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

Here, as at the close of the preceding chapter, and generally in this entire first part, we presume the amount of profit falling to a given capital to be equal to the total amount of surplus-value produced by means of this capital during a certain period of circulation. We thus leave aside for the present the fact that, on the one hand, this surplus-value may be broken up into various sub- forms, such as interest on capital, ground-rent, taxes, etc., and that, on the other, it is not, as a rule, identical with profit as appropriated by virtue of a general rate of profit, which will be discussed in the second part.

So far as the quantity of profit is assumed to be equal to that of surplus-value, its magnitude, and that of the rate of profit, is determined by ratios of simple figures given or ascertainable in every individual case. The analysis, therefore, first is carried on purely in the mathematical field.