IDEA (Gr. [Greek: idea], connected with [Greek: idein], to see; cf. Lat. _species_ from _specere_, to look at), a term used both popularly and in philosophical terminology with the general sense of "mental picture." To have no _idea_ how a thing happened is to be without a mental picture of an occurrence. In this general sense it is synonymous with concept (q.v.) in its popular usage. In philosophy the term "idea" is common to all languages and periods, but there is scarcely any term which has been used with so many different shades of meaning. Plato used it in the sphere of metaphysics for the eternally existing reality, the archetype, of which the objects of sense are more or less imperfect copies. Chairs may be of different forms, sizes, colours and so forth, but "laid up in the mind of God" there is the one permanent _idea_ or type, of which the many physical chairs are derived with various degrees of imperfection. From this doctrine it follows that these _ideas_ are the sole reality (see further IDEALISM); in opposition to it are the empirical thinkers of all time who find reality in particular physical objects (see HYLOZOISM, EMPIRICISM, &c.). In striking contrast to Plato's use is that of John Locke, who defines "idea" as "whatever is the object of understanding when a man thinks" (_Essay on the Human Understanding_ (I.), vi. 8). Here the term is applied not to the mental process, but to anything whether physical or intellectual which is the object of it. Hume differs from Locke by limiting "idea" to the more or less vague mental reconstructions of perceptions, the perceptual process being described as an "impression." Wundt widens the term to include "conscious representation of some object or process of the external world." In so doing he includes not only ideas of memory and imagination, but also perceptual processes, whereas other psychologists confine the term to the first two groups. G. F. Stout and J. M. Baldwin, in the _Dictionary of Philosophy and Psychology_, i. 498, 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.