Father of modern philosophy. René Descartes was a 17th-century French philosopher, mathematician, and scientist often called the “Father of Modern Philosophy.” He believed that true knowledge had to be built on clear, rational thinking rather than tradition or assumptions. His most famous idea is “Cogito, ergo sum” — “I think, therefore I am” — which means that the act of thinking proves your own existence. Descartes also made huge contributions to mathematics by creating analytic geometry, whic