Having a passing understanding of Herodotus, Aristotle, and Cicero, it always struck me as odd that for peoples that seem to be so concerned with their rights, freedoms and the nature of man, no one in the Roman or Greek worlds seems to have come up with a philosophical opposition to slavery. Aristotle even wrote a justification for slavery in The Politics in the terms of it being a positive good for "barbarians", to whom he presumably thought the Greeks were doing a big favour by enslaving.

However, recently I've been reading Seneca's Letters to Lucilius, and in Letter XLVII he spends a long time expressing the equality between master and slave, how both are human beings and both are worthy of respect, couching the argument in Stoic philosophy. While he doesn't outright say "slavery is evil", this seemed to me to be a rather radical view for the Classical world. How common were sentiments like this and did anyone take it to the point of opposing slavery utterly?