An interesting tidbit (mind you, I'm not saying I agree, just that it's interesting):
Adam Smith made the argument that free labor was economically better than slave labor, and argued further that slavery in Europe ended during the Middle Ages, and only then after both the church and state were separate, independent and strong institutions, that it is nearly impossible to end slavery in a free, democratic and republican forms of governments since many of its legislators or political figures were slave owners, and would not punish themselves, and that slaves would be better able to gain their freedom when there was centralized government, or a central authority like a king or the church. Similar arguments appear later in the works of Auguste Comte, especially when it comes to Adam Smith’s belief in the separation of powers or what Comte called the "separation of the spiritual and the temporal" during the Middle Ages and the end of slavery, and Smith's criticism of masters, past and present. As Smith stated in the Lectures on Jurisprudence, "The great power of the clergy thus concurring with that of the king set the slaves at liberty. But it was absolutely necessary both that the authority of the king and of the clergy should be great. Where ever any one of these was wanting, slavery still continues."