Socrates had taught that virtue is one. And Plato in his earlier writings adopted this view. But later on he came to see that every faculty of man has its place and its function, and the due performance of its function is a virtue. He did not, however, surrender the unity of virtue altogether, but believed that its unity is compatible with its plurality. There are four cardinal virtues. Three of these correspond to the three parts of the soul, and the fourth is the unity of the others. The virtue of reason is wisdom, of the noble half of {224} the mortal soul courage, of the ignoble appetites, temperance or self-control, in which the passions allow themselves to be governed by reason. The fourth virtue, justice, arises from the others. Justice means proportion and harmony, and accrues to the soul when all three parts perform their functions and co-operate with each other. Following Zeller, we may add to this account of the virtues some of Plato's views upon the details of life. And first, his opinion of women and marriage. Here Plato does not rise above the level of ordinary Greek morals. He has nothing specially original to say, but reflects the opinions of his age. Women he regards as essentially inferior to men. Moreover, the modern view of woman as the complement of man, as possessing those special virtues of womanliness, which a man lacks, is quite alien to Plato. The difference between men and women is, in his view, not one of kind but only of degree. The only specific difference between the sexes is the physical difference. Spiritually they are quite the same, except that woman is inferior. Hence Plato would not exclude women from the same education which man receives. He would educate them in exactly the same way, but this involves the imposition upon them of the same burdens. Even military duties are not outside the sphere of women. His views of marriage flow from the same principle. Since woman is not the complement of man, she is in no special sense fitted to be his companion. Hence the ideal of spiritual companionship is absent from Plato's view of marriage, the sole object of which, in his opinion, is the propagation of children. The natural companion {225} of a man is not a woman, but another man. The ideal of friendship, therefore, takes the place of the spiritual ideal of marriage in Plato and, indeed, among the ancients generally. Slavery is not denounced by Plato. He takes no trouble to justify it, because he thinks it so obviously right that it needs no justification. All that can be said to his credit is that he demands humane and just, though firm and unsentimental, treatment of slaves.