Cogito Ergo Sum. This was the conclusion of
Descates' quest to find out if anything can be known with absolute certainty. He proceeded to do this by doubting everything.
Unknown to many, this line of thinking has a precedent in St. Augustine, who came to roughtly the same conlusion as Descartes. Praphrasing: "Of this last doubt, I cannot doubt: that I doubt."
See also:
The Method of doubt
Descartes was wrong