It is from this eschatology that Plato derives the moral sanctions which he employs in his State. It is true that no one has insisted with greater force than he upon the truth that virtue is, in and for itself, the highest human good; he believed, however, that this could be appreciated only by the philosopher, who had experience of it, and that for the lower orders of men a more powerful, though less noble, sanction was necessary. Accordingly, he depicts the joys of Elysium in images that could not but appeal to the Hellenic imagination, and paints Tartarus in gruesome colors that would do honor to a St. Ignatius. In order fully to understand the method of Plato's political education, we must revert to Chapter III of Book I. There we saw that, according to the Greeks, a complete education demanded three things, (1) a noble nature, (2) training through habit, (3) instruction. For the first Plato would do what can be done by artificial selection of parents; for the second, he would depend upon music and gymnastics; for the third, upon philosophy. In these last two divisions we have the root of the mediæval _trivium_ and _quadrivium_. The Platonic pedagogical system seeks to separate the ignoble from the noble natures, and to place the former in the lowest class. It then trains the noble natures in music and gymnastics, and, while this is going on, it tries to distinguish those natures which are capable of rising above mere training to reflective or philosophic thought, from those which are not. The latter it assigns to the military class, which always remains at the stage of training, while the former are instructed in philosophy, and, if they prove themselves adepts, are finally admitted to the ruling class, as sages. Any member of either of the higher classes who proves himself unworthy of that class, may at any time be degraded into the next below.