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Aristotle's illusion

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A tactile illusion that is created when the eyes are closed, two fingers of one hand are crossed, and a small object such as an acorn is pressed (especially by another person) into the cleft between the tips of the crossed fingers. The sensation is that of touching two objects rather than one. The first written account of it was given by the Greek philosopher Aristotle (384–322bc) in the essay On Dreams in the Parva Naturalia: ‘When the fingers are crossed, one object seems to be two; but yet we deny that it is two; for sight is more authoritative than touch. Yet, if touch stood alone, we should actually have pronounced the one object to be two’ (Chapter 2). Aristotle mentioned it also in Metaphysics: ‘Touch says there are two objects when we cross our fingers, while sight says there is one’ (Book 4, Chapter 6). The explanation for the illusion, apparently overlooked by Aristotle, is that when the fingers are crossed the outsides of two fingers are touched simultaneously, and in ordinary circumstances and past experience that requires stimulation by two separate objects. This explanation was first advanced in Problems, a spurious work often attributed to Aristotle, probably written by one of his followers: ‘When we hold the hand in its natural position, we cannot touch an object with the outer sides of two fingers’ (Book 35, Chapter 10). Further references to the illusion are to be found in Book 31, Chapters 11 and 17 of Problems. Also called Aristotle's experiment. See also diplaesthesia.

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