Consider this. I look outside an see a tree. How do I know it's real tree and not an illusion imposed on my mind by a demon?

Well, I can remember this tree being there yesterday and the day before. If someone messed with my mind just now I would've noticed the inconsistency.

This doesn't discard *all* the possible hypothesises involving deceptive demons. For instance, what if the demon not only created the appearence of the tree now but likewise edited all my memories? But it discards *some* of them.

We can keep discarding more and more hypothesises this way. I go outside and touch the tree and notice that it's not just a visual illusion. This requres the demon to make his illusion not only visual but also tactile. I can ask people around me whether they think there is a tree. If they say yes, this requres the demon to falsify their replies as well.

And so on and so forth. Every test I apply discards more and more sceptic explanations leaving only the ones consistent with all the tests, requiring the demon to consistently account for more and more things. And therefore even if I was initially 50:50 between a naturalistic and sceptic explanation for the tree I end up more and more confident that the tree actually exists...

Except, what if the demon messed up with my epistemology and I only think that this is the valid way to reason about matters, while actually it's not? But this is just a matter of more tests - whether my epistemology systematically works or not. And now the demon has to falsify this as well. What began as a question about one specific thing became entangled with all kind of facts about reality. In the end the illusion that demon creates has to be all-encompasing. It has to systematically produce the same results as a naturalistic universe for *every* possible test.

At which point... what is the actual difference between the "reality" and such "illusion"? They work according to the same rules and produce all the same observations. We just so happened to give them different names in natural language, but their substance is the same. Therefore our sceptic description of our universe is isomorphic to the naturalistic one. And the disagreement is just in semantic.

The only difference is that sceptic scenario supposes that there is a "demon" outside our reality who created everything. Which is a specific claim that I do not have any evidence for, so according to the epistemic techniques that systematically produce good results in our reality I assume it to be unlikely. Even if the epistemology of meta-reality worked completely differently, it doesn't seem likely that I would be able to just guess a fact about it for no reason. In any case, this is a separate topic.