Born on March 31: Rene Descartes -- originated "Cogito Ergo Sum"

from The Writer's Almanac (Garrison Keillor, American Public Media):

March 31 is the birthday of philosopher Rene Descartes, born in La Haye en Touraine, France (1596), called the father of modern philosophy, but he considered himself a mathematician and scientist. He became interested in philosophy when he heard that the church persecuted Galileo for his scientific theories. Descartes realized some of his own theories were also controversial, so he wrote a book called Discourse on Method (1637), about the necessity of doubt in scientific inquiry. He also wrote about beginning to doubt everything about his life, even the fact of his own existence. But in the process of doing so, he realized that he couldn't doubt the existence of his own thoughts, and he produced his most famous line: "I think, therefore I am."