In contrast are illusions where the mistake is about the properties, position, or identity of some object actually in view. In pure hallucinations -for example, the pink elephant a drunkard sees, the apparitions of delirium, Macbeth's dagger -some physical object is "perceived" when neither it nor anything at all like it is present.
These are not strictly illusions (they usually do not deceive), and they vary around a norm in which the objects are perceived accurately. In all these cases the apparent properties of an object vary relative to the position of the percipient, the distance and media between him and the object, the lighting, the state of his health, body, or sense organs, etc. Further examples are color blindness, shortsightedness, and other physical defects that alter the appearance of things. The same water may feel cool to one person and warm to another the same wine may taste sweet or dry, depending on what one has just been eating green hills may look blue in the distance and as a train rushes past, the pitch of its whistle may seem to vary. relativity of perceptionsĪ round plate that looks elliptical when seen from an angle and a square table that looks diamond shaped illustrate the relativity of perception. The perception of motion introduces many more: At the cinema a rapid succession of slightly different stills on a flat screen makes us see a scene with a three-dimensional perspective in which people move about the wheels of a coach may seem to be going backward when really they are moving rapidly forward (stroboscopic effect). Other examples are mirages, mirror effects, and conjurers' tricks. Diseases or drugs, including alcohol, may produce other illusions, such as double images or the unearthly colors and multiple shapes an object may assume for one who has taken mescaline. Psychologists have produced a number of optical illusions, such as equal lines that appear to be of unequal length a stationary balloon that when inflated and then deflated seems to advance and then recede and a specially constructed Distorted Room, in which a man looks smaller than a boy.
Illusions proper occur when the percipient is deceived or is liable to be deceived in identifying the object perceived or its properties. The term illusion is used by philosophers to cover a range of phenomena approximately classifiable as follows. This entry will describe illusions and set forth and examine the argument from illusion that perception cannot be trusted as a source of knowledge of the external world but affords direct awareness only of appearances or sensa. Most of the major philosophical problems of perception derive from the fact of "illusions." These problems center on the question whether perception can give us true and direct knowledge of the world, and thus they are basic to epistemology.
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