Meditations on First Philosophy begins with two introductions. The first is addressed to the theology faculty at the Sorbonne (a university in Paris), the second to his lay readers. He outlines some of the objections to the Discourse and asserts that his critics generally ignored his chains of logic and only attacked his conclusions. He pledges to return to the two criticisms he finds worth considering. He asks his readers to approach the rest of the book with an unbiased mind.

The first meditation reiterates material from the Discourse. Responding to an objection to his critique of the senses, Descartes agrees that he would seem a madman if he argued he was not sure that he possessed a body. But he also points out that in his dreams he experiences a reality as convincing as his waking reality. He can find no sure way to distinguish between waking life and sleep. He then goes on to argue that if we dream of hands, feet, eyes, and bodies, then they must actually exist. When we dream, he continues, we use information we gathered from reality. Even if particular complex objects do not exist, at least the basic colors and shapes that compose them exist. In the same way, we can say the physical sciences are uncertain because they study composites. Arithmetic and geometry study simple objects (shapes, angles, numbers) and are therefore trustworthy. He trusts his perceptions of self-evident truths such as simple shapes and numbers because he believes in an all-powerful God that created these things.

Descartes admits that he cannot be sure that God is not playing some sort of trick on him. However, because he believes that God is good, he knows that God would not deliberately deceive him. Therefore, to rebuild his knowledge on the basis of doubt, he decides to pretend that a “malignant demon” is bent on tricking him. This powerful demon has created the illusions of the physical world to deceive him. With this in mind, Descartes sets out to prove, using only reason, that some things are beyond doubt.

Most of meditation II is devoted to discovering whether there is anything about which Descartes can be absolutely certain. First he decides he can be certain that he exists, because if he doubts, there must be a thinking mind to do the doubting. He does not yet accept that he is a thinking mind inside a body. After all, the demon could have convinced him that his body and the physical world exist. He moves to another question: what is the “I” that is doing the thinking?

The answer is that the mind is a purely thinking thing. Descartes concedes, however, that though what he perceives with his senses may be false, he cannot deny that he perceives. So the human mind is capable of both thought and perception. He explains this using the example of a piece of wax. We understand that solid wax and wax melted by a candle are both wax. Therefore perception is not strictly a function of the senses. It must be the reasoning mind that makes this judgment. Because the senses can be deceived, physical objects, including bodies, are properly perceived only by the intellect, and the mind is still the only thing he can be certain exists.

In meditation III, Descartes says he can be certain that perception and imagination exist, because they exist in his mind as “modes of consciousness,” but he can never be sure whether what he perceives or imagines has any basis in truth. He then expands on his argument for the existence of God from the Discourse. He examines his own mind to see whether there is anything in him that would allow him to make God up. Not only is God perfect, but God is also infinite and all powerful. Descartes knows that he himself is finite. He reasons that it is not possible for a finite being to dream of infinity. Therefore the idea of the infinite must come before the idea of the finite, before any person can begin to think of what he or she is.