In French: "Je pense donc je suis".

I like Augustine though:

"If they say, "What if you are mistaken?" -- well, if I am mistaken, I am. For, if one does not exist, he can by no means be mistaken. Therefore, I am, if I am mistaken that I am, since it is certain that I am, if I am mistaken. And because, if I could be mistaken, I would have to be the one who is mistaken, therefore, I am most certainly not mistaken in knowing that I am. Nor, as a consequence, am I mistaken in knowing that I know. For, just as I know that I am, I also know that I know."