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In the Timaeus, which in the series of Plato’s works immediately follows the Republic, though probably written some time afterwards, no mention occurs of the doctrine of ideas. Geometrical forms and arithmetical ratios furnish the laws according to which the world is created. But though the conception of the ideas as genera or species is forgotten or laid aside, the distinction of the visible and intellectual is as firmly maintained as ever. The IDEA of good likewise disappears and is superseded by the conception of a personal God, who works according to a final cause or principle of goodness which he himself is. No doubt is expressed by Plato, either in the Timaeus or in any other dialogue, of the truths which he conceives to be the first and highest. It is not the existence of God or the idea of good which he approaches in a tentative or hesitating manner, but the investigations of physiology. These he regards, not seriously, as a part of philosophy, but as an innocent recreation (Tim.).

Passing on to the Parmenides, we find in that dialogue not an exposition or defence of the doctrine of ideas, but an assault upon them, which is put into the mouth of the veteran Parmenides, and might be ascribed to Aristotle himself, or to one of his disciples. The doctrine which is assailed takes two or three forms, but fails in any of them to escape the dialectical difficulties which are urged against it. It is admitted that there are ideas of all things, but the manner in which individuals partake of them, whether of the whole or of the part, and in which they become like them, or how ideas can be either within or without the sphere of human knowledge, or how the human and divine can have any relation to each other, is held to be incapable of explanation. And yet, if there are no universal ideas, what becomes of philosophy? (Parmenides.) In the Sophist the theory of ideas is spoken of as a doctrine held not by Plato, but by another sect of philosophers, called ‘the Friends of Ideas,’ probably the Megarians, who were very distinct from him, if not opposed to him (Sophist). Nor in what may be termed Plato’s abridgement of the history of philosophy (Soph.), is any mention made such as we find in the first book of Aristotle’s Metaphysics, of the derivation of such a theory or of any part of it from the Pythagoreans, the Eleatics, the Heracleiteans, or even from Socrates. In the Philebus, probably one of the latest of the Platonic Dialogues, the conception of a personal or semi-personal deity expressed under the figure of mind, the king of all, who is also the cause, is retained. The one and many of the Phaedrus and Theaetetus is still working in the mind of Plato, and the correlation of ideas, not of ‘all with all,’ but of ‘some with some,’ is asserted and explained. But they are spoken of in a different manner, and are not supposed to be recovered from a former state of existence. The metaphysical conception of truth passes into a psychological one, which is continued in the Laws, and is the final form of the Platonic philosophy, so far as can be gathered from his own writings (see especially Laws). In the Laws he harps once more on the old string, and returns to general notions:–these he acknowledges to be many, and yet he insists that they are also one. The guardian must be made to recognize the truth, for which he has contended long ago in the Protagoras, that the virtues are four, but they are also in some sense one (Laws; compare Protagoras).

So various, and if regarded on the surface only, inconsistent, are the statements of Plato respecting the doctrine of ideas. If we attempted to harmonize or to combine them, we should make out of them, not a system, but the caricature of a system. They are the ever-varying expression of Plato’s Idealism. The terms used in them are in their substance and general meaning the same, although they seem to be different. They pass from the subject to the object, from earth (diesseits) to heaven (jenseits) without regard to the gulf which later theology and philosophy have made between them. They are also intended to supplement or explain each other. They relate to a subject of which Plato himself would have said that ‘he was not confident of the precise form of his own statements, but was strong in the belief that something of the kind was true.’ It is the spirit, not the letter, in which they agree–the spirit which places the divine above the human, the spiritual above the material, the one above the many, the mind before the body.