ABSTRACT
Illusions are discrepancies from truth. All the senses can suffer illusions; but as we know most about visual perception so visual illusions are the best known and fully understood – though much remains mysterious and controversial. All perceptions are subject to errors of many kinds; but illusions pass unnoticed except when strikingly inconsistent with what is accepted as true, or when there are internal inconsistencies – such as contradictory sizes or shapes, ambiguities, or paradoxes. And, as for our conceptual understanding (or misunderstanding), failed predictions can signal inadequacies and errors – though only backwards in time.
