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==Definition of terms==

The first term that needs to be defined in discussing visual illusions is 'percept.' The simplest definition of visual percepts is "visual experience" which works well in ordinary discourse, vision science, and thinking about visual illusions. Of course many visual stimuli result in appropriate behavioral responses even when we are not aware of having seen them; indeed the majority of visual processing and visually guided behavior falls in this category (think of all the visual information processed and responded unconsciously to keep your car on the road when driving to work while preoccupied with other thoughts). Perception is thus used here in the conventional sense of our visual experience or phenomenology. 

The second term that must be defined is 'illusion.' The usual concept of an illusion is a percept that fails to agree with the real world measurements made with devices such as photometers, spectrophotometers, rulers, protractors, and so on. Although this definition is a good start, the evidence discussed here leads to the counterintuitive conclusion that all visual percepts are illusory in this sense, and that the textbook illusions that people have made so much of are only the more obvious examples of the normal discrepancy between stimuli and percepts (the more flagrant discrepancies arising for the reasons explained below).