Argument from Illusion

The argument from illusion is a line of reasoning in epistemology and the philosophy of perception used to challenge naive or direct realism, the view that perception puts a person in immediate contact with physical objects exactly as they are. Its classic illustration is a straight stick that looks bent when part of it is submerged in water: since the stick is not actually bent, what a person sees in that moment cannot simply be the stick itself, but must instead be some intervening mental item, often called a sense-datum, that can diverge from the object's real properties. Because ordinary, non-illusory perception seems continuous with illusory perception and the two cannot reliably be told apart by introspection alone, defenders of the argument conclude that all perception, not only the illusory kind, is mediated by sense-data rather than being a direct encounter with physical objects, a conclusion associated in the twentieth century with philosophers such as A.J. Ayer.