Meditations on First Philosophy

Descartes published the Meditations in 1641 as six days of concentrated doubt and reconstruction. The project is radical: tear down every belief that admits of the slightest uncertainty, find something that cannot be doubted, and rebuild knowledge from that foundation.

The First Meditation pushes doubt as far as it will go. The senses deceive, dreams are indistinguishable from waking life, and an evil genius might be fabricating the entire world of experience. The Second Meditation finds the floor: even if everything else is illusion, the act of thinking proves the existence of the thinker. "I am, I exist" holds every time it is conceived. What this "I" is turns out to be a thinking thing (res cogitans), known more certainly than any body. The Third Meditation argues from the idea of God, a being of infinite perfection, to God's actual existence: the cause of the idea must have at least as much reality as the idea represents. The Fifth Meditation offers a second proof, the ontological argument. The Sixth Meditation, with God's veracity secured, recovers the existence of the external world, though the world known through clear and distinct ideas of mathematics, not through the unreliable testimony of the senses.

The Meditations is short enough to read in an afternoon, but the problems it raises occupy the next four centuries. The mind-body split, the criterion of clear and distinct perception, the role of God as guarantor of knowledge, the method of radical doubt as philosophical starting point: these become the terms in which Spinoza, Leibniz, Locke, Hume, and Kant conduct their own investigations.