In Discourse on Method (1637), his first published work, French philosopher and scientist René Descartes combined an intellectual autobiography with a popular presentation of the system he was to develop more rigorously in his Meditations (1641) and Principles of Philosophy (1644). Meditations begins with a radical attempt to doubt all past beliefs, but finds a proposition that resists doubt in the existence of the self as a thinking thing. It then uses this initial certainty as a basis for arguing that God exists, that mind and body are distinct, and that we can achieve certainty in the sciences if we assent only to clear and distinct ideas, provided we have first shown that God would not deceive us about those ideas. The Principles uses the metaphysics and epistemology laid out in the Meditations as the foundation for an ambitious attempt to provide a...