You've missed the point. We are trying to defend direct realism. Direct realism is the claim that we directly perceive rocks, trees and tables. NOT neural inputs. If you are going to claim the BIV directly perceives neural inputs then you have to say the same for yourself right now - and that isn't direct realism.
You're right - direct realism is indefensible, precisely because of how the brain and senses work. We
don't directly see the tree, the monitor, the flying spaghetti monster. We see photons in patterns and wavelengths and such. Our senses convert those patterns etc. into neural signals, which are then transmitted to the brain. The BIV thought experiment demonstrates conclusively how direct realism can be fatally incorrect, after a fashion; unless the thinker concedes that the only objects of direct realism are neural signals themselves.
Since the brain also can generate the same neural signals, without use of the sensory organs, this can sound pretty confusing for any form of realism. Dreams? Imagination? Hallucination? Our entire lives could be some extended dream sequence, and for our brain, there'd be no difference whatsoever.
But there is a quality to the waking world, a consistancy and a continuity, that seems to provide us with a referential about what is 'real' and what is not. Sadly, it is only an assumption, an inference, that allows us to accept physical reality as it is, rather than presuming it is all fake.
I'm not entirely sure I comprehend the semantic differences between direct and indirect realism. I'm fairly certain, though, that what seems to be described as 'direct realism' is false, for several reasons. The first being that the brain does not directly perceive anything, save neural inputs; the second being that the senses do not perceive reality as it truly is (when's the last time you saw the U-V markings on a flower, or sensed the gamma radiation in the area, or were directly aware of the magnetic field of the earth??), but only as they are equipped to sense. The third being the number of experiments that show that neither our senses nor our brains are foolproof. Remember Ian's checkerboard illusion? He insists, against all proof, that the two squares in question are absolutely the same color, even though they aren't. This is just one small example of why I think direct realism is silly and indefensible.
I'm completely lost on 'indirect realism' - seems to me that's a closer description to what's really going on, that we don't sense reality directly, but only through the process of sensation, neural input, and interpretation. Reality is there, but we perceive it in bits and pieces, and miss so much. Like being blind, really, and having to figure out what the world is like with only our hands, ears, etc.
What is non-realism?
