[And that's a good thing! Because really, as far as he's concerned it's all theories, nothing more - an interesting intellectual exercise, maybe, but that's about it.]

Well, of course I know they're there! But the theory is based on how you only have your experience to base it off of. You may be interacting with highly advanced machines that happen to look like people, for example! Or it may all be some sort of hologram projection - you have no way to know whether anyone you're talking to is really real and alive, you can only answer for yourself!

I don't believe in such things myself, but they're interesting to study regardless.