Since Aristotle’s notes on slavery frequently attract controversy, it may be well worth it to spend some time figuring out exactly what he means.
Even before Aristotle starts talking about slavery, he quotes “the poets” to whom he ascribes the dictum, “It is meet that the Hellenes should rule over the barbarians.” (1.2.8) Aristotle quotes this in a section wherein he considers proper place of slaves and women within a society, and this appears to lay the foundation for his slightly later discussion on slavery. Aristotle starts, then, by illustrating with the example of Greek dominion over barbarians the rightfulness of a slave’s disordered soul to the more ordered soul of his master.
The Philosopher spends a while describing instrumentality through the slave and various justifications of slavery, after which he arrives at an analogy: just as some are born with the bodies of slaves and some are born with the bodies of freemen, so are some born with the minds of slaves and some born with the minds of freemen. Aristotle concludes thusly:
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.
(1.5.36-40) That is to say, so much as some may have a body intended for servitude, and some a body intended for the work of a freeman, even more so are free and slave distinguished in their souls.
Chapter 6 contains a discussion on relative freedom and nobility, according to which Greeks are reluctant to deign anyone a slave but barbarians, save when in the barbarians’ own land. Concluding that both Greek and barbarian believe that “from good men a good man springs,” (1.6.40-42) the Philosopher continues to discern a relationship between slave and master the abuse of which constitutes an injury to both.
Aquinas comments on a “good man” springing from other “good men” in the following:
And sons may also differ from parents in goodness or wickedness not only because of natural bodily disposition but also because of an aspect that does not necessarily result from a natural inclination. And so human beings who are similar to their parents in natural disposition may also, because of a different education and upbringing, be dissimilar even in morals. Therefore, if the children of good parents are good, they will be both reputedly and really good. But if the sons of good parents are wicked, they will be reputedly noble but actually base. And the converse is true about the sons of wicked parents.
Then [Aristotle] shows how it is expedient or inexpedient for some to be slaves, summarily concluding form the foregoing that the difficulty previously posed has some plausibility. And so there is in some cases a distinction between freedom and slavery by law, not by nature. But in other cases, there is a distinction between the two by nature, and it is advantageous in such cases for one person to be a slave and another to be his master. And this is also just.
(Commentary on Aristotle’s Politics, comment on 1255a3-b15.)
Thomas earlier quotes the more succinct and poignant passage from Proverbs: “The stupid will serve the wise.” (Prov. 11:29)
This indicates that slavery, as imagined by Aristotle and briefed by Thomas, is entirely just when dealing with the submission of lazy or wicked souls to better souls. This, it appears, is why Aristotle includes a discussion of the natural subservience of barbarians – as imagined as disordered souls – at the beginning of Politics. The barbarians described at 1.2.8 have no order, and so no ability to establish a polis in which they might work towards the Good. When Aristotle considers the arguments for and against slavery starting in 1.4, he finds a middle ground between slavery by human law and absolute abolition in finding that some people are indeed less suited for for freedom owing to the quality of their souls. As Aristotle believes it is just that someone establish dominion over the barbarians to inculcate virtue within them, so it is just that less ordered souls within a polis be led toward virtue by “masters.”
Those are only some quick thoughts that throw out some terms such as “justice” and “virtue” that may require further background in Politics or Nicomachean Ethics, thought I welcome and encourage additional thoughts on this.