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G. F. Stout and J. M. Baldwin, in the Dictionary of Philosophy and Psychology, define idea as "the reproduction with a more or less adequate image, of an object not actually present to the senses."

They point out that an idea and a perception are by various authorities contrasted in various ways. "Difference in degree of intensity", "comparative absence of bodily movement on the part of the subject", "comparative dependence on mental activity", are suggested by psychologists as characteristic of an idea as compared with a perception.

It should be observed that an idea, in the narrower and generally accepted sense of a mental reproduction, is frequently composite.

That is, as in the example given above of the idea of a chair, a great many objects, differing materially in detail, all call a single idea.

When a man, for example, has obtained an idea of chairs in general by comparison with which he can say "This is a chair, that is a stool", he has what is known as an "abstract idea" distinct from the reproduction in his mind of any particular chair (see abstraction).

Furthermore, a complex idea may not have any corresponding physical object, though its particular constituent elements may severally be the reproductions of actual perceptions. Thus the idea of a centaur is a complex mental picture composed of the ideas of man and horse, that of a mermaid of a woman and a fish.