§ 47 The Sources of Law as Constitutive and Abrogative
The process of legal evolution is threefold. It comprises in the first place the increase or growth of law—that is to say, the substitution of legal principles for the discretion of courts, and the transformation of fact into law. It involves in the second place the opposite process of the decrease of law—the reconquest by the arbitrium judicis of domains formerly occupied by legal principle—the transformation of law into fact. Finally it includes the alteration of law—that is to say, the destruction of one legal principle and the substitution of another in its stead.
To carry out this threefold process, it is clear that we require instruments of legal development which are capable not merely of creating new law, but of destroying old. It is not sufficient to obtain new law which stands side by side with the old, as a supplement to it; it is necessary to obtain new law which excludes the old, as a substitute for it. We must possess instruments of abrogative, and not merely instruments of constitutive power. So far we have considered the sources of law only in respect of this latter operation. We have yet to consider to what extent they possess the power of destroying law, as well as of creating it. The conservative virtue of the law has at all times been very great. We find, accordingly, that the constitutive operation of the sources is much more general than the abrogative. It by no means follows that, because a certain fact is capable of giving rise to a new rule, it is equally capable of getting rid of an old one. Legislation, indeed, is pre-eminent in this respect above all other legal sources. Alone among the instruments of legal development, it works with equal facility in both ways; and it is this peculiarity which makes it so efficient a method of legal reform.
In the strict theory of the law, precedent is wholly constitutive, being quite destitute of abrogative power. When the law is already settled, the judges have no authority save to obey and administer it. Their power of making new law by way of judicial decision is limited to those vacant spaces where there is as yet no other law which they can apply. Precedents make law, but cannot alter it.
Mercantile custom resembles precedent. So long as the ground is vacant—so long as there is no rule of the common law in pari materia—the proved custom of merchants will be allowed by the courts as a source of new law. But so soon as from this or any other source principles have been once established in the matter, there is no longer any room for new rules thus arising. Immemorial custom, on the other hand, has full power to derogate from the common law, though the statute law is beyond its operation.
Agreement possesses considerable, though not complete, abrogative power. A great part of the law is subject to supersession and modification by the consent of all persons interested. Modus et conventio vincunt legem. It is law only until and unless there is some agreement to the contrary. The residue of the law, however, is peremptory, and not to be thus excluded by consent. Agreements which attempt to derogate from it, and to establish special law in place of it, are illegal and void.