Why is the Adelson's checkerboard illusion (shown above) an illusion? Nothing looks weird about this image. Right? For instance, checker A is a darker checker and B is a light checker. View the same image below, but now with a homogeneous patch of grey overlaying the checkers. A and B are in fact identical in luminance, but our perception of them is quite different from their physical reality. This is an illusion of lightness constancy.
I'm teaching (as a GSI) a course in Perception this semester with Steve Palmer, and I taught Sensation & Perception last fall with my advisor, Bill Prinzmetal. One of the most striking things I've learned from doing this is just how many illusions there are, and not just visual (if I can figure it out, I'll post some auditory illusions I have). The more I learn about how the body is formed at the cellular level and how the body functions both biologically and perceptually, the more I am amazed by how amazing humans are (and other animals). Yes, there are a slew of illusions that humans are prey to, but we also have overwhelmingly accurate perceptions most of the time. There are so many areas and ways that our system/body could breakdown, and it does don't get me wrong, but it also doesn't a lot. I don't know, I don't really have a point except that evolution has done a pretty good job.
(images acquired from this MIT website)
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