Sorry ;) It's an obscure subset of philosophy. I took a class on it during undergrad. A lot of are we brains in a jar? Are we living in heaven? Is everything an illusion kind of thing? There's a famous philosopher who supposedly refuted the theory that everything is an illusion by kicking a rock. That kind of thing.

Also a Chinese Philosopher wrote about "Am I a man dreaming I was a butterfly or a butterfly dreaming that I am a man" kind of thing.