Capital, Volume Three: The Process of Capitalist Production as a Whole by Karl Marx

Suppose, as is assumed in this part, the amount of profit in any particular sphere of production equals the sum of the surplus-value produced by the total capital invested in that sphere. Even then the bourgeois will not consider his profit as identical with surplus-value, i. e., with unpaid surplus-labour, and, to be sure, for the following reasons:

1) In the process of circulation he forgets the process of production. He thinks that surplus-value is made when he realises the value of commodities, which includes realisation of their surplus- value.