Chapter VI — The Sources of Law
§ 44 Formal and Material Sources
The expression source of law (fons juris) has several meanings which it is necessary to distinguish clearly. We must distinguish in the first place between the formal and the material sources of the law. A formal source is that from which a rule of law derives its force and validity. It is that from which the authority of the law proceeds. The material sources, on the other hand, are those from which is derived the matter, not the validity of the law. The material source supplies the substance of the rule to which the formal source gives the force and nature of law.
The formal source of the whole body of the civil law is one and the same, namely, the will and power of the state as manifested in courts of justice. Whatever rules have the sanction and authority of the body politic in the administration of justice have thereby the force of law; and in such force no other rules whatever have any share. The matter of the law may be drawn from all kinds of material sources, but for its legal validity it must look to the tribunals of the state and to them alone. Customary law, for example, has its material source in the usages of those who are subject to it; but it has its formal source in the will of the state, no less than statutory law itself.