Descartes! "I think, therefore I am." (I've seriously considered tattooing "cogito ergo sum" on my body.)
There's really only one thing that you need to know, and it's that he thinks nothing can be proven other than that you, yourself exist. He tried to disprove everything that he could and go back to the roots of what he could no for sure, and in doing so he only knew one thing for certain. That being that he was actually able to question and doubt beliefs, which meant that he was thinking. And if he's thinking, then he, himself, must exist, for even if something was deceiving him... the fact that he was actually being deceived proved that there was something to deceive. (If that makes sense.)
And if I'm not mistaken, I do believe that "I think, therefore I am," is a misquote... much the same way "play it again, Sam" is.
