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.