The Basis of Modern Economics Is the Cognition That It Is Precisely the Disparity in the Value Attached to the Objects Exchanged That Results in Their Being Exchanged

An inveterate fallacy asserted that things and services exchanged are of equal value. Value was considered as objective, as an intrinsic quality inherent in things and not merely as the expression of various people’s eagerness to acquire them. People, it was assumed, first established the magnitude of value proper to goods and services by an act of measurement and then proceeded to barter them against quantities of goods and services of the same amount of value. This fallacy frustrated Aristotle’s approach to economic problems and, for almost two thousand years, the reasoning of all those for whom Aristotle’s opinions were authoritative. It seriously vitiated the marvelous achievements of the classical economists and rendered the writings of their epigones, especially those of Marx and the Marxian school, entirely futile. The basis of modern economics is the cognition that it is precisely the disparity in the value attached to the objects exchanged that results in their being exchanged. People buy and sell only because they appraise the things given up less than those received. Thus the notion of a measurement of value is vain. An act of exchange is neither preceded nor accompanied by any process which could be called a measuring of value. An individual may attach the same value to two things; but then no exchange can result. But if there is a diversity in valuation, all that can be asserted with regard to it is that one a is valued higher, that it is preferred to one b. Values and valuations are intensive quantities and not extensive quantities. They are not susceptible to mental grasp by the application of cardinal numbers.

--Ludwig von Mises, Human Action: A Treatise on Economics, ed. Bettina Bien Greaves (Indianapolis: Liberty Fund, 2007), 1:203-204.