The word "idea" is a transliteration of a Greek word of which the root meaning is "see." In classical Greek it never lost the possible meaning "visual aspect"; thus Plato writes of a person as being "very beautiful in idea," meaning "beautiful in visual aspect" or "good-looking" (Protagoras 315E). Very often visual shape is primarily involved, as when Plato refers to the "idea of the earth," meaning "the visible shape of the earth" (Phaedo 108D). The transferred sense of "type" or "kind" springs quite naturally from this use. Thus Thucydides writes of "many ideas [kinds] of warfare" (Histories I, 109).

In Plato's more technical use, the Ideas or Forms are always spoken of as (1) the objects of intelligence, in contrast with the objects of perception; (2) things that truly are, in contrast with changing objects of perception, which are in a state of becoming; (3) eternal, in contrast with the perishable...