Alan, have you heard of the brain-in-a-vat (or jar) thing? It's the sort of philosophical problem they give first-year philosophy students to chew on, to see how well they can construct an argument and counterargument. In essence it postulates that you don't actually exist at all; everything about you, everything you perceive and have ever perceived, every single memory of everything, exists only inside a brain being kept floating in a vat, created by a mad scientist. Everything that you think you perceive is in fact just a virtual reality simulation occurring in said brain in a tank; not just now, but always, because all your memories have been created and implanted. They didn't actually happen; they were made up.
The interesting thing about this is that it can't actually be disproven - if the simulation is good enough, then it's literally impossible to distinguish between that sort of virtual reality and everything that you think you have ever seen, heard, touched, tasted, smelt or experienced in any way. Yes, I know that it looks and it feels as though there's an external world out there of lots of different kinds of stuff, but can you rigorously prove that that's the case? Actually, no, you cannot. Can't be done.
I use this an example to show that while issues about perception constitute a huge and fascinating area of philosophical speculation, nobody who's allowed out in public and is deemed to be able to dress themselves really takes scepticism about the external world to that degree. Nobody actually believes that we're all just brains in vats. Nobody believes it. Nobody thinks that germs don't cause disease. Nobody thinks that objects subject to gravity (as here on Earth) won't fall when dropped. And nobody sane, once acquainted with the evidence, denies evolution either.
