The perfect example of : dubito, ergo cogito, ergo sum ("I doubt, therefore I think, therefore I am")
But, I think Descartes is straight up wrong here. He doesn't doubt, he believes that he doubts. Descartes never got away from his evil demon - it simply blinded him. He didn't take radical skepticism far enough and if he did he would realize that the Cogito is not good enough for a proof of ontology!
The universe of the evil skepticism demon is as follows: "There is exactly one truth, that there are no other truths". If you hold that axiom, than how can one unironically conclude that the cogito is a neat proof of ontology? Seems like the philosophical equivalent of "wet pavement causes rain"