What the working man sells is not directly his labor, but his laboring power, the temporary disposal of which he makes over to the capitalist. This is so much the case that I do not know whether by the English Laws, but certainly by some Continental Laws, the maximum time is fixed for which a man is allowed to sell his laboring power. If allowed doing so for any indefinite period whatever, slavery would be immediately restored. Such a sale, if it comprised his lifetime, for example, would make him at once the lifelong slave of his employer.

Wage-Labour and Capital/Value