A theory of vision must explain how information crosses the space between the perceiver and the object she is perceiving. Early theories of sight offered three major opinions. One opinion held that the eye sends out rays to objects, and that these rays give the viewer information. Among the proponents of this view were the Pythagoreans. A second opinion held that sight depended upon an interaction between images that were ejected from the eye and the perceiver's own spirit. Socrates and Plato supported this theory. Finally, another opinion held that when people see, they actually make contact with the objects they see, or, rather, with replicas of those objects.
In the fifth century B.C., the Greek philospher Democritos maintained that every object contained numerous replicas of itself. These replicas were called eidola (singular, eidolon). It was believed that every object continuously emits eidola. That if a person looks at an object, its replicas fly toward the viewer and enter her eyes. According to this scheme, one is able to see some object only because one of the object's eidola has winged its way into her eye. Several observations were taken as support of the eidolon theory. For instance, the fact that a person could see her reflection in water or in a mirror proved that the viewer was emitting eidola.
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