On this day 389 years ago, the French philosopher and mathematician René Descartes had the dreams which inspired his famous work, Meditations on First Philosophy. Descartes is most famous for the statement cogito ergo sum, which means “I think, therefore I am.” In Meditations on First Philosophy, Descartes discusses the difference between things which can be called into doubt and the things which can be known for sure.
In Meditation III, Descartes argues that the existence of God can be known for sure. He says that there are three types of ideas: innate (ideas which have always been with us), invented (ideas which come from our imagination), and adventitious (ideas coming from experiences of the world). We understand the perfect idea of a perfect being, and this cannot have been cause by anything less than a perfect being, so the idea of God is innate and placed in us by God, and God must exist.
Descartes’s philosophical legacy is known as Cartesian philosophy. In the field of mathematics, he created the theoretical basis for calculus, and he created the field of analytic geometry through the merger of algebra and Euclidean geometry. The Cartesian co-ordinate system is used to define each point in a plane uniquely.