Half a century later Marx went into this much more thoroughly and dealt with it in Chapter XIV of the 3rd Volume of Capital. He held that there is a general tendency for the rate of profit to fall because of the greater quantity of constant capital in production (plant, machinery, raw material, etc.) and the relatively smaller quantity of variable capital, i.e., that part spent on wages. He explained in the opening paragraph of the chapter that whereas other economists were looking for an explanation of the falling rate of profit the problem for him was the opposite one, namely of finding out why the fall is not greater and more rapid. He wrote: “There must be some counteracting influences at work, which thwart and annul the effects of this general law, leaving to it merely the character of a tendency.” In Chapter XIV he dealt briefly with these counteracting factors which included raising the intensity of exploitation, and the cheapening of the elements of constant capital. He therefore expected the fall to be slow.

https://www.marxists.org/archive/hardcastle/1960/rateofprofit.htm