(1) I see something, and I recognize it as a tree.
(2) What I’m looking at might be a real tree, or might be an illusion. (let’s ignore other possibilities)
(3) If the tree is real, I’m looking at a tree.
(4) If it is an illusion, what I’m seeing is an illusion.
(5) Phenomenally, on the start there is just what I’m looking at. I need to do further inspection, to figure out if what I’m looking at is real, or an illusion.
(6) It is hence a contingent property of what I’m looking at, to be real or illusionary.
(7) Only existent things can have properties (in this case of “being real” and “being illusion” )
( 8 ) Hence what we see exists (as what we see), and it has contingent property of being real, or being illusion.
A comment by Justin made me realize, that maybe I need to further explain the argument… Here is short explanation…
One of the things I want to point to by the argument is the difference between exists/doesn’t exists and real/illusionary distinctions.
In the argument, I am thinking of existence in Russellian sense, addressed formally through existential quantifier.
And in such sense, in case of illusion there is something, and it is somehow.
Take for example two states
(a) where person sees yellow-circle afterimage
(b) where person doesn’t see yellow afterimage
The situations are different because in (a) there is yellow circle afterimage, while in (b) there isn’t. So existential quantifier can be used on illusions (there is illusion vs. there isn’t illusion), but even when there are illusions, those are not real. So, in this way of using existence, it seems to me there is clear difference of nonexistent and illusionary.
I guess the proponents of qualia had already made this kind of arguments, as if one accepts qualia, then one can say that it is the same quale in both cases (illusionary or real tree).
On other hand, I don’t know how qualia-people address the possibility for transparency, i.e. the case where the tree we are seeing is real.
Any comments and pointers on those issues are welcome!