Do you exist?
“What? Of course I exist. What a stupid question.”
Okay, now that I’ve proven that I have psychic powers by predicting your answer ahead of time (You believe me, right?), take a step back from the question. How do you know that you really exist? It is, after all, possible that you are nothing but a Matrix-style AI simulation of consciousness being studied by a highly advanced future society. Can you prove that you’re not? It’s entirely conceivable that a society could develop the technology necessary to simulate reality—it’s not even that much of a stretch to suggest that we may be able to do this within our lifetimes (combine modern video games with Japan’s Earth Simulator, then extrapolate). Once you’ve accepted that, how can you know for sure that you’re not an AI operating within a highly advanced reality simulator? Granting that such a thing is theoretically possible, you really can’t. If you can’t entertain the mere possibility, however, I have bad news for you: you’re suffering from a chronic mental illness called certainty.