When there’s nothing left he still is left with himself and nothing else. Regardless of whether or not he is being deceived by some demon or his beliefs are wrong, he is able to see that even if he has the ability to doubt something he must be existing to even doubt it in the first place. The fact that he can think is what assures himself of his own existence, and a deceiving god cannot negate that. From this point on, Descartes can continue in his examination of reality without worry that he is by all means existing.
What Descartes means by his quote “I think, therefore I am.” Is he alluding to verification and validation of his existence through his ability to think. When there’s nothing left he still is left with himself and nothing else. Regardless of whether or not he is being deceived by some demon or his beliefs are wrong, he is able to see that even if he has the ability to doubt something he must be existing to even doubt it in the first place. The fact that he can think is what assures himself of his own existence, and a deceiving god cannot negate that. In addition his theory of his mind existing validating his existence he acknowledges the idea that if he and individual who has thought; means that there is an entity who has a prime and perfect mind of thought, who he thought to be was God.
