The question of slavery in the ancient world requires careful distinction between different forms of unfree labor and different cultural contexts. What we call 'ancient slavery' was not a single institution but a spectrum of practices varying enormously by time, place, legal status, and practical condition.

Athenian chattel slavery — the form most discussed in ancient philosophy — was a complete legal and social exclusion from the polis. Aristotle's defense of natural slavery in the Politics is the most systematic ancient argument for the institution, and it remains uncomfortable reading. The Athenian economy was substantially dependent on slave labor in the silver mines at Laurion, where conditions were brutal, and in domestic service, where they were much more variable.

Roman slavery was legally absolute but socially more fluid: manumission was relatively common, and freedmen could achieve considerable wealth and influence. The Stoic philosophers argued that external conditions including enslavement were morally irrelevant — true freedom was internal — which was a radical position and also, critics then and now have pointed out, a position that conveniently rationalized the status quo.

The question of ancient moral discourse about slavery is interesting. There was no ancient abolitionist movement, but there were critics of the institution and arguments about the moral arbitrariness of slave status.