Descartes: Rebuilding Knowledge from the Foundations

Where We Are

Module 4 left modern thought standing in rubble. The Aristotelian cosmos was broken (Lesson 20), Christendom was split, and Montaigne's "Que sais-je?" (Lesson 19) had made educated doubt fashionable: perhaps nothing can be known for certain at all. Most people found this unsettling. One man found it useful. René Descartes decided that if he could doubt a belief, he would throw it out — and keep throwing, ruthlessly, until either nothing was left or he hit something that could not be doubted no matter how hard he tried. On that one thing, if it existed, he would rebuild everything. This lesson follows that demolition and reconstruction, and meets the princess who found the crack in it.

The Historical Moment

René Descartes (1596–1650) was a product of the best schooling the age could offer — the Jesuit college of La Flèche, where he absorbed the scholastic Aristotelianism of Suárez (Lesson 19) and, by his own later account, the conviction that almost none of it was certain. He was a soldier, a traveler, and one of the great mathematicians in history: analytic geometry, which lets you write a curve as an equation and so fuse algebra with geometry, is his (we still call the grid of x and y "Cartesian"). That achievement shaped his whole philosophy — for in mathematics he had seen what certainty looks like, and he wanted it everywhere. He embraced the new mechanical philosophy (Lesson 20) so fully that he wrote a heliocentric physics, The World — then quietly shelved it in 1633 when he heard Galileo had been condemned (a caution he never entirely lived down). Instead he published the method first: the Discourse on the Method in 1637, in plain French so that even, he said, women might read it. It distilled the road to truth into four deceptively plain rules — accept nothing as true that is not clearly and distinctly so; divide each difficulty into as many parts as needed; ascend in order from the simplest objects to the most complex; and review the whole so completely as to be certain of omitting nothing. Four years later came the application that made him immortal, the Meditations on First Philosophy (1641), in Latin for the scholars — which he circulated beforehand to the age's sharpest minds, printing their objections, with his replies, alongside it.

The Method of Doubt

The Meditations is staged as six days of solitary thinking, and Descartes invites you to do it with him, in the first person. The plan is radical: to find the indubitable, he will treat as false anything that admits the slightest doubt. The senses sometimes deceive (a straight oar looks bent in water), so they go. Even the conviction that I am sitting by the fire could be a dream — there is no certain mark dividing waking from dreaming. Surely mathematics survives? Two plus three makes five whether I wake or sleep. To dislodge even that, Descartes deploys his most famous weapon, a worst-case scenario engineered to doubt the undoubtable:

I will suppose, then, not that Deity, who is sovereignly good and the fountain of truth, but that some malignant demon, who is at once exceedingly potent and deceitful, has employed all his artifice to deceive me; I will suppose that the sky, the air, the earth, colours, figures, sounds, and all external things, are nothing better than the illusions of dreams, by means of which this being has laid snares for my credulity.

— René Descartes, Meditations on First Philosophy I, 1641 (trans. John Veitch, 1901)

If an all-powerful deceiver is rigging even my arithmetic, then everything is in doubt at once. This is methodic doubt (doubt used as a tool, not embraced as a conclusion) pushed to its hyperbolic limit — and it is the skeptics' own acid (Lesson 19), turned up to maximum and aimed, deliberately, at finding what can survive it. The modern descendants of the demon — the brain in a vat, the simulation, the machine feeding you a fake world — are all his children (Lesson 50).

The First Certainty

Something survives. However mightily the demon deceives me, he cannot make me, who am being deceived, not exist — for to be deceived, I must be there to be deceived. The act of doubting is itself a case of thinking, and thinking cannot happen without a thinker:

[T]his proposition, I am, I exist, is necessarily true each time it is expressed by me, or conceived in my mind.

— René Descartes, Meditations II (trans. John Veitch, 1901)

In the Discourse he had put it in the four words everyone remembers — je pense, donc je suis, "I think, therefore I am," later Latinized as cogito, ergo sum. This is the Archimedean point, the single fixed place on which a world can be levered up. (Descartes was not quite the first here: Augustine, facing the ancient skeptics, had written si fallor, sum — "if I am deceived, I exist" — Lesson 11. Descartes knew the line, and insisted, fairly, that he had made of a passing remark a foundation.) And the cogito tells him not just that he is but what he most certainly is: not a body — that he can still doubt — but a thing that thinks, res cogitans. He drives the point home with a lump of wax: melt it, and every sensory quality (shape, scent, hardness, sound) changes, yet I judge it the same wax — so I grasp it not by the senses or imagination but by the intellect alone. Even bodies, it turns out, are known by the mind. The thinking self is better known than anything physical.

The Escape Hatch: God

There is a catch, and it is severe. All Descartes has so far is himself and his own ideas; he is sealed inside his own mind, with the demon still potentially rigging everything outside. To break out, he needs to prove that a good God exists who would not let him be systematically deceived. His main argument is the trademark argument: I find in myself the idea of a perfect, infinite being, and a mere finite, imperfect mind like mine could not have manufactured such an idea — an effect cannot exceed its cause — so it must have been stamped in me by an actually existing perfect being, as a craftsman's mark. (He also revives Anselm's ontological argument, Lesson 12: a supremely perfect being must have existence, since existence is a perfection.) A perfect God is no deceiver; deception is a defect. So God becomes the guarantor of the one rule Descartes now trusts:

I now seem to be able to lay it down as a general rule, that all things which I very clearly and distinctly conceive are true.

— René Descartes, Meditations III (trans. John Veitch, 1901)

With clear and distinct perception certified by a non-deceiving God, the world comes flooding back — mathematics, the external bodies, the sciences — now on a foundation no skeptic can crack. But there is a famous worm in the structure, pointed out at once by the theologian Antoine Arnauld in the fourth set of objections: Descartes uses clear and distinct ideas to prove God, then uses God to guarantee clear and distinct ideas. This is the Cartesian circle, and whether Descartes can escape it has been argued for almost four centuries. His system's certainty rests on a step that may beg the question.

The Ghost and the Machine

The reconstruction leaves Descartes a dualist. There are two utterly different kinds of stuff: mind (res cogitans) — thinking, conscious, unextended, indivisible — and body (res extensa) — extended, mechanical, unthinking, and infinitely divisible. The entire physical universe, living bodies included, is res extensa: pure machine, to be explained by the mechanical philosophy (Lesson 20) without any appeal to soul. Only the human mind stands outside the machinery. Descartes states the split as a real distinction:

[I]t is certain that I am truly distinct from my body, and that I can exist without it.

— René Descartes, Meditations VI (trans. John Veitch, 1901)

This is the move that founds modern philosophy of mind — and it immediately creates the problem that still defines the field. If mind and body are completely different substances, how do they interact? When I decide to raise my arm, how does an unextended thought push extended matter? Descartes gestured at the pineal gland, a small structure deep in the brain where, he proposed, the two meet — but a location is no answer to the real puzzle, and his most formidable critic saw straight through it. Princess Elisabeth of Bohemia (1618–1680), exiled, brilliant, and reading the Meditations closely, wrote to him in 1643 and asked the question he could never properly answer:

Tell me, please, how the soul of a human being — being only a thinking substance — can determine the bodily spirits to produce voluntary actions. For it appears that every determination of movement happens by the pushing of the thing moved, or else by the surface and shape of whatever pushes it. The first two require contact, the third requires extension — but you exclude extension entirely from your notion of the soul, and contact seems to me incompatible with a thing's being immaterial.

— Princess Elisabeth of Bohemia, letter to Descartes, May 1643 (our close rendering)

Causation in the mechanical world happens by contact and push; but a mind has no surface to push with and nothing to push. Descartes admitted, in effect, that the union of mind and body is something we feel but cannot clearly conceive — a remarkable confession from the philosopher of clear and distinct ideas. Elisabeth had found the crack in the foundation, and it never closed. Hers was only the most penetrating of the responses Descartes printed: Hobbes (Lesson 22) objected that "thinking thing" might just be a body, and Gassendi (Lesson 20), in the fifth set, needled him as "O Mind" to Descartes's "O Flesh." The Objections and Replies are philosophy conducted as a live, public argument — and the argument was not over.

Legacy: Who Listened

Descartes did not live to defend his system for long: invited to Stockholm to tutor Queen Christina of Sweden, the lifelong late-riser was made to discuss philosophy at five each morning in a freezing library, caught pneumonia, and died there in February 1650, aged fifty-three. But the title posterity gave him — father of modern philosophy — is earned by a single decisive shift: he made epistemology first. Before him, philosophy began with being — what exists; after him, it begins with the knowing subject — what can I be certain of, and how do I get from my mind to the world? That "veil of ideas," the worry that we are each locked behind our own perceptions, drives everything downstream: Locke, Berkeley, and Hume wrestling with whether we can reach the external world at all (Module 6), and Kant's whole Copernican turn (Lesson 29). The cogito makes the self the bedrock of certainty; the dualism makes it a ghost in a machine — a phrase Gilbert Ryle will coin three centuries later precisely to mock it (Lesson 51), as the mind–body problem Descartes opened becomes the spine of the modern philosophy of mind. His proofs of God revive Anselm only for Kant to demolish them again (Lesson 29). And the interaction problem Elisabeth exposed becomes the engine of the rest of this module: Spinoza will abolish the two substances into one (Lesson 23), Malebranche and Leibniz will keep them apart and let God do the coordinating (Lesson 24) — three escape routes from one stranded ghost. Even the evil demon refuses to die: it returns as the brain in a vat in Putnam (Lesson 50) and as every "what if reality is fake?" you have ever entertained. Descartes wanted to end doubt forever. Instead he gave it its sharpest modern forms — and a method so powerful that even those who rejected his answers could not escape his questions.

Reflection Questions

Descartes treats anything dubitable as if it were false, hoping to reach bedrock. Is this a legitimate route to certainty, or does it smuggle in a standard — "only the indubitable counts as knowledge" — that guarantees skepticism about ordinary life? What would you be unwilling to doubt even for the sake of the experiment?
Elisabeth argued that an unextended mind could not move an extended body, because all physical causation is push and contact. Was she right that interaction is unintelligible — or has she assumed that all causation must look like collision? Does anything in modern physics loosen her premise?
The Cartesian circle accuses Descartes of using clear ideas to prove God and God to validate clear ideas. Can you think of any foundation for knowledge that does not eventually have to vouch for itself? Is circularity here a fatal flaw or an unavoidable feature of any starting point?

Self-Check Quiz

What is the role of the "evil demon" in the Meditations?
- A. A real being that Descartes believes exists and torments the mind, so that it must be defeated before knowledge can proceed
- B. A symbol of the Church's authority over reason, which Descartes uses to show that thinkers must submit to received doctrine
- C. A worst-case hypothesis designed to cast doubt on even mathematics, so that whatever survives it will be genuinely indubitable
- D. An explanation of why the senses are sometimes deceived, as in illusions and dreams, which Descartes thinks accounts for error
Why does Descartes think the cogito survives even the most radical doubt?
- A. Because the very act of doubting is an act of thinking, and thinking cannot occur without a thinker who exists
- B. Because God guarantees it, and Descartes can trust his own existence only after proving that God exists and is no deceiver
- C. Because his senses confirm that he has a body, and so even a deceiver cannot deny that a living person is there
- D. Because mathematics shows that thinking is a kind of calculation, and calculation cannot occur unless something exists to carry it out
Explain the Cartesian circle. Why does Arnauld's objection threaten the certainty Descartes was trying to establish?
State Descartes's mind–body dualism, and explain the interaction problem that Princess Elisabeth pressed against it. Why is the pineal gland not an adequate answer?
Augustine, facing the skeptics, wrote si fallor, sum — "if I am deceived, I exist" (Lesson 11). How does Descartes's cogito resemble Augustine's insight, and what does Descartes claim to have done with it that Augustine did not?

Show answers

C — The demon is a tool, not a belief: by supposing an all-powerful deceiver who could rig even "2 + 3 = 5," Descartes manufactures the most extreme doubt conceivable, precisely so that anything still standing afterward is certain beyond any skeptical attack.
A — To be deceived, dreaming, or doubting, one must exist to undergo it; each is a mode of thinking, and thinking entails a thinker. So "I exist" is necessarily true every time I think it — the one proposition the demon cannot falsify.
The circle: Descartes uses clear and distinct perception to prove that God exists, then invokes God's non-deceiving nature to guarantee that clear and distinct perceptions are true. The criterion underwrites the proof, and the proof underwrites the criterion. Arnauld's worry is that this is question-begging — if clear and distinct ideas need God's guarantee, they cannot be trusted before God is proved, but the proof itself relies on them. The whole rebuilt edifice of certainty may rest on a circular step.
Dualism holds that mind (res cogitans) is thinking, conscious, and unextended, while body (res extensa) is extended, mechanical, and unthinking — two wholly distinct substances. The interaction problem: if the mind has no extension and no surface, how can it move the body, given that physical causation works by contact and push (Elisabeth's point)? Pointing to the pineal gland only names a place where mind and body supposedly meet; it does not explain how an unextended thing could exert or receive a physical push, which is the actual difficulty.
Both notice that the very occurrence of error or deception guarantees the existence of the one erring: you cannot be deceived into thinking you exist if you do not. Descartes's claim to originality is that, where Augustine used the point as a local rejoinder to skepticism, he made it the single foundation — the Archimedean fixed point — on which an entire reconstruction of knowledge is deliberately built.