Illusions and knowing what is real

Geometrical illusions are displays that give false impressions that observers take to be accurate. They have traditionally been cited as evidence against the naive realist claim that people see the world as it "really " is. Such illusions, however, often depend on their being viewed from a single vantage point (Kennedy & Portal, 1990). In Gibsonian terms, they depend on the availability of impoverished information. In this study, spatial transformations were applied to line-length (Sander parallelogram) and area (Jastrow curves) illusions to provide information to observers about veridical size. In particular, the reversal of certain parts of the displays resulted in informative invariants that specified the veridical nature of the parts. Most observers were able to use the information. In this article we consider t h e status of geometrical illusions a n d possible informative perceptual transformations vis-a-vis problems of knowledge. We show how information for veridical size arises via t h e movement of parts of illusions, even though t h e illusory impressions are not dispelled. Geometrical illusions are usually flat displays of lines o r black-and-whit