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Delusions

R. Gregory

Published 2003 in Perception

ABSTRACT

Illusions are failures of perception, delusions are failures of conception. So illusions are errors of the perceived here and now; but delusions are errors of understanding that may be very general, even to madness. Illusions of perception can create delusions when the cause of the illusion is not recognised. Thus, a rainbow is a delusion if we expect to walk under it as through an arch or brick or stone. For, unlike an architectural arch, a rainbow recedes as we approach it, never to be reached. Can physical phenomena, such as refractions and reflections of light, give perceptual illusions? This is really a matter of terminology. But as it is useful to put phenomena into categories, let's try to decide. If we define illusions as disturbances of normal perception, then we might well say that rainbows and mirages are illusions. But for the physicist, interested in the phenomena of refraction and so on, these disturbances of vision are the evidence needed for discovering and investigating these phenomena of physics. Perceptually the rainbow is a fictional arch; but it is a particularly interesting phenomenon of optics well worth studying and explaining by the physicist. The mirage shows to eye and brain touchable objects, but displaced; so this is a distortion illusion. It is also a delusionöwhen we are fooled by this disturbance of vision to a false belief, perhaps to walk for miles through the desert, to die. We have defined illusions as perceptual departures from truths, pointing out that these are selected truths, serving as references for perceptual departures. For a mirage, the reference is the map position of the oasis, or whatever, seen in a different place. The rainbow does not have such a reference, for its appearance departs from nothing. As the reference truth is nothing, though something is seen, this is an illusion of fiction. When an optical (or physiological or cognitive) illusion is accepted as true, we are deluded. Thus a straight line seen as curved in an illusion figure, is a delusion when accepted as really curved though it is straight. Such errors can be life-threatening, as in driving or flying. A well-known visual distortion illusion could upset the concepts of a geometer (figure 1).

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