Rene Descartes, the French philosopher and mathematician who invented the Cartesian coordinate system, is considered the father of modern analytical geometry. His work was crucial to the discovery of infinitesimal calculus by Gottfried Liebniz and Isaac Newton.
In addition to his extraordinary contributions to mathematics, Descartes is probably best known to the general public for his famous philosophical proposition, "cogito ergo sum," meaning "I think, therefore I am." Much of Descartes philosophical work dealt with the question of how man can know whether he or anything else really exists. According to his famous argument, thinking about his own existence proves that he exists because there has to be someone there to do the thinking. This argument is so fundamental and familiar that philosophers often refer to it simply as "cogito."