Note to the Nature of Wealth by John Ruskin

The question of equivalence (namely, how much wine a man is to receive in return for so much corn, or how much coal in return for so much iron) is a quite separate one, which we will examine presently. For the time let it be assumed that this equivalence has been determined, and that the Government order in exchange for a fixed weight of any article (called, suppose, a), is either for the return of that weight of the article itself, or of another fixed weight of the article b, or another of the article c, and so on.