Hi, Graham. In barter exchange all goods and services can have infinite number of prices in terms of other goods and services. Pricing in barter is marginal or relative. Economic calculation in barter exchange is ordinal according individual subjective valuation. When there was no money, cardinal or precise monetary calculation was impossible. Regression theorem states that before a certain good is chosen as money, it had only ordinal value. After its recognition as money, a good acquires exchange value which becomes prevailing. Prices of all other goods then can be expressed in terms of money. Money allows monetary or cardinal valuation.