René Descartes was a French philosopher and mathematician whose work laid the foundation for much of Western philosophy and mathematics. Ghost in the Machine. 

“But I observed that, while I was thus resolved to feign that everything was false, I who thought must of necessity be somewhat; and remarking this truth — I think, therefore I am — was so firm and so assured that all the most extravagant suppositions of the sceptics were unable to shake it, I judged that I could unhesitatingly accept it as the first principle of the philosophy I was seeking. I could feign that there was no world, I could not feign that I did not exist.”

– René Descartes, Discourse on Method