Locke followed Descartes who, instead of employing the words image, species, phantasm in speaking of the mental representations of sense-perceptible objects, used the word idea. Hume tells us that all the perceptions of the human mind resolve themselves into ideas and impressions. With him the idea is a purely mental structure or content which is a less vivid copy of an original sensation, emotion or volition. With Kant an idea becomes a trans cendental conception of the pure reason, such as the idea of God. With Hegel the highest thought of the logic is named by him Idee, suggesting Plato's highest principle. Idee thus suggests the final category of the Hegelian dialectic — the realized ideal, energy in its su preme manifestation; it is the Absolute Idea, or the Absolute. At the present time many psychologists use the word idea only for a com plex that does not arise from direct outward impressions. They thus use it for the so-called memory-images. For ideas formed from out ward sense-impressions they generally use the word perception. Wundt does not regard this distinction as of any importance, since, as he says, there is really no valid difference between memory ideas and the so-called sense percep tions. Yet, according to Professor Stout, the existence of ideas is subsequent to that of per cepts, and even implies some complexity of per ceptual cognition. Perhaps, however, all agree that ideas are symbols, symbols having a mean ing. The whole realm of mental representa tions may be viewed with regard to either their symbolism or their significance. Considering their symbolism ideas of things as well as ideas of relations are not unreasonably regarded as products of the mind. But considering their meaning, ideas represent realities. In other words, their contents or that which they signify is real. There are indeed some ideas that are not even representations. Such are solely of an auxiliary nature, comparable to tools of the mind. Such are hypotheses. This brings us to the concept of content. Content, obiter dicta, is absolutely incomprehensible from a merely mechanical view of the world. For who could explain the meaning of a word mechanically? Content involves a principle of order—that of objective reality, meaning and dependency. We perceive this for example in the relationship of the characteristic marks of a logical concept to one another. Only grossest misunderstanding can confound the inner structure of such a con cept with the juxtaposition within a mere sense presentation. The fundamental form of con nection in the case of such a concept is one of system. Each element stands within a whole, under the influence of a whole, and remains subject to its compelling power, while the vari ous elements mutually determine one another.
Nowadays it is not seldom that one comes across the word idea combined in phrasal con nection with other words. A consideration of the more important of these cannot be out of place. Some psychologists speak of a dominant idea. They mean by this an imperative or in sistent idea, such as besets the mind, and this in spite of all effort to inhibit it and in spite of one's assurance of its unreasonable char acter. We may remark here that every idea considered, not as a mere feeling, but as a brain-movement fit to serve as an irritation to action will, if not inhibited, pass into an action, whether it be connected with consciousness or not. Thus a persistent or obsessing idea may become dangerous to the one holding it. It is usually a train of thought which a subject can not banish or escape, though he recognizes in falsity or triviality, Such imperative or dom inant ideas exist in all degrees of intensity, from the 'sound that sings in the earl' to such oh sessions as agoraphobia. In this sense it is the same with a fixed idea. Fixed ideas are delusional ideas or trains of thought which dominate the mind in certain forms of insanity or monomania.
By a free idea experimental psychologists understand an idea or representation which is dissociated from sense-perception or presenta tion and from the organic impulses connected with sense-perception. A free idea is thus one which may take its place in an associative train, and may be used in the process of discrimina tion.
By an implicit idea, Hoffding understands the idea or group of ideational elements that fuses with the presentation in the act of per ception. It is thus the ideational associate that raises a datum of sensation to the rank of a per ception. An extrinsic idea is a temporal or spatial idea. Spatial and temporal ideas are immediately distinguished from intensive ideas by the fact that their parts are united in a definitely fixed order. Ideas with such a fixed arrangement are called in general extensive ideas.
Thus no word in philosophy or psychology has been responsible for more confusion than the word idea. Yet what the word signifies is of utmost importance. Its sense in the minds of some philosophers is the key 'to their entire system. Nowadays it sometimes means merely an opinion; sometimes mental images; and we even find it employed merely as an element in a periphrasis (as in ((to have an idea on”. It is thus one of the most important words in the history of thinking and at the same time one of the least understood. See INNATE IDEAS.