uses the examples of written words and a sword cutting the skin to illustrate how different motions give rise to different passions and sensations, such as pain, color, sound, odor, and taste.
Chapter 137
CXCVIII
Descartes argues that the senses only perceive figure, magnitude, and motion of external objects, and that all sensations (including light, sound, pain) arise from local motion of the nerves. He concludes that qualities like color, smell, taste, and heat are not in objects themselves but are merely dispositions of objects to move our nerves in various ways.
Chapter 138
CXCIX
Descartes asserts that no natural phenomenon has been omitted from his treatise, as all phenomena perceived by the senses reduce to motion, magnitude, figure, and the dispositions of objects, with secondary qualities like light, color, and taste being merely subjective perceptions of these primary attributes.
Chapter 139
The Universality of Mechanical Principles
Descartes argues that his philosophy of material things is not new but is based on universally received principles, specifically those of Aristotle and other philosophers. He explains that he only considers figure, motion, and magnitude of bodies, and examines their interactions through mechanics, which is confirmed by daily experience. He contrasts these clear and distinct ideas with sensory qualities like colors and sounds, which only provide confused images.
Chapter 140
CCI
Descartes argues that sensible bodies are composed of insensible particles, asserting that reason can extend beyond sensory perception. He uses the example of a tree's growth to illustrate that minute particles are added imperceptibly, and criticizes those who rely solely on the senses as the measure of knowledge. He advocates for explaining nature through analogy with observable phenomena rather than inventing obscure entities like substantial forms.
Chapter 141
CCII
Descartes distinguishes his philosophy from that of Democritus, acknowledging shared assumptions about corpuscles with diverse sizes, figures, and motions, but rejecting Democritus's indivisible atoms, vacuum, and inherent gravity. He argues that his own system is no more aligned with Democritus than with Aristotle or other sects, emphasizing coherence and the rejection of unproven suppositions.
Chapter 142
CCIII: How we may arrive at the knowledge of the figures, magnitudes, and motions of the insensible particles of bodies
Descartes explains how he deduces the properties of insensible particles from clear and distinct notions of figures, magnitudes, and motions, using the principles of geometry and mechanics. He compares natural bodies to artificial machines, arguing that the same mechanical rules apply to both, and that from sensible effects one can infer the nature of insensible causes.
Chapter 143
CCIV
Descartes argues that for phenomena not perceived by the senses, it is sufficient to propose causes that could produce them, without needing to determine the actual means used by the Supreme Maker. He compares this to an artisan making two different clocks that appear the same externally, and notes that Aristotle himself only aimed to show that things could be as he explained them.
Chapter 144
CCV
Descartes distinguishes between moral certainty and absolute certainty, arguing that while moral certainty is sufficient for practical life, it may still be false in light of God's absolute power. He illustrates this with examples of common beliefs and deciphering a cipher, and suggests that the coherence of many deductions from a few principles makes it unlikely that those principles are false.
Chapter 145
CCVI
This chapter argues that we possess more than moral certainty of certain truths, achieving absolute certainty when we judge that a thing cannot be otherwise. This certainty is grounded in God's supreme goodness and veracity, ensuring our faculty of distinguishing truth from error is not fallacious when used correctly. Mathematical demonstrations, the existence of material things, and clear reasonings about them are cited as examples. The author suggests that the results of this treatise may be admitted as absolutely certain, being deduced in a continuous series from first principles, especially given the necessity of local motion in nerves for perceiving external objects.
Chapter 146
CCVII
The author submits all opinions to the authority of the church and the judgment of the wise, asserting that nothing should be believed unless compelled by reason and evidence.
Chapter 147
Appendix
Descartes presents a geometrical demonstration of the existence of God and the real distinction between mind and body, providing definitions, postulates, axioms, and propositions to establish metaphysical truths through clear and distinct perception.
Chapter 148
Endnotes
This chapter provides extensive endnotes clarifying Cartesian terminology, particularly the concept of 'idea' as used by Descartes and his followers. It distinguishes between idea as a mental act (perception) and as a representative object, discusses the twofold reality (formal and objective), and explores the classification of ideas by origin (sense, imagination, pure intellect) and by object (individual/picturable vs. general/unpicturable). The notes also address the relation between mental ideas and corporeal species, arguing against interpretations that Descartes allowed direct consciousness of organic impressions.
