Meditations on First Philosophy

In six meditations, written as though over six days, Descartes resolves to doubt everything that can be doubted: the senses, which sometimes deceive, the waking world, which might be a dream, even mathematics, which an evil demon might falsify. One thing survives the doubt, that he who doubts must exist, I think, therefore I am. From that point he tries to rebuild knowledge, proving to his own satisfaction the existence of God, the distinction between mind and body, and the reality of the physical world.

Published in Latin in 1641, the Meditations opened modern philosophy, and readers have argued about their proofs ever since; al-Ghazali's account of his own doubt in al-Munqidh, also in this library, makes a fascinating comparison. D. E. Wittkower reads it in about three and a half hours.