The law of association tells us that when multiple participants in a market produce two or more of the same goods, they can almost always realize mutual gains by specialization and exchange. Only in rare instances is the principle inapplicable: if production rates are exactly proportional mathematically from one participant to the next; or if the quantities of goods produced are too small to establish a convenient exchange rate in whole units. This law can be proven by straightforward mathematics (

) without recourse to assumptions about the theory of value. Consequently, it remained intact after the marginalist revolution of the 1870s (p. 4.4:38), when much of the rest of classical economic theory had to be discarded.

The law of association does not apply merely to bamboo poles and berries, of course, but potentially encompasses a wide variety of goods and services. The opportunities for mutual benefit from voluntary exchange and cooperation are therefore vast. Because Crusoe and Friday always maximize their value scales, they will take advantage of such mutually beneficial opportunities through voluntary exchange and cooperation—unless, of course, such agreements are precluded by personal animosities (i. e., if a psychic disutility factor affects one or both parties). The economic benefits of association, of course, create a strong motive for them to overcome any such ill feelings—just as they provide a powerful incentive to harmonious relationships and benevolent attitudes among human beings in a larger society or across the globe.

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