When a country has substituted credit money or fiat money for metallic money, because the legal equating of the over-issued paper and the metallic money sets in motion the mechanism described by Gresham's Law, it is often asserted that the balance of payments determines the rate of exchange. But this also is a quite inadequate explanation. The rate of exchange is determined by the purchasing power possessed by a unit of each kind of money.

Theory and History: An Interpretation of Social and Economic Evolution