Illusions we should have (But don't)

Illusions have been much studied in psychology and cognitive neuro- science. In addition, they continue to intrigue and amuse scientists and laymen. But what exactly are “illusions”? What is their use in the study of perception? Why are we fascinated by illusions? In this chapter I attempt to provide a novel answer to these questions. I will start by considering two extreme views, attempts to understand illusions by categorization, and the common sense view that illusions are perceptual mistakes. Next, I will propose a definition of illusions as perceptual inconsistencies. While this definition is based on phenomenological analysis, I suggest that it also has psycho-physical implications. By stud-ying the relationship between perceptual and stimulus inconsistency, we can learn a great deal about perception, not only about its intriguing inconsistencies that we call illusions, but also – and most importantly – about its adaptive function of providing us with a behaviorally useful representation of the external environment.