Descartes

It's the birthday of the man who wrote, "I think, therefore I am," René Descartes, (books by this author) born in Touraine, France (1596). Though he's often been called the father of modern philosophy, he considered himself more of a mathematician and a scientist than a philosopher. He conducted all kinds of experiments. He studied refraction and the properties of rainbows. He dissected animals and wrote about how they were constructed like machines. He invented analytic geometry, a precursor to calculus. And he worked for a long time on a theory of science that was similar to what became the scientific method.

Descartes only got into philosophy after he learned that Galileo had been persecuted by the church. He worried that some of his scientific ideas could be similarly controversial, so he decided to write a book to prove that skepticism about the laws of nature was a necessary step in understanding nature. And that book became his Discourse on Method (1637), in which he described his own experience of coming to doubt everything, even his own existence, until he realized that the one thing he could not doubt was the existence of his own thoughts. He decided that if he was able to think then he must exist, and he so he wrote the famous line, "I think, therefore I am."

René Descartes said, "If you would be a real seeker after truth, it is necessary that at least once in your life you doubt, as far as possible, all things.