Capital, Volume One: The Process of Production of Capital by Karl Marx

On the surface of bourgeois society the wage of the labourer appears as the price of labour, a certain quantity of money that is paid for a certain quantity of labour. Thus people speak of the value of labour and call its expression in money its necessary or natural price. On the other hand they speak of the market-prices of labour, i.e., prices oscillating above or below its natural price.

But what is the value of a commodity? The objective form of the social labour expended in its production. And how do we measure the quantity of this value? By the quantity of the labour contained in it. How then is the value, e.g., of a 12 hour working-day to be determined? By the 12 working-hours contained in a working day of 12 hours, which is an absurd tautology. [Footnote 1]