It's pretty easy to extend this further. The photons that bounce into your eyes when you look at a tree are real, but who's to say they got to your eyes by bein

It's pretty easy to extend this further. The photons that bounce into your eyes when you look at a tree are real, but who's to say they got to your eyes by being emitted from a big ball of fusion, through the 8 minute trip to Earth, and off that tree into your eyes? If someone put a sufficiently detailed screen there, we wouldn't know. Similarly, how do you know you're not a brain in a jar? etc. GP's point, that there's a hierarchy to truth/reality, is probably more useful. All we have are observations, and we can use those observations to draw conclusions about the state of the world. We must always be aware that it's always a map, though -- we can never know the territory.