Where then there is such a difference as that between soul and body, or between men and animals (as in the case of those whose business is to use their body, and who can do nothing better), the lower sort are by nature slaves, and it is better for them as for all inferiors that they should be under the rule of a master.
For he who can be, and therefore is, another’s and he who participates in rational principle enough to apprehend, but not to have, such a principle, is a slave by nature. Whereas the lower animals cannot even apprehend a principle; they obey their instincts.
And indeed the use made of slaves and of tame animals is not very different; for both with their body’s minister to the needs of life. Nature would like to distinguish between the bodies of freemen and slaves, making the one strong for servile labour, the other upright, and although useless for such services, useful for political life in the arts both of war and peace.
But the opposite often happens-that some have the souls and others have the bodies of freemen. And doubtless if men differed from one another in the mere forms of their bodies as much as the statues of the Gods do from men, all would acknowledge that the inferior class should be slaves of the superior. And if this is true of the body, how much more just that a similar distinction should exist in the soul? But the beauty of the body is seen, whereas the beauty of the soul is not seen. It is clear, then, that some men are by nature free, and others slaves, and that for these latter slavery is both expedient and right.
So the theory is that natural slaves should have powerful bodies but be unable to rule themselves. Thus, they become very much like beasts of burden, except that unlike these beasts human slaves recognize that they need to be ruled. The trouble with this theory, as Aristotle quite explicitly states, is that the right kind of souls and bodies do not always go together so, one could have the soul of a slave and the body of a freeman, and vice versa.
Nonetheless, apparently because there are some in whom the body and soul are appropriate to natural slavery, that is a strong body and a weak soul, Aristotle holds that there are people who should naturally be slaves. It also seems that men naturally rule women and that bararians are naturally more servile than Greeks! This seems like an odd, indeed arbitrary, way for the virtues of the soul to be distributed! Las Casas deals with a similar problem in regard to the native peoples of the Americas.