Ricardo's and Mill's 'profit' is made more precise by the concept of the marginal efficiency of capital (the expression, though not the concept, is due to Keynes), which may be defined as the annual revenue which will be yielded by an extra increment of capital as a proportion of its cost. So the interest rate "r" in equilibrium will be equal to the marginal efficiency of capital "r". Rather than work with "r" and "r" as separate variables, we can assume that they are equal and let the single variable "r" denote their common value.