Michael Munger, interviewed by Russ Roberts:
"the original justification for slavery, which was the Roman one wasn’t good enough. And so Southerners cast about and found basically an alternative, which was the Greek justification for slavery. And let me just say very briefly what those two are. The one justification for slavery, and it was pretty common in Rome, was that if you lost a battle and were captured, then you might either be killed or kept as a slave. And there is a mutually beneficial exchange, if you will, in the sense that you’ve already lost. So, me saying, ‘I tell you what: I won’t kill you if you will agree to act as my slave for the rest of your life. And I may free you; I may not; but that’s up to me.’ And you say, ‘Killed/be a slave: I’m going to go with the slave thing.’ But, it meant that some slaves were very excellent. And in Roman society some slaves occupied very high positions, positions of respect. It’s just that they made this promise. It was an economic institution. And that was the way that slavery had existed in Africa: if you lost a battle, then you would be captured by the other side. It was almost like indentured servitude: you could work it off.
Well, that didn’t work in the American South because they wanted to maintain slaves, to be able to identify slaves and to have a justification that would allow them to enslave the children — which the old Roman justification would never have allowed. ...
So, the Southerners needed a different way, so they were looking for the Aristotelian notion of slavery, which is that slaves are people who are either morally inferior or lack the judgment to make independent choices. They are like children or like horses. That means that you actually have a positive-good justification for enslaving them: if I have a thoroughbred horse or a fancy dog, it would be cruel of me to set it loose to let it run around, because it’s not capable of taking care of itself. I have obligations to take care of it. My ownership actually gives me obligations. And what’s interesting and what this paper is about is how Southerners worked that out between about 1815 and 1835, and started to understand the implications for how they had to change the economic institutions of slavery to match this new ideology that they were creating. ...
what we’re looking at is how Southerners manage to persuade themselves. ... They actually came to believe that slavery was, first, a necessary evil; and then, later, a positive good: that not only could they not do without it, but that slaves themselves were better off as slaves than they would have been in Africa. ...
I think, that if I were born to a slave-owning wealthy family in the South in 1830, 1835, I would have defended slavery. And that’s terrible. But, the fact that you are raised in this system where people take it for granted; where it’s a kind of convention; and they had these justifications — these elaborately worked-out justifications — does make you wonder what 200 years from now, people will look back at our society and say, ‘How could they have thought that?’"
Many more interesting points, including: a) would Britain have given up slavery in 1830 (i.e., after the value of slave labor had risen)? b) the emergence of the rationale for racism 1815-1835 (e.g., "since I owned them I have a much better reason to take care of them because they are still going to be valuable to me 5 years from now; whereas if I rent labor, I don’t care: that guy can die"). c) slaves as "lazy" = trying to get better working conditions. d) Adam Smith discussing Greek justifications for infanticide, its parallels to justifications for slavery.
Appendum. Michael Karp writes: "the book is less about whether slavery was or was not “modern,” and more about the fact that leading slaveholders believed it was. ... It wasn’t just that slaveholders believed Britain and other European powers would come to their aid in a war against the North, although they did believe that. It was that their entire ideological and strategic worldview depended on a belief in the global necessity of slave labor. European states might oppose slavery in the abstract, but they could not escape the deeper principle of racial inequality upon which slavery rested."