René Descartes was a French mathematician, natural scientist. and philosopher best remembered for coining the phrase 'Cogito, ergo sum' (1 think therefore I am). He wrote in physiology, cosmology, coordinate geometry, and optics, but he is best known as the "father of modern philosophy. Descartes was not a direct Enlightenment participant, but his legacy would be his effect on others who made contributions to the scientific, political and social transformations that took place during this age of reason.