Je pense, donc, je suis.

"I convinced myself that there is nothing at all in the world, no sky, no earth, no minds, no bodies; is it not therefore also true that I do not exist? However, I certainly did exist, if I convinced my self of something. There is some unidentified deceiver, however, all powerful and cunning, who is dedicated to deceiving me constantly. Therefore it is indubitable that I also exist, if he deceives me…he will still never bring it about that I am nothing as long as I think I am something."

- from the 2nd of 6 meditations in 1636 by René Descartes (1596 - 1650)