Except it's "Cogito ergo sum", not "I think therefore I am" "I" here is whatever is the origin of these thoughts, and nothing more. Not you. He explained it a

Except it's "Cogito ergo sum", not "I think therefore I am" "I" here is whatever is the origin of these thoughts, and nothing more. Not you. He explained it as "we cannot doubt of our existence while we doubt" So no, there is no postulate, just observation. However far the doubt goes... well, there is still something doubting, and thus thinking (doubts are thoughts), that has to be. Otherwise there would be no doubt. That's all. Wether it's an individual human's mind or the self-deceiving / self-ignorant projection of a 6 dimensional octopus, wether thinking really is an independent creative process or just the deterministic result of outside events doesn't matter.