Marx’s Economic Manuscripts of 1861-63
Capital and Profit

2) Profit always expresses Surplus Value too Small

It follows from the characteristic distinction of form between surplus value and profit that profit always expresses a smaller proportion than that of real surplus value, hence the rate of profit always represents the ratio in which capital appropriates alien labour as much smaller than it really is. This (tautological) law, once understood, does away with all incorrect statistics, and it has bigger merits. It is essential for the comprehension of phenomena which would otherwise remain incomprehensible and limp along beside the theory as indigestible fragments of reality.

It goes without saying that the magnitude a expresses a smaller ratio if it is measured against b+c+a than if it is measured against c+a, or that a magnitude expresses a larger or smaller part of a third magnitude according to whether that latter magnitude is itself larger or smaller. The total capital is therefore always larger than the part of it which is exchanged for wages.