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(Gk., eidos, visible form) A notion stretching all the way from one pole, where it denotes a subjective, internal presence in the mind, somehow thought of as representing something about the world, to the other pole, where it represents an eternal, timeless unchanging form or concept: the concept of the number series or of justice, for example, thought of as independent objects of enquiry and perhaps of knowledge. These two poles are not distinct meanings of the term, although they give rise to many problems of interpretation, but between them they define a space of philosophical problems. On the one hand, ideas are that with which we think, or in Locke's terms, whatever the mind may be employed about in thinking. Looked at that way they seem to be inherently transient, fleeting, and unstable private presences. On the other hand, ideas provide the way in which objective knowledge can be expressed. They are the essential components of understanding, and any intelligible proposition that 184 is true must be capable of being understood. Plato's theory of forms is a celebration of the objective and timeless existence of ideas as concepts, and in his hands ideas are reified to the point where they make up the only real world, of separate and perfect models of which the empirical world is only a poor cousin. This doctrine, notable in the Timaeus, opened the way for the Neoplatonic notion of ideas as the thoughts of God. The concept gradually lost this other-worldly aspect, until after Descartes ideas become assimilated to whatever it is that lies in the mind of any thinking being.

Together with a general bias towards the sensory, so that what lies in the mind may be thought of as something like images, and a belief that thinking is well explained as the manipulation of images, this was developed by Locke, Berkeley, and Hume into a full-scale view of the understanding as the domain of images, although they were all aware of anomalies that were later regarded as fatal to this doctrine (see abstraction ). The defects in the account were exposed by Kant, who realized that the understanding needs to be thought of more in terms of rules and organizing principles than of any kind of copy of what is given in experience. Kant also recognized the danger of the opposite extreme (that of Leibniz ) of failing to connect the elements of understanding with those of experience at all (Critique of Pure Reason, A270).

It has become more common to think of ideas, or concepts, as dependent upon social and especially linguistic structures, rather than the self-standing creations of an individual mind, but the tension between the objective and the subjective aspect of the matter lingers on, for instance in debates about the possibility of objective knowledge, of indeterminacy in translation, and of identity between the thoughts people entertain at one time and those that they entertain at another.