Cases in which what we apprehend by sensation does not correspond with the way things really are. Thus, for example, the apparent discontinuity between the portions of a spoon in and out of a glass of water is a visual illusion caused by the different indices of refraction of water and air. Representationalists commonly try to account for such cases by appeal to the distinction between primary and secondary qualities, but skeptics and idealists use perceptual illusion to raise more general doubts about the reliability of sensory knowledge.

    Recommended Reading: John W. Yolton, Perception & Reality: A History from Descartes to Kant (Cornell, 1996) {at Amazon.com}; William P. Alston, The Reliability of Sense Perception (Cornell, 1996) {at Amazon.com}; and Mark B. Fineman, The Nature of Visual Illusion (Dover, 1996) {at Amazon.com}.