There is no dispute over the fact that slaves are to some extent different from the others, but there is sharp disagreement as to whether or not stress on this distinction is mere pedantry. Schematically, the alternative is between viewing slavery as one species of the genus "dependent (or involuntary) labour" and viewing slavery as the genus, the others as species. Retention of the slave/serf distinction even by those who reject further differentiation provides a clue to the answer, which, in Marxist terms, is embedded in the concepts of MODE OF PRODUCTION and SOCIAL FORMATION. Serfs were the appropriate form of labour under feudalism, slaves in ancient society, a major element in the social relations of production along with private property and commodity production.
The test of the dominance of a slave mode of production lies not in the numbers of the slaves but in their location, that is, in the extent to which the elite depended on them for their wealth.
Capitalism has reduced the vast majority of human beings to a condition of following someone else’s orders, with no control over their own circumstances that for most of human history would have been seen as fit only for slaves.