The earliest significance of the Greek original was what the eye recognized in the object, the form. Hence in common usage it signified the type. In the philosophy of Plato and Aristotle it was the universal, which had a real existence of its own and constituted the reality of particular objects of sensuous experience. By way of medieval Platonism it passed into the psychological usage of modern thought, carrying both significations, that of the form and that of the nature of the thing known, as these appeared in the mind of the knower. Thus Locke used it indifferently for the immediate object in the mind, whether this was sensation or concept,
(216) a double usage which was quite sympathetic to the nominalism of Berkeley. Hume confined idea to the mental image of the sensation. In current usage it has come to answer very generally to the concept of the thing, though it is still haunted by the ghost of the image of the thing. It is no longer a sharply defined technical term in current English philosophy and psychology. The various significations of idea are now assigned to different terms, such as sensation, image, presentation, representation, meaning, and concept. In German philosophy Kant revived the objective signification of the term, in the "ideas of the reason;" in his system these were directive principles of thought that assumed the existence of eternal realities, which were independent of our phenomenal experience. In the systems of Romantic Idealism, that succeeded Kant's Transcendental Idealism the object of rational thought was given in thought itself, and the idea in Hegel's doctrine stood for the complete expression of all reality in the Absolute. The influence of German idealism in England and America carried over this significance of the word into the terminology of the neo-Hegelians in English and American Universities.