The social studies through which we have accompanied Plato seem to have reacted on his more abstract speculations, and to have largely modified the extreme opposition in which these had formerly stood to current notions, whether of a popular or a philosophical character. The change first becomes perceptible in his theory of Ideas. This is a subject on which, for the sake of greater clearness, we have hitherto refrained from entering; and that we should have succeeded in avoiding it so long would seem to prove that the doctrine in question forms a much less important part of his philosophy than is commonly imagined. Perhaps, as some think, it was not an original invention of his own, but was borrowed from the Megarian school; and the mythical connexion in which it frequently figures makes us doubtful how far he ever thoroughly accepted it. The theory is, that to every abstract name or conception of the mind there corresponds an objective entity possessing a separate existence quite distinct from that of the scattered particulars by which it is exemplified to our senses or to our imagination. Just as the Heracleitean flux represented the confusion of which Socrates convicted his interlocutors, so also did these Ideas represent the definitions by which he sought to bring method and certainty into their opinions. It may be that, as Grote suggests, Plato adopted this hypothesis in order to escape from the difficulty of defining common notions in a satisfactory manner. It is certain that his earliest Dialogues seem to place true definitions beyond the reach of human knowledge. And at the beginning of Plato’s constructive period we find the recognition of abstract conceptions, whether mathematical or moral, traced to the remembrance of an ante-natal state, where the soul held direct converse with the transcendent realities to which those conceptions correspond. Justice, temperance, beauty, and goodness, are especially mentioned as examples of Ideas revealed in this manner. Subsequent investigations must, however, have led Plato to believe that the highest truths are to be found by analysing not the loose contents but the fixed forms of consciousness; and that, if each virtue expressed a particular relation between the various parts of the soul, no external experience was needed to make her acquainted with its meaning; still less could conceptions arising out of her connexion with the material world be explained by reference to a sphere of purely spiritual existence. At the same time, innate ideas would no longer be required to prove her incorporeality, when the authority of reason over sense furnished so much more satisfactory a ground for believing the two to be of different origin. To all who have studied the evolution of modern thought, the substitution of Kantian forms for Cartesian ideas will at once elucidate and confirm our hypothesis of a similar reformation in Plato’s metaphysics.