PERCEPTUAL ILLUSIONS

Analysis suggests that the term illusion refers to discrepancies between reality and perception which occur as a result of normal sensory functioning and which are as yet unexplained. As a result, attempts to classify illusions must rely on theory rather than mechanism and consequently such classificatory schemes tend to be theory laden. As an example, a recent classification by Gregory is examined and found wanting. It is suggested that premature classification can hinder the search for explanation and that terms like ''top down'' or ''cognitive'' may be obscurantist.