Bernard Williams, in Shame and Necessity, says that the ancient Greeks – here talking primarily of the Classical period – did not generally attempt to morally justify slavery.
Instead, since they believed that it was essential to the functioning of their society, particularly domestic slavery, they just hoped that this fate would never fall of themselves or their relatives, then otherwise ignored the problem. “If someone is a slave, he has the same flesh, for no one was ever born a slave by nature: it is chance that has enslaved his body.” (p. 109)
(Under Roman law apparently, if a woman conceived a child when she was free, but became a slave by the time that it was born, it was recognised as free.)
Later antiquity “seems to have given up on the question of slavery as a moral problem, trying, unedifyingly instead, to claim that real freedom was freedom of the spirit, also, perhaps even especially, available to slaves. So said Seneca: “It is a mistake to think that slavery goes all the way down into a man. The better part of him remains outside it. The body belongs to the master and is subject to him, but the soul is autonomous and is so free that it cannot be held in any prison … ” (p. 115)
(See where dualism gets you!)
But lest we indulge in a “reflex of self-congratulation”, Williams says: “We have social practices in relation to which we are in a situation much like that of the Greeks with slavery. We recognise arbitrary and brutal ways in which people are handled by society, ways that are conditioned, often, by no more than exposure to luck. … [But are uncertain whether to act] partly because we have seen the corruption and collapse of supposedly alternative systems, partly because we have no settled opinion on the question … how far the existence of a worthwhile life for some people involves the imposition of suffering on others.” (p. 125)
Some other snippets:
Quoting Wilamowitz: “To make the ancients speak, we must feed them with our own blood.” (p. 19)
On the lack of the mind/body dualism in Homer: “the word that came to mean something like ‘soul’ by the time of Plato, psuche, stands in Homer for something that is mentioned only when someone is fainting, dying or dead; when the person is dead, it is pictured as existing in a very flimsy, deprived and unenviable condition …. The later Greek word for the body as opposed to the soul, soma, means a corpse in Homer. (p. 23) The sensibility of Homer “was basically formed by the thought that this thing that will die, which unless it is properly buried will be eaten by dogs and birds, is exactly the thing that one is.” (p. 24)
“The Greeks tended to regard the capacity to hold out against feeling or desire as the same capacity, whatever the feeling or desire and however it originated — whether it was sexual desire or the desire to yield to pain or to run away or to take revenge. For this reason, they tended to put together strengths and weaknesses in ways different from those that have been familiar to what has been, at least until very recently, conventional modern opinion … they thought that men were better at resisting both fear and sexual desire than women were.” (p. 40)
To the claim that Homer’s heroes lack and inner life: “we must come back to the question of what the poem is doing. Some characteristics of these figures – the dignity, the distance, the grave acceptance of a fate or fortune that is given … are features of how they are presented, artefacts of the epic style.” (p. 46-7)