I think the reader A Sinner provides all of the major points that should be considered, referring to the old Catholic Encyclopedia entry. He adds:
“I use the term slavery to connote chattel slavery, but I most definitely include in this the Roman institution as well as medieval slavery (as distinguished from serfdom). I simply cannot give any credence to an argument that this is one socio-economic arrangement among many in this fallen world, and so a priori ethically neutral.”
Again, I think it really depends on what you are viewing, and what those societies viewed, as essential to the institution called “slavery.”
If included the idea of “owning” a person, reducing them to an object, or in any way reducing on principle (as opposed to just often in practice, as an abuse) what the moral law says is owed in justice and charity towards another human being…of course this is wrong.
But this isn’t necessarily what slavery was in Old Testament Israel, for example, nor in medieval Christendom (even as opposed to “mere” serfdom).
Nothing can be considered a “human right” that is not granted to children vis a vis their parents. If one’s only objection to certain forms of slavery (albeit largely “theoretical” forms) is based on “freedom” or on masters subjecting slaves to the same sorts of limitations to which parents may legitimate subject their children…one should also, then, start arguing against the power of parents over children (on a family farm, for example).
As it stands, as Cardinal Dulles said, “more or less moderate forms of subjection and servitude will always accompany the human condition.”
Some have been critical of Aristotle for his acceptance and justification of slavery in the Greek world. I would just to take this away from what he said: a slave is distinguished from someone who is free or self-directing with respect to domestic/economic matters. He is a tool or an instrument of another. As far as I can remember, Aristotle does not say that the slave does not have any "rights" or that he is mere "property" of the master, or that the master is morally free to do with the slave as he wishes. Aristotle justifies slavery with the claim that some are naturally inferior and incapable of directing themselves; consequently, they should be directed by others. I do not think he talks of slavery as a punishment for a crime or as a consequence of losing a war. I don't think he talks about those who become deficient in the use of reason through the acquisition of vice or some other character flaw.
