What if someone says, "Hang on Callahan, did you just 'prove' that illusions don't exist, period? I mean, if I can't trust my eyes in the desert when I think I see water, how can I trust me eyes when I read in a book that it talks about 'mirages'?"
My post certainly did not mean to imply that perceptual illusions never occur. But this does not negate my general point, because consider how we decide that something is a perceptual illusion: Let us say we think we see an oasis in the desert ahead of us. Then we walk closer to what we thought we saw, and find only an empty stretch of sand. We walk back to where we were before, and again see the oasis. We again walk forward, and see only sand. If we are still unsure about our perception, we ask someone else: "What do you see here?" It is only based upon other, better perceptual evidence that I decide that some of my perceptual evidence has been illusory.
If we are presented with one of the many famous perceptual illusions on a piece of paper, how do we convince ourselves that the figure in one drawing is not really smaller than the figure in another? We take a ruler and measure, and perceive that the ruler measures each figure to be the same height.
In other words, we only can declare certain perceptions to be "illusions" by taking other perceptions to be accurate. What we do is to check one anomalous perception against our whole perceptual world, in order to form a coherent world of perceptual ideas. If someone suggests that every last one of our perceptions is illusory, that eliminates all possibility of declaring any particular perception to be mistaken. It is only by assuming that most of our perceptions are accurate that we can declare certain of them to be mistaken.
A true achievement of the physical sciences is their procedures for checking and double checking the accuracy of our perceptions. But every one of those checks relies entirely on other perceptions. If some meter has always displayed a value between one and two, but late one night, when I am very tired, I perceive it to be reading 10, I check that reading by performing the measurement over and over again. When I consistently perceive the meter to always read between one and two, I decide that my reading of 10 was an illusion.