Because the law of association derives from mathematics rather than value theory, it applies not only to individuals but also to whole countries. For instance, it assures that the free exchange of goods between two countries will permit the citizens of both countries to enjoy those goods in equal abundance at lower total cost. (The fact that such exchanges are effected through money rather than barter renders the analysis more complex, but does not alter the conclusion.) Of course, these benefits may not accrue equally to all industries in either country. Friday's agreement with Crusoe negatively impacted his personal bamboo-pole industry, requiring that he reallocate some of his labor and other means to berry production. Similarly, while free trade among countries enables the same quantities of goods to be attained at lower total costs, it may also alter the balance among various economic sectors, as means are more efficiently allocated to ends. In a free market, of course, producers cannot forcibly interfere with foreign competitors, since their would-be customers enjoy self-ownership and hence freedom of choice.