Cogito Ergo Sum

With God established as a non-deceiver, Descartes is finally in a position to address what Meditation I suspended: does the external world exist? His argument in Meditation VI is essentially this. He has a strong, involuntary tendency to believe that his sensory experiences are caused by material objects outside him. He did not choose this tendency; it simply comes with being the kind of creature he is. If God is not a deceiver, and if God gave him a faculty that spontaneously produces beliefs about an external world, then that world must actually exist — because otherwise God would have created in him an irresistible inclination toward falsehood, which a good God would not do.

Descartes adds an important qualification: his senses do not tell him everything about the nature of material things with precision. They are reliable guides to what is beneficial or harmful to him, but not to the exact mathematical properties of objects. For precise knowledge of the external world, he still needs the intellect, not the senses alone. This is consistent with what the wax argument showed in Meditation II.

The central metaphysical conclusion of Meditation VI is the real distinction between mind and body. Descartes argues that the mind and body are not merely different aspects of one thing, or different ways of describing the same substance. They are genuinely, fundamentally different kinds of things — two distinct substances.

His argument turns on what he can and cannot clearly and distinctly conceive. He can clearly and distinctly conceive of himself as a purely thinking thing, with no spatial extension whatsoever. He can clearly and distinctly conceive of his body as a purely extended thing, with no thought whatsoever. Since he can conceive of each without the other, and since God could create anything he can clearly conceive, each could in principle exist without the other. Therefore they are genuinely distinct substances.

The mind is a thinking thing (res cogitans): unextended, indivisible, essentially characterized by thought. The body is an extended thing (res extensa): spatial, divisible, essentially characterized by size, shape, and motion. This position is called substance dualism, and it is Descartes’ most influential and most controversial contribution to philosophy of mind.

Descartes acknowledges, in Meditation VI, that the mind and body are not merely joined like a pilot in a ship — occupying the body from the outside, observing it at a distance. They are deeply, intimately intermingled. When the body is damaged, the mind feels pain; when the mind is afraid, the body trembles. There is clearly some causal interaction between the two.

But here is the problem. If mind and body are genuinely distinct substances — one unextended, one extended; one thinking, one merely mechanical — how can they interact? Causation, as Descartes understands it, involves a transfer of reality or force from cause to effect. How can something with no spatial properties whatsoever produce a change in something that is entirely defined by spatial properties? How does a thought move a hand?

Descartes suggested that interaction occurred through the pineal gland, a small structure at the center of the brain. But this is not really an explanation — it just relocates the problem. Pointing to the pineal gland tells us where mind and body meet, not how they can meet given what Descartes says they are. His contemporaries noticed this immediately, and it generated a century of philosophical responses: occasionalism (God mediates every interaction), parallelism (mind and body run in parallel without actually interacting), and eventually the rejection of dualism altogether.

At the end of the Meditations, Descartes has done the following: stripped all beliefs that could possibly be doubted, found one that cannot be doubted (the cogito), used that to establish the existence of God, used God to validate the faculty of clear and distinct perception, and used that faculty to establish the existence of the external world and the real distinction between mind and body. The project he set out in Meditation I is, in his view, complete.

Whether it actually succeeds is another matter. The causal argument for God relies on a principle many find questionable. The ontological argument is contested. The Cartesian Circle may or may not be avoidable. The interaction problem is genuinely unresolved. And the cogito itself, as we saw, is not as simple as it looks.

But even where Descartes fails, he fails productively. The questions he raised — Can we be certain of anything? What is the relationship between mind and body? What role does God play in underwriting knowledge? — are among the most important questions in the history of philosophy. Every philosopher who comes after him in this course is, in one way or another, responding to what he started.

Descartes believed that reason, properly deployed, could deliver certainty about the nature of the world, the existence of God, and the structure of the mind. Less than a century later, David Hume will argue that this faith in reason is almost entirely misplaced. For Hume, the mind is not a transparent window onto reality. It is a bundle of perceptions, governed by habit and association, capable of far less than Descartes imagined — and honest about it.