Descartes; If I don't remember badly, he was also the author of the 'onthological proof' of God's existence (which in my humble opinion is not a proof at all); I think his contributions to Mathematics and Geometry was more relevant.

Bonjour Cogito, ergo sum est une expression latine du philosophe René Descartes qui signifie « je pense, donc je suis ». ... Tout un programme !

Hi Aiboud: It was DESCARTES and, like Mike just said, it is: "Cogito ERGO sum" or in spanish: "Pienso, por lo tanto existo" (I think, therefore I exist). In french: " Je pense, donc je suis".

it's "cogito ERGO sum"