Okay, so we all know what René Descartes once said: "I think therefore I am." The real question is: so what? The guy becomes an immortal genius because he proved he exists? One time, we successfully identified the color of the sky. Do we get to be remembered forever, too?
Actually, Descartes did a little bit more than assert his own existence. You remember learning the Cartesian coordinate system in high school? That was another of René's little discoveries. Given that the Cartesian coordinate system is the basis of analytic geometry, this discovery alone would have been enough to assure him a place among the All Time Greats.
But there's more:
Not too shabby.
And about that "I think, therefore I am" business… No, that is not Descartes's entire contribution to philosophy—not by a long shot. Descartes's actual interest is in attaining absolute certainty, a framework of knowledge that is incapable of being doubted. What he shows is that even if we agree to reject every claim about which the slightest, tiniest, teensy-weensy doubt can be raised, even then your own existence cannot possibly be questioned.
So, is Descartes all about being a sort of 17th-century yoga teacher, telling us that now all of us can fling open our arms and joyously proclaim: "I am! Yes, I am"? Not at all. The existence of the "I" is only the starting point, the foundation of Descartes's whole philosophical system. Assuming only the existence of himself, Descartes goes on to prove first the existence of God and then the existence of an external world of material objects.
By the end of Descartes's argument, laid out most memorably in his Meditations on First Philosophy, you are supposed to know, with absolute certainty, quite a few things:
Who knew that the insult you're just taking up valuable space was actually a philosophical position?
Descartes's is a radically new view of the world, but there is one big problem with it: it doesn't explain how the mind and the body can connect with one another. Why is it that when we smash our material finger with a material hammer our immaterial mind experiences pain, as well as an overwhelming desire to sharply kick the dog quietly lying at our feet? (He could have had the decency at least to warn us that we were about to hurt ourselves.)
This is the mind-body problem, the one problem that even smarty-pants Descartes could never solve. And the attempt to deal with this problem, in one way or another, occupied nearly every philosopher who came after him.
No wonder Descartes is called the Father of Modern Philosophy. You can love him or hate him; for most people (especially those who knew him), it's the latter. You can agree with him or reject everything he said; somehow almost everyone now goes for door number two. But, father that he is, you can't ignore him.