The Step Leading from Classical to Modern Economics Is the Realization that Classes of Goods in the Abstract Are Never Exchanged and Valued, But Always Only Concrete Units of a Class of Goods

Only the disintegration of the universalistic mentality brought about by the methodological individualism of the seventeenth and eighteenth centuries cleared the way for the development of a scientific catallactics. It was seen that on the market it is not mankind, the state, or the corporative unit that acts, but individual men and groups of men, and that their valuations and their action are decisive, not those of abstract collectivities. To recognize the relationship between valuation and use value and thus cope with the paradox of value, one had to realize that not classes of goods are involved in exchange, but concrete units of goods. This discovery signalized nothing less than a Copernican revolution in social science. Yet it required more than another hundred years for the step to be taken. This is a short span of time if we view the matter from the standpoint of world history and if we adequately appreciate the difficulties involved. But in the history of our science precisely this period acquired a special importance, inasmuch as it was during this time that the marvelous structure of Ricardo’s system was first elaborated. In spite of the serious misunderstanding on which it was constructed, it became so fruitful that it rightly bears the designation “classical.”

The step that leads from classical to modern economics is the realization that classes of goods in the abstract are never exchanged and valued, but always only concrete units of a class of goods. If I want to buy or sell one loaf of bread, I do not take into consideration what “bread” is worth to mankind, or what all the bread currently available is worth, or what 10,000 loaves of bread are worth, but only the worth of the one loaf in question. This realization is not a deduction from Gossen’s first law. It is attained through reflection on the essence of our action; or, expressed differently, the experience of our action makes any other supposition impossible for our thought.

--Ludwig von Mises, Epistemological Problems of Economics, trans. George Reisman, ed. Bettina Bien Greaves (Indianapolis: Liberty Fund, 2013), 139-140.