Abstract Studies of irrationality in cognitive psychology have usually looked at areas where humans might be expected to be rational, yet appear not to be. In this paper the other extreme of human irrationality is examined: the delusion as it occurs in psychiatric illness. A parallel is suggested between the delusion as an aberration of cognition and some illusions which result from aberrations within optics. It is argued that, because delusions are found predominantly within certain limited areas of cognitive functioning, (...) they may represent a form of mental aberration which may tell us something about the abstract properties of mind. Possible implications of this model for several areas of cognitive science are discussed. (shrink)