In the “Phaedo,” as in the “Meno,” the origin of ideas is sought for in a previous state of existence. There was no time when they could have been acquired in this life, and therefore they must have been recovered from another. The process of recovery is no other than the ordinary law of association, by which in daily life the sight of one thing or person recalls another to our minds, and by which in scientific enquiry from any part of knowledge we may be led on to infer the whole. It is also argued that ideas, or rather ideals, must be derived from a previous state of existence because they are more perfect than the sensible forms of them which are given by experience (74 and following). But in the “Phaedo” the doctrine of ideas is subordinate to the proof of the immortality of the soul. “If the soul existed in a previous state, then it will exist in a future state, for a law of alternation pervades all things.” And, “If the ideas exist, then the soul exists; if not, not.” It is to be observed, both in the “Meno” and the “Phaedo,” that Socrates expresses himself with diffidence. He speaks in the “Phaedo” (114 D, 115 D) of the words with which he has comforted himself and his friends, and will not be too confident that the description which he has given of the soul and her mansions is exactly true, but he “ventures to think that something of the kind is true.” And in the “Meno,” after dwelling upon the immortality of the soul, he adds, “Of some things which I have said I am not altogether confident” (compare 86 C, and “Apology,” 40 and following; “Gorgias” 527 B). From this class of uncertainties he exempts the difference between truth and appearance, of which he is absolutely convinced (98 B).
In the Republic the ideas are spoken of in two ways, which though not contradictory are different. In the tenth book (596 and following) they are represented as the genera or general ideas under which individuals having a common name are contained. For example, there is the bed which the carpenter makes, the picture of the bed which is drawn by the painter, the bed existing in nature of which God is the author. Of the latter all visible beds are only the shadows or reflections. This and similar illustrations or explanations are put forth, not for their own sake, or as an exposition of Plato’s theory of ideas, but with a view of showing that poetry and the mimetic arts are concerned with an inferior part of the soul and a lower kind of knowledge. On the other hand, in the 6th and 7th books of the Republic we reach the highest and most perfect conception, which Plato is able to attain, of the nature of knowledge. The ideas are now finally seen to be one as well as many, causes as well as ideas, and to have a unity which is the idea of good and the cause of all the rest. They seem, however, to have lost their first aspect of universals under which individuals are contained, and to have been converted into forms of another kind, which are inconsistently regarded from the one side as images or ideals of justice, temperance, holiness and the like; from the other as hypotheses, or mathematical truths or principles.
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 (30, 37). 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 (“Timaeus” 59 D).
Passing on to the “Parmenides” (128–136), 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
