We perceive objects and events by their effects on our sense organs—the light they provide the eye, the pressure waves that enter the ear, and so on. An object's physical attributes, such as its size, shape, and reflectance, are distal stimuli: properties that are relatively invariant for a given object, and important to our dealings with the world. To perceive these properties, we must extract information about them from the patterns of stimulus energies at our sense or-gans—the proximal stimulation, which changes constantly. Thus, varying the viewing distance, viewing angle, and il lumination changes the size, shape, and brightness of the object's proximal stimulus. Nevertheless, things do not generally seem to expand when approached or become less reflective when moved from light to shade. These phenomena are given appropriate names: size constancy, lightness constancy, position constancy, and so forth.
This general rule, as formulated by Hermann von Helmholtz, can be paraphrased as follows: One perceives that state of affairs in the world that would, under normal conditions, have given rise to the pattern of proximal stimulation that one's sense organs receive.
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