By its very nature, the pure theory of international trade treated in Book I does not require any reference to money nor to the money prices of goods (expressed in terms of one or other of the national currencies: US dollar, UK pound, Italian lira, etc.). This derives from the fact (see Chap. 1) that the distinctive feature of the pure theory of international trade is the assumption that trade takes place in the form of barter, or that money, if present, is only a veil, having no influence on the underlying real variables but serving only as a reference unit, the numeraire. An important consequence of this assumption is that, as we have seen (for example, in Sect. 3.3) the international accounts of any country vis-a-vis all the others invariably balance: that is, no balance-of-payments problem exists.