René Descartes

It's the birthday of the father of modern philosophy and modern mathematics: the rationalist René Descartes, (books by this author) born in La Haye, Touraine, France (1596). His most famous phrase is "Cogito ergo sum," or "I think, therefore I am."

He was sent away to study at a Jesuit school. The priests allowed him to stay in bed all morning because of his delicate health, and this became a routine that he followed throughout his life. When he met his fellow mathematician Blaise Pascal in 1647, he told Pascal that the only way to stay in excellent health and to do good mathematical work was to not allow anyone to get you up in the morning until you were ready.

It was during one of his mornings in bed that Descartes invented coordinate geometry, while he watched a fly hovering in the air. He realized that the fly's position at any moment could be described as its distance from three intersecting lines or axes.