Descartes: Doubt, Reason and Certainty

René Descartes, a French philosopher and mathematician, sought a foundation for knowledge that could not be doubted. In his 1637 Discourse on the Method he resolved to doubt everything that could possibly be false, and found one thing he could not doubt: that he was thinking. His conclusion, 'I think, therefore I am', became the starting point for rebuilding knowledge through reason. Descartes stressed clear and distinct ideas and mathematical reasoning, an approach later called rationalism, in contrast with Bacon's emphasis on experience.

He also pictured the physical world as a vast machine explained by matter in motion, and he sharply separated mind from body. Many of his specific theories, such as his explanation of planetary motion by swirling vortices, were later rejected by followers of Newton. Yet his method of systematic doubt encouraged readers to test every inherited belief, a habit central to the Enlightenment.