§ 18 Conventional Law
By conventional law is meant any rule or system of rules agreed upon by persons for the regulation of their conduct towards each other. Agreement is a law for the parties to it. Examples are the rules and regulations of a club or other society, and the laws of whist, cricket, or any other game. What are the laws of whist, except the rules which the players expressly or tacitly agree to observe in their conduct of the game?
In many cases conventional law is also civil law; for the rules which persons by mutual agreement lay down for themselves are often enforced by the state. But whether or not these conventional rules thus receive recognition and enforcement as part of the law of the land, they constitute law in the wide sense of a rule of human action.65
The most important branch of conventional law is the law of nations, which, as we shall see later, consists essentially of the rules which have been agreed upon by states, as governing their conduct and relations to each other.