Rene Descartes, widely regarded as the father of modern philosophy, broke with the Aristotelian tradition, helping establish modern rationalism. He argued for a mechanistic universe in opposition to Aristotle's views on causality. He also made important contributions to mathematics and physics.
Descartes is most famous for his statement, "Cogito, ergo sum," often translated as, "I think, therefore I am." He argued that this provided an irrefutable foundation for knowledge. Even when humans are doubting, they are still thinking, so they can therefore always know that they exist. On this premise Descartes constructed his system of philosophy, later known as Cartesian Dualism.
In mathematics, Descartes invented the system of Cartesian coordinates that allows geometric statements to be expressed in algebraic form. He also invented the practice of using "x" and "y" to represent unknown variables in equations. His early work on optics was the first to mention the law of reflection, which states that light is reflected at the same angle at which it approaches a surface. He supported the idea of conservation of momentum and laid out the principle of inertia. Descartes worked in other areas of science as well, producing some early attempts to explain the origin and development of the universe in purely mechanistic terms without resorting to supernatural intervention.