The Making of Modern Civilization: Savings, Investment, and Economic Calculation-Mises

“It was one of the fundamental errors of Aristotle that he believed that in exchanges the things which were exchanged must have the same value. Since the days of Aristotle, for two or three thousand years, the same error has prevailed again and again, leading great thinkers, as well as simple men, astray. The same error appears in the first pages of Marx’s Das Kapital, making everything Marx said about these problems useless. This error was repeated even much later in the writings of Henri Bergson2, the eminent French philosopher. 

There is no equivalence in exchange. On the contrary, it is differences that bring about exchange. You cannot reduce the terms of exchange and of trade to equivalence; you can only reduce them to differences in evaluation. The buyer values what he gets more highly than what he gives away; the seller values what he gives less highly than what he gets…”