His name was Jean-Paul Sartre. He looked at Descartes's statement "I think, therefore I am" very deeply and suddenly realized, in his own words, "The consciousness that says ‘I am' is not the consciousness that thinks." What did he mean by that? When you are aware that you are thinking, that awareness is not part of thinking. It is a different dimension of consciousness. And it is that awareness that says "I am." If there were nothing but thought in you, you wouldn't even know you are thinking. You would be like a dreamer who doesn't know he is dreaming. You would be as identified with every thought as the dreamer is with every image in the dream. Many people still live like that, like sleepwalkers, trapped in old dysfunctional mind-sets that continuously re-create the same nightmarish reality. When you know you are dreaming, you are awake within the dream.
— Eckhart Tolle