A Nation in the Economic-Commercial Sense Can Be Defined as the Space within Which Factors of Production Are Perfectly Mobile, although They Are Totally Immobile between Nations

In fact, for a theory of international trade to be developed, there must be differences between international trade and intra-national trade. Of course, in both cases, the same general theory is used, that is the general theory of exchange. But the theory of international trade has to add a specification, so that it can be a particular application of the general theory of exchange.

Most generally, economic theory assumes that goods and factors of production are perfectly mobile, which means that they can move without costs. One may study the consequences of a restriction on mobility (for example, arising from transport costs or information costs). Traditionally the trade theory assumes – as hypothesized by David Ricardo – that commodities are internationally mobile, whereas factors of production are mobile within a nation, but immobile internationally. In other words, a nation in the economic-commercial sense can be defined as the space within which factors of production are perfectly mobile, although they are totally immobile between nations. There is no need at this stage to investigate the specific causes of this immobility (natural or linguistic barriers, state controls and bans on trade, and so on).

--Pascal Salin, The International Monetary System and the Theory of Monetary Systems, New Thinking in Political Economy (Cheltenham, UK: Edward Elgar Publishing, 2016), 3.