Everyone agrees that slavery is evil. Everyone knows that slavery existed. Is there any understanding of why it existed? Is it just because people were too ignorant/greedy to see the problem with it? I have heard that Karl Marx has a theory that slavery was necessary (and evil in the same time) because every society needs some sort of forced labor. Nowadays, technology is our source of forced labor, but when technology was very primitive, slavery played the role of the required forced labor. Does such a theory (or anything somehow similar to this) exist? If yes, what is the reasoning behind it.

I don't think you are right, many people in the past thought that slavery was unproblematic, and based on the natural differences that exist between people. In just the same way that today we think it is OK for animals to work for us and be eaten by us, and for plants to be used by us, so some argued that slaves and the group they came from were so different from their owners that different treatment was appropriate.

I have been to countries where slavery still exists, informally if not entirely legally, and the same justification is used today as was used in the past.

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