René Descartes

“I may lay this down as a principle, that whenever this sentence I am, I exist, is spoken or thought of by me, it is necessarily true.”

“They could never use words or other signs arranged in such a manner as is competent to us in order to declare our thoughts to others.”

“Because on this side I have a clear and distinct idea of my self, as I am only a thinking thing, not extended; and on the other side because I have a distinct idea of my body, as it is only an extended thing, not thinking, it is from hence certain, that I am really distinct from my body.”

“I will suppose, not an infinitely perfect God, the fountain of truth, but that some evil spirit which is very powerful and crafty has used all his endeavours to deceive me.”