This false appearance distinguishes wages labor from other historical forms of labor. On the basis of the wages' system even the unpaid labor seems to be paid labor. With the slave, on the contrary, even that part of his labor which is paid appears to be unpaid. Of course, in order to work the slave must live, and one part of his working day goes to replace the value of his own maintenance. But since no bargain is struck between him and his master, and no acts of selling and buying are going on between the two parties, all his labor seems to be given away for nothing.

Wage-Labour and Capital/Value