I think about this sometimes, usually when writing. This is because different characters see things in different ways, especially when one of them is mentally ill. I've concluded that reality is what you make it, and that no one lives in the exact same reality as anyone else.

Note: When I say reality is what you make it, I don't mean to imply that we think ourselves into/out of hallucinations and delusions. It was more of a "believe what you want to believe" kind of thing.

Okay, I'm just going to offer an example, because I'm not explaining my thoughts well.
Basically, if you're religious and you believe in God, He's real to you even no one else believes in Him. Does that convey the thought?

I don't know what I'm talking about...