Rene Descartes, (1596-1650), French philosopher and mathematician. While travelling in Europe as a young man, Descartes formulated his system of Cartesian coordinates and analytical geometry. This meant that for the first time geometrical problems could be solved algebraically, and vice versa. Descartes' philosophical theories were rational in nature, appealing to reason as a source of knowledge and believing that information received through the senses was unreliable. His unshakeable belief in the existence of thought underpins his famous line 'cogito ergo sum,' 'I think, therefore I am.'