"But you can't explain the concept 'illusion' without reference to 'consciousness'" Of course you can. What you see is just a model the brain constructs based

"But you can't explain the concept 'illusion' without reference to 'consciousness'" Of course you can. What you see is just a model the brain constructs based on visual input. The brain squares up corners, sees depth in converging lines. You can hardly see at all outside your focus point, but the brain makes you think you do. You have a blind spot in each of your eyes but the brain fills in the spaces based on the surrounding images so you don't notice it. None of these effects are the least bit dependent upon consciousness. It is conceivable that consciousness is just the brain's model of itself, and since the brain can construct these models without the need for consciousness, the argument is not circular.