Slavery - Modern Focus

I mentioned slavery while discussing Aristotle's 'Politics'. I said that slavery was an unchallenged norm of the time and I was curious when that really changed. It seems that centuries later, Aristotle still had a seat at the debate. In 'Politics' he says:

those who are as different [from other men] as the soul from the body or man from beast—and they are in this state if their work is the use of the body, and if this is the best that can come from them—are slaves by nature. For them it is better to be ruled in accordance with this sort of rule, if such is the case for the other things mentioned.

So some people are simply made for slavery; those that are as different from others as the man from the beast. Unfortunately this is hardly a clear distinction. When the Spanish met up with the natives in the New World they must have seemed to qualify and the Spanish virtually enslaved them. This was the argument of a 16th century theologian.
Before the New World was found, there was a much smaller racial component to slavery. In ancient Greece slaves came from a few different categories. They were sometimes conquered peoples. They were also something like indentured servants. Fellow Greeks were slaves. Some slaves were used as teachers and tutors. It's hard to imagine the conquistadors bringing back Aztecs for use as teachers and tutors . . .
Over many centuries, Western civilization had been more or less comfortable with slaves and quasi slaves such as serfs. Then with exploration they discovered groups of people that they could comfortably regard as beasts. I don't know how much attention people had paid to Aristotle's ideas on slaves before then but it certainly would have made sense then.
Only something else happened at the same time. A Spanish settler named De Las Casas found himself shocked at the treatment of natives/slaves in Cuba. He worked and worked and in 1542, Spain enacted laws outlawing slavery in their colonies. Pro slavery forces weakened the laws in a few years but now their was real argument against the practice. It would take some centuries, but eventually slavery would become illegal throughout the West.

When Western explorers came in contact with a more obvious 'other' they felt free to use them as slaves. They also treated them much worse than they had their countrymen. In fact, they treated them so badly that it shocked the conscious of some of the society's thinkers and led them to develop universal arguments against the practice of slavery.