Aristotle wrote quite a bit about slavery. He claimed it was justified in cases where the slave is not capable of taking control of his own life. We don’t let children decide whether they eat their broccoli or not, and similarly we make decisions for slaves about what sort of work they are to do etc. Aristotle recognized that not everyone who is enslaved is this helpless, and therefore not everyone who is enslaved ought to be enslaved. But once slavery was limited to people of a certain skin color, (it wasn’t in Aristotle’s time) it was possible to claim that all “those people” were naturally inferior, and therefore all of them deserve to be enslaved.

Until the United States came along, It was assumed that there was a natural order of things in Which everyone obeyed their superiors. Slaves obeyed their masters, who obeyed their liege lords, who obeyed their King, who obeyed God. Thomas Jefferson was one of the first people to widely popularize the idea (first expressed by John Locke), that all people are created equal, and that they had the right to depose a king if they chose to. Despite him being a slave owner, and a fairly harsh one, I think he deserves some moral credit for putting that idea out there. We take that idea for granted now, but it was radical when he said it. More on this here.

https://teedrockwell.medium.com/save-our-imperfect-moral-heroes-e490dced158c