Yesterday I asked a question on slavery regarding whether Romans or the Ancients in general had any movement to end slavery/improve the welfare of slaves. The general consensus I was given that Roman and Greek society at least, lacked the perspective of slavery being something "wrong" (even among slaves themselves) even if slaves individually fought to improve their lot in life.
This is a follow up question then. I would like to know then when historically did we see this movement towards slavery being seen as something inherently evil. (Apologies to any admins if I was meant to continue posting/asking the question in the original post. I felt this question, while similar tangentially to my original post, was different enough and required different expertise knowledge warrant a seperate question. If there are any issues, please tell me and I'll try avoid it in the future! I'm new to this incredible subreddit)
Well, it's not that some Classical thinkers didn't hold anti-slavery views - a full range of opinions on any given subject can typically be found in any population. For example: Alcidamas, in his Messeniakos, argued for emancipation and wrote "God has left all men free; nature has made no man a slave". And when Diogenes' slave Manes ran away, the philosopher remarked "It would be absurd if Manes can live without Diogenes, but Diogenes cannot get on without Manes". See also: Epictetus, Antisthenes, and Zeno.
The thing is, most of the Cynics and Stoics saw the negatives of slavery as being more abstract than the mere socio-economic condition of permanent servitude. It was a common sentiment amongst them that it was worse to be a "slave to desire" or something than to be an actual literal slave, since a literal slave could still have a free heart and purse virtuous ideals, while a master might find himself constantly enraptured with futile pursuits. So they didn't see slavery as inherently evil so much as simply being another of the myriad imperfections in Greek civil life, and were more concerned about the moral attributes of the individuals themselves than of the legal institution.
Worth noting is that institutional slavery was different in the ancient Mediterranean than in the colonial era, which is when the most explicit moral abolitionists emerged.
You know what, this is actually a much more complicated subject than I can tackle. I recommend David Brion Davis' The Problem of Slavery in Western Culture, which deals with precisely this topic in several of its chapters. It's probably horribly out of date, but it did win a Pulitzer in '67.