René Descartes was a polymath who fundamentally altered the course of Western thought. His genius was dual-faceted: in mathematics, he created the Cartesian coordinate system, which bridged the gap between algebra and geometry (analytic geometry). In philosophy, he sought a foundation for knowledge that was beyond doubt, leading to his famous dictum 'Cogito, ergo sum' (I think, therefore I am). This established the 'mind-body dualism,' the idea that the mind and body are distinct substances. His work encouraged a shift toward rationalism and the use of the scientific method, emphasizing that human reason is the primary tool for understanding the universe.