ILLUSION, the experience and the result of misconstruing or misinterpreting some real sense stimulus or stimuli, as when a wax figure at Madame Tussaud's or similar exhibitions is mistaken for a real policeman, or when a piece of suitably modelled plasti cine is mistaken for a sausage. Something is actually there to stimulate the senses, and the sense experience itself is produced in a normal manner, but, owing to established habits of rapid association or the temporary "set" of the mind, the observer mistakes the thing for something different. Many optical and other illusions are perfectly normal and can be experienced by a large number of people at the same time. This happens, for example, at the familiar entertainments consisting of juggling and conjuring tricks, also ordinarily in public places, thanks to the luminous "moving" advertisements which make modern civiliza tion so gay and dazzling at night. The normality and indeed the inevitableness of so many illusions constitute one of the serious problems in any attempt to vindicate the validity of human knowledge. (See HALLUCINATION ; ABNORMAL PSYCHOLOGY ; PSY CHOLOGY [and the bibliography given there] ; KNOWLEDGE, THEORY OF.) See J. Sulby, Illusions 0880.