René Descartes

Descartes set out to rebuild knowledge by refusing to accept anything he could not establish for himself. He doubted the evidence of the senses, since they sometimes deceive; he doubted even mathematics, by imagining an all-powerful deceiver who might make him err in the simplest reasoning.

What survived was the act of doubting itself. To be deceived, he had to be thinking, and to be thinking he had to exist. Cogito, ergo sum — I think, therefore I am — became the first certainty, and the foundation from which he tried to prove God and the reality of the material world. The method made the knowing subject, rather than the world, the starting point of philosophy.

Descartes concluded that there are two kinds of substance: thinking substance, which is not extended in space, and extended substance, which occupies space and obeys mechanical laws. The mind is the first, the body the second, and on his account a person is a union of the two.

The separation gave later science a free hand to describe the body as a machine, but it left an obvious difficulty. If mind and body are utterly different, how does a decision move a limb, or a wound produce pain? Descartes located the interaction in the pineal gland, an answer that satisfied few, and the mind-body problem he framed has occupied philosophy ever since.