At the age of twenty he would select the most promising youths and give them a ten years' course in severe study of science. This systematic study corresponds to the graduate and professional period in modern education, only he extends it over ten years, where we confine it to three or four. Again at thirty there is another selection of those who are most steadfast in their learning and most faithful in their military and public duties, and these are given a five years' course in dialectic or philosophy. They are trained to see the relation of the special sciences to each other and how each department of truth is related to the whole. At the age of thirty-five they must be appointed to military and other offices. "In this way they will get their experience of life, and there will be an opportunity to try whether, when they are drawn all manner of ways by temptation, they will stand firm or stir at all." And when they have reached the age of fifty, after fifteen years of this laboratory work in actual public service, holding subordinate offices and learning to discriminate good and evil, not as we find them done up in packages and labelled in the study, but as they are interwoven in the complicated texture of real life, "those who still survive and have distinguished themselves in every deed and in all knowledge, come at last to their graduation; the time has now arrived at which they must raise the eye of the soul to the universal light which lightens all things and behold the absolute good; for that is the pattern according to which they are to order the state and the lives of individuals and the remainder of their own lives also, making philosophy their chief pursuit; but when their turn comes, also toiling at politics and ruling for the public good." The wisdom which comes of this prolonged and elaborate education is the third of Plato's four cardinal virtues. In the state it is the ruling principle, and its agents are the philosophers. As Plato says in a famous passage: "Until then philosophers are kings, or the kings and princes of this world have the spirit and power of philosophy, and political greatness and wisdom meet in one, and those commoner natures who follow either to the exclusion of the other are compelled to stand aside, cities will never cease from ill,--no, nor the human race, as I believe,--and then only will this our state have a possibility of life and behold the light of day." Precisely so, no individual will attain his true estate until this philosophic principle, which sees the good, through training has been so developed that it can bring both appetite and spirit into subjection to it, as a charioteer controls his headstrong horses. V RIGHTEOUSNESS THE COMPREHENSIVE VIRTUE