The information gathered by the eye is processed in the brain to give a perception that does not tally with a physical measurement of the stimulus source. There are three main types:
literal optical illusions that create images that are different from the objects that make them, physiological ones that are the effects on the eyes and brain of excessive stimulation of a specific type (brightness, colour, size, position, tilt, movement), and cognitive illusions, the result of unconscious inferences.
There are things being revealed by illusions that that teach us things about ourselves and our relation to the world around us; that we don’t know much about our relation to the world until we discover them through illusions. Which also illustrates a more general important lesson about ourselves; that the “informed guessing” is being done by processes in our nervous system of which we are largely unaware. Not only do we not know the basis of the informed guessing, until we notice and study it, but we aren’t in fact usually aware that what we see involves guessing at all.
The “cognitive unconscious” makes informed guesses and delivers them to the conscious part, which supports our awareness of what we are seeing while knowing little or nothing of how what we see has been constructed. By trying to explain illusions, wondering how we come to see what we see, we can, using consciousness, deconstruct the process of perception, gaining a useful ability to appreciate things in ways other than those made immediately available by the informed guessing of the unconscious. But still, after we have deconstructed the process of the false perception… its still hard not to see the illusion. Thats probably because the brain is more concerned with making sense of the world than to see ambiguous information as it really is: confusing. And it does so by organizing incoming sensations into information which is meaningful.