" The wage-worker sells to the capitalist his labour-force for a certain daily sum. After a few hours’ work he has reproduced the value of that sum; but the substance of his contract is, that he has to work another series of hours to complete his working-day; and the value he produces during these additional hours of surplus labour is surplus value, which cost the capitalist nothing, but yet goes into his pocket. "

― Friedrich Engels , The Condition of the Working Class in England