Karl Marx' 'Capital' in Lithographs page 50. What actually confronts the owner of money in the market is not labor, but the worker. What the worker sells is his labor power. As soon as his work really begins, it has already ceased to belong to him and can therefore no longer be sold by him.
Labor is the substance and the immanent measure of value, but it has itself no value.
It is an expression as imaginary as when we speak of the value of the earth. Yet these imaginary expressions arise out of the very relations of production. They are categories for the phenomenal forms of essential relations. Since the value of labor is only an irrational expression for the value of labor power, it is self-evident that the value of the labor must invariably be less than the value it creates, for the capitalist always arranges that labor power shall continue in operation for a longer time than is necessary for the reproduction of its own value. . . . . . . In slave labor, even that part of the working day in which the slave is merely replacing the value of his own means of subsistence, and during which, therefore, he is really working for himself, assumes the aspect of work for his master.