Sure, there might be only one person on the universe, and all of us are in his dream, and we do arbitrary things, plus he changes the past. That will be difficult to refute to prove something. Or magic exists (which we can't prove it doesn't) and changed the facts of a cases -- e.g. it was X that stabbed Y, but a magician transformed as X.

That said, why would you want to prove something beyond "reasonably"?