The philosopher’s frank admissions, in which he opposes the doctrine that slavery is founded on mere force, are fatal to his first argument on the natural slave. He admits that nature does not always consummate her purpose; that the souls of freemen are sometimes found in the bodies of slaves, and vice versa;[653] that it is difficult to distinguish the quality of the soul, in any event;[654] that the claim that slavery is neither natural nor beneficial has in it a modicum of truth, as there are sometimes merely legal slaves, or slaves by convention;[655] that slavery based on mere might without virtue is unjust;[656] that captives of war may be wrongly enslaved;[657] that only those who actually deserve it, should meet this fate;[658] that the accidents of life may bring even the noblest of mankind into slavery;[659] and that only non-Greeks are ignoble and worthy of it.[660] He even insists that the terms “slave-master,” “freeman,” “slave,” when rightly used, imply a certain virtue or the lack of it, and therefore that to be justly a master, one must be morally superior.[661] The question of the possession of the higher virtues by slaves is recognized by him to be a difficult problem, for an affirmative answer breaks down his distinction of “natural” slave, yet it seems paradoxical to deny these virtues to him as a human being.[662] Nor can the difficulty be avoided by positing for the slave a mere difference in degree of virtue, for the distinction between ruler and subject must be one of kind.[663] In any event, temperance and justice are necessary even for good slave service.[664] Aristotle therefore evades the difficulty, and begs the question by concluding that both master and slave must share in virtue, but differently, in accord with their respective stations.[665] With this admission, he places slaves on a higher plane than free artisans, in that he denies virtue to such classes, since it cannot be produced in them, except as they are brought into contact with a master.[666] He thus makes slavery a humanitarian institution, and the slave a real member of the family.[667] But the admission most fatal to his theory is in agreeing that the slave _qua_ man may be a subject of friendship,[668] and in advocating his manumission as a reward for good behavior. With this, the attempted distinction between him, _qua_ slave and _qua_ man, utterly breaks down, and the existence of natural slaves is virtually denied.[669] Thus the great champion of slavery in the ancient world, by his very defense of it, repudiates its right as a natural institution. His actual conception of the relation is, indeed, not far from the ideal of Plato, a union for the best mutual service of rulers and ruled, in which the slave receives from his master a moral exchange value for his physical service.[670]