Dear Andrew,
Perhaps we should have two kinds of the profit rate conception. By definition,
of course, the rate of profit may well be a ratio between the new value increment and the original (old) value as it is to be estimated over a certain period.
An interest rate, too, is not to be calculated on the two terms of the same points of time.
However, when we think of the competitive capitalists, they have to compute the
profit rate, if he/she intends to enter a new branch of production, by reference to the present (new) input and output values. So, Marx's equal rates of profit
are to be calculated on the (new) input values if it is related to the capitalist competition. So, I think there are two kinds of profit rates. One is actual, customary profit rate, the other is an economic (competition-inspired) profit
rate. How do you think?
With regards
Chai-on