Labour-power was not always a commodity (merchandise). Labour was not always wage-labour, i.e., free labour. The slave did not sell his labour-power to the slave-owner, any more than the ox sells his labour to the farmer. The slave, together with his labour-power, was sold to his owner once for all. He is a commodity that can pass from the hand of one owner to that of another. He himself is a commodity, but his labour-power is not his commodity. The serf sells only a portion of his labour-power. It is not he who receives wages from the owner of the land; it is rather the owner of the land who receives a tribute from him. The serf belongs to the soil, and to the lord of the soil he brings its fruit. The free labourer, on the other hand, sells his very self, and that by fractions. He auctions off eight, 10, 12, 15 hours of his life, one day like the next, to the highest bidder, to the owner of raw materials, tools, and the means of life – i.e., to the capitalist. The labourer belongs neither to an owner nor to the soil, but eight, 10, 12, 15 hours of his daily life belong to whomsoever buys them. The worker leaves the capitalist, to whom he has sold himself, as often as he chooses, and the capitalist discharges him as often as he sees fit, as soon as he no longer gets any use, or not the required use, out of him. But the worker, whose only source of income is the sale of his labour-power, cannot leave the whole class of buyers, i.e., the capitalist class, unless he gives up his own existence. He does not belong to this or that capitalist, but to the capitalist class; and it is for him to find his man – i.e., to find a buyer in this capitalist class.

-Karl Marx, Wage Labour and Capital, Ch. 2