Idea. This word has borne very distinct meanings in the history of philosophy. Down to the 17th century it had the signification given to it by Plato, and referred to the Platonic doctrine of eternal forms existing in the Divine mind, according to which the world and all sensible things were framed. The word was used in this sense in literature as well as in philosophy down to the 17th century, as in Spenser, Shakespeare, Hooker, and Milton.
In speaking of the mental representation of external things, Descartes, instead of employing the various terms image, species, phantasm, &c., which had been the words formerly in use for that particular signification, used the word idea. In this he was followed by other philosophers, as, for example, Locke, who states that he has adopted the word to stand for 'whatever is the object of the understanding, when a man thinks.' Thus, the mental impression that we are supposed to have when thinking of the sun without seeing the actual object is called our idea of the sun. The idea is thus in contrast with the sensation, or the feeling that we have when the senses are engaged directly or immediately upon the thing itself. But the word has been very variously used, as by Berkeley, Hume, Kant, Hegel (see these articles). For innate ideas, see COMMON SENSE, LOCKE.—Idealism is a term used almost as variously as Idea. Idealism may be a theory concerning our knowledge of external existence, restricting mind directly to knowledge of its own state, whereas the opposed realism implies a direct knowledge of the external. Idealism may be also a theory as to the nature of the universe, and be spoken of (rightly or wrongly) as subjective idealism, as in Fichte (q.v.), critical as in Kant (q.v.), or absolute as in Hegel (q.v.). See also BERKELEY. In the medieval controversies between nominalism and realism, realism was a kind of idealism (see NOMINALISM). Idealism is also used for ethical and æsthetic systems which adopt an ideal standard of estimating character, human possibilities, or subjects in art (see REALISM). The word realism has a further peculiar sense in Herbart (q.v.).