§ 19 Customary Law
By customary law is here meant any rule of action which is actually observed by men—any rule which is the expression of some actual uniformity of voluntary action. Custom is a law for those who observe it—a law or rule which they have set for themselves, and to which they voluntarily conform their actions. It is true that custom is very often obligatory; that is to say, it is very often enforced by some form of imperative law, whether the civil law or the law of positive morality; but, irrespective of any such enforcement, and by reason solely of its de facto observance, it is itself a law in that wide sense in which law means a rule of action.66
Some writers regard international law as a form of customary law. They define it as consisting of the rules actually observed by states in their conduct towards each other. We shall consider this opinion in a later section of the present chapter. Civil law, as we have defined it, is a form of customary law, inasmuch as it consists of the rules actually observed by the state in the administration of justice. It is the custom of the judicature. The relation between popular custom and the civil law is an important matter which will be considered in a later chapter. It is sufficient here to make the following remarks with regard to it:—
(1) Popular custom has not in itself the nature of civil law; for the essence of civil law lies in its recognition by the state in the administration of justice.
(2) Popular custom is one of the primitive substitutes for civil law, men being governed by custom before the state has been established or has undertaken the function of making and administering law.
(3) Popular custom is one of the sources of the civil law; for that law, when it comes into existence, is largely modelled on the pre-existing customs of the community. Civil law, which is the custom of the state, is based to a large extent on that precedent customary law which is merely the custom of the society.