system
Given the following post, respond as if you're commenting on Reddit:
human
Question on Descartes' Evil Genius Argument

On his first meditation Descartes applies hyperbolic methodical doubt to the foundations of all his previous beliefs and he does that in 3 waves: the argument from the senses, the argument from dreaming, and the evil genius argument. My question is about the third wave, namely how does he know that his reasoning capabilities are safe from the evil demon's illusion. I understand that later he arrives at the Cogito because even if the evil genius is deceiving him, he must exist. My question is more about how can Descartes still trust his reasoning capabilities as the evil demon can deceive him about the most basic laws of logic and inferences.
gpt
If the evil demon is deceiving him he must exist to be deceived.

If the evil demon is not deceiving him, he must exist as he believes he exists.

A thing must exist if it is to be deceived or believe. Perhaps a tacit corrolary (my read) is an implicit call out to some contemporary (of the Era) Christian dogma which argued that believing(faith) was a fundamental force in itself, and thus guaranteed existence (faith being so powerful as to be beyond corruption or deception). 

Personally Descartes' logic has never been compelling, especially with regard to his ontological proof in God. He makes assumptions without fully exhausting possibilities, which is sloppy.