There is a difficulty in understanding what Plato meant by ‘the longer way’: he seems to intimate some metaphysic of the future which will not be satisfied with arguing from the principle of contradiction. In the sixth and seventh books (compare Sophist and Parmenides) he has given us a sketch of such a metaphysic; but when Glaucon asks for the final revelation of the idea of good, he is put off with the declaration that he has not yet studied the preliminary sciences. How he would have filled up the sketch, or argued about such questions from a higher point of view, we can only conjecture. Perhaps he hoped to find some a priori method of developing the parts out of the whole; or he might have asked which of the ideas contains the other ideas, and possibly have stumbled on the Hegelian identity of the ‘ego’ and the ‘universal.’ Or he may have imagined that ideas might be constructed in some manner analogous to the construction of figures and numbers in the mathematical sciences. The most certain and necessary truth was to Plato the universal; and to this he was always seeking to refer all knowledge or opinion, just as in modern times we seek to rest them on the opposite pole of induction and experience. The aspirations of metaphysicians have always tended to pass beyond the limits of human thought and language: they seem to have reached a height at which they are ‘moving about in worlds unrealized,’ and their conceptions, although profoundly affecting their own minds, become invisible or unintelligible to others. We are not therefore surprized to find that Plato himself has nowhere clearly explained his doctrine of ideas; or that his school in a later generation, like his contemporaries Glaucon and Adeimantus, were unable to follow him in this region of speculation. In the Sophist, where he is refuting the scepticism which maintained either that there was no such thing as predication, or that all might be predicated of all, he arrives at the conclusion that some ideas combine with some, but not all with all. But he makes only one or two steps forward on this path; he nowhere attains to any connected system of ideas, or even to a knowledge of the most elementary relations of the sciences to one another.