§ 46 A List of Legal Sources
We cannot deduce from the nature of law the nature of its sources, for these are merely contingent, not necessary; they differ in different systems and even in the same system at different periods of its growth. It is possible, however, to distinguish five sources which in England or elsewhere have possessed predominant influence. These are Legislation, Custom, Precedent, Professional Opinion, and Agreement. Legislation is the declaration or enunciation of a principle by some adequate authority in the body politic; custom is the realisation or embodiment of a principle in a uniformity of practice; precedent is the judicial application of a principle to its appropriate facts; professional or expert opinion is the approval or recognition of a principle by the general voice of those whose business it is to know the law; agreement is the adoption of a principle by the consent of those whose interests are affected by it. Such declaration, realisation, application, approval, and adoption determine in each case the judicial recognition as law of the principle so dealt with, and therefore constitute the sources of the law.
Law which has its source in legislation is called statute, enacted, or written law. That which is based on custom is customary law. Precedent produces case-law, and agreement conventional law. That which is created by professional or expert opinion has no recognised title, but in analogy to German usage we may call it juristic law (Juristenrecht).
There are two chief reasons for allowing law-creative operation to these various sources. In the first place there is a presumption that principles proceeding from them are principles of truth and justice, worthy of adoption by the judicature. A statute is an attempt made by the legislature to formulate the rules of right for the use and direction of the judicature. This attempt is not always successful, for law and justice are sometimes far apart; yet no better device has been discovered, and the courts accept the rules so formulated as authoritative and final. A similar presumption of truth and justice is one of the grounds of the operation of precedent also. When one of the superior courts of law has, after solemn argument and full consideration, laid down a certain principle as one fit to be applied to the case in hand, there is a reasonable presumption that this decision is correct, and that the principle is a just one fit to be applied to all similar cases in the future, that is to say, fit to receive permanent recognition as a new rule of law. Res judicata pro veritate accipitur.108 So also in the case of custom. Customary law has as one of its foundations the presumption that whatever is customary is just and expedient. The popular conscience embodies itself in popular usage, and the law courts accept as authoritative the principles so sanctioned and approved. Professional opinion—the opinion of lawyers—is merely an historical, not a legal source of English law. In other systems, however, and chiefly in that of Rome, it has shown itself capable of serving as one of the most important of legal sources. Almost all that is of special value in Roman law has this as its origin; the Digest of Justinian consists wholly of extracts from the writings of Roman lawyers. It is clear that one of the grounds for the allowance of such opinion as a source of law is to be found in a reasonable confidence in the skill and knowledge of the expert. Cuique in sua arte credendum est. Finally we may see the same influence at work in the case of the fifth and last source, namely agreement. Every man may be trusted to see to his own interests and to claim his own rights. Whatever rule, therefore, is freely agreed upon by two or more persons as defining their mutual rights and obligations may be confidently accepted by the law courts as a true and just rule between those who have so consented to it. As to them, it is fit and proper to be applied as law.
There is, however, a second ground of not less importance on which the efficacy of these legal sources rests. They are not merely presumptive evidence of the justice and truth of the principles proceeding from them, but they are the basis of a rational expectation on the part of all persons concerned that these principles will be consistently acted on in the future. Justice demands that such expectations shall be fulfilled. Even when a rule does not accurately conform to the ideal standard, it may be a right and reasonable thing to adhere to it, when it has once been formulated. For men act on the faith of it; and to overturn an imperfect rule with all the expectations built upon it will often do more harm than can be counterbalanced by any benefits to be derived from the substitution of a better principle. Thus legislation is an announcement to all the world that in future certain principles will be applied in the administration of justice. Forthwith the expectations, dealings, and contracts of all men concerned are based upon the principles so declared, and the disregard of them by the judicature would be a breach of faith and an ill service to the cause of justice. Similarly the decision of a court may not be perfectly wise or just; but whether it is or not, all men expect that like decisions will for the future be given in like cases. It is often more important that the course of judicial decision should be uniform and within the limits of human foresight, than that it should be ideally just. So with all the other sources of law. That which has always been customary in the past is entitled for this reason alone to a certain measure of allowance and recognition in the future. That which is approved by the general opinion of the legal profession serves so largely as the basis of the actions and expectations of men, that the courts of law will not lightly depart from it. That which all parties interested have agreed to, and which they have declared as valid law to bind them, may not, for all that, be absolutely just and reasonable; but they must be held bound by it none the less, otherwise there will be no certainty of dealing among mankind.