That the family cannot easily organize itself by a process of explicit bargaining does not mean there will not grow up within it reciprocal expectancies of the sort that, on a more formal level, would be called “customary law.” Indeed, the family could not function without these tacit guidelines to interaction; if every interaction had to be oriented afresh and ad hoc, no group like the family could succeed in the discharge of its shared tasks. At the midrange, it should be observed that the most active and conspicuous development of customary law in modern times lies precisely in the field of commercial dealings. Finally, while enemies may have difficulty in bargaining with words, they can, and often do, profitably half-bargain with deeds.

Lon Fuller, “Law and Human Interaction,” p. 82.