The guy who in the end just said "Fuck it, a Wizard (God) did it."? It's really interesting to see the logical hole he digs for himself in the book but then cops out in the end with a "Well, God is good so he wouldn't let something bad happen to us so we must exist."

https://en.m.wikipedia.org/wiki/Trademark_argument. You kind of have it backwards, he first concluded that he existed (I think therefore I am). Then because of that (and other arguments), he believed god exists and is good, so he wouldn’t deceive his senses.

He ended with that about non-first-person existants. His argument works for proving oneself exists to oneself.

He basically defined existence as thinking, then turned it into a tautology. And then it blew up in his face when he realized that he couldn't prove anything else was thinking. And in fact even if there wasn't some devil tricking him (ala brain in a box hooked to the Matrix), things like rocks and water could never exist.

His entire philosophy was flawed right from the start. But it usually is. That's why so many philosophy texts take the first chapter (or volume) refuting philosophers who came before them.