Most Greek slaves were barbarians, and their inability to speak Greek kept them from talking back to their masters and also indicated their slave status. Aristotle, who despite being spectacularly wrong about almost everything, was incredibly influential, believed that some people were just naturally slaves, saying, It is clear that there are certain people who are free and certain people who are slaves by nature, and it is both to their advantage and just for them to be slaves. This idea, despite being totally insane, remained popular for millennia. The Greeks popularized the idea that slaves should be traded from far away, but the Romans took it to another level. Slaves probably made up 30% of the total Roman population, similar to the percentage of slaves in America at slavery's height. The Romans also invented the plantation, using mass numbers of slaves to work the land on giant farms called latifundia, so-called because they were not fun-dia. The Judeo-Christian world contributed as well, and while we are not going to venture into the incredibly complicated role that slavery plays in the Bible, because I vividly remember the comment section from the Christianity episode, the Bible was widely used to justify slavery and in particular the enslavement of Africans, because of the moment in Genesis when Noah curses Ham, saying, Cursed be Canaan, the lowest of slaves shall he be to his brothers.