Yet notwithstanding the loftiness and nobility of much of Plato’s ethical thought, still, like Socrates, he stood almost wholly on Greek ground. His ethics is scarcely more than a justification of the common Greek morality of his time. He destroys the family in the interest of the state; he approves of the exposition of ill-formed, unpromising infants; he makes morality to be a class thing--only select and cultured souls are with him capable of genuine virtue. He accepts slavery as a necessary institution of the state; he practically shuts out the non-Greek world from the sphere of morality;[505] and with the common Greek he believes that to do evil to one’s enemies is an imperative duty.[506] Nor does Plato, like Hebrew seer, rise high enough above the general Greek viewpoint to discern the great law of moral progress, and to prevision the historical goal--ethical world unity. [Sidenote: Aristotle and his ethics] Aristotle makes Plato’s classification of the virtues the basis of his well-rounded system of ethics. In one important respect, however, he differs from Plato; he did not believe that knowledge of the right necessarily leads to its practice. He recognized the fact that man though knowing the good often perversely follows evil. The great defect of Aristotelian ethics is its failure to rise to the ethical conception of collective humanity. In the moral inequality of men, which he assumes as the presupposition of his ethics, he even exaggerated the common Greek view. He divided men so rigorously into classes with varying grades of moral capacity that his moral system was ethically like the caste system of the Indian Brahmans. To affirm the moral equality of men seemed to him to be a species of treason against the true humanity, a crime against Greek civilization. According to Aristotle the slave was a being so morally different from the freeman as to constitute practically another species. He was not wholly incapable of virtue, but could practice only such servile virtues as obedience and humility. The last, though a virtue in a slave, was in a freeman an unworthy weakness.