It is often assumed that the differences among people give rise to war and other conflicts, while peace and friendship can only be built on commonalities. The law of association, however, shows that differences can also engender peaceful, harmonious relationships among individuals and countries. The diversity in skills between Crusoe and Friday generated opportunities for mutually beneficial cooperation and trade—opportunities which of course would be vastly multiplied in a larger market. Similar opportunities may arise among countries, not only because of differences in labor skills, but also because of access to different natural resources and other factors of production. Free trade and other forms of economic cooperation require only that the participating parties agree to deal with one another in a peaceful manner, without resorting to force. Provided that this single common principle has been established, natural variations in abilities, resources, and values enable all participants to achieve their ends more efficiently through exchange and other kinds of voluntary agreements.