alt Hacker NewsSlavery was already being abolished in the West when Adam Smith wrote the Wealth of Nations. But what was notable was that Adam Smith was really the first to make a strong case and prediction that it was not just the moral thing to do, but would lead to prosperity.
Adam Smith also differentiated between different levels of slavery - that Roman slavery was different than Serfdom was different from chattel slavery in the US.
It's worth noting that Adam Smith did not think total abolition was possible. One of his concerns about free markets was that people deeply desired control of other people, and slavery would increase as a byproduct of wealth.
And effectively it did: many people are kept in their place by the combined pressure points of debt and employment to stay (barely) afloat.
This is of course nothing compared to the cruelty of real slavery but the effect is much the same, a lot of people are working their asses of for an upper class that can ruin their lives at the drop of a hat. That there are no whips involved is nice but it also clearly delineated who was the exploiter and who were the exploited. That's a bit harder to see today.