§ 56 Reasons for the Reception of Customary Law
The reasons for attributing to custom the force of law have been already briefly indicated in relation to legal sources in general. We have seen that, in the first place, custom is the embodiment of those principles which have commended themselves to the national conscience as principles of truth, justice and public utility. The fact that any rule has already the sanction of custom raises a presumption that it deserves to obtain the sanction of law also. Via trita via tuta. Speaking generally, it is well that the courts of justice, in seeking for those principles of right which it is their duty to administer, should be content to accept those which have already in their favour the prestige and authority of long acceptance, rather than attempt the more dangerous task of fashioning a set of principles for themselves by the light of nature. The national conscience may well be accepted by the courts as an authoritative guide; and of this conscience national custom is the external and visible sign.
Custom is to society what law is to the state. Each is the expression and realisation, to the measure of men’s insight and ability, of the principles of right and justice. The law embodies those principles as they commend themselves to the incorporate community in the exercise of its sovereign power. Custom embodies them as acknowledged and approved not by the power of the state, but by the public opinion of the society at large. Nothing, therefore, is more natural than that, when the state begins to evolve out of the society, the law of the state should in respect of its material contents be in great part modelled upon and coincident with the customs of the society. When the state takes up its function of administering justice, it accepts as true and valid the rules of right already accepted by the society of which it is itself a product, and it finds those principles already realised in the customs of the realm. As those customs develop and alter with change of circumstance and the growth of public enlightenment, the state is wisely content to allow such development and modification to reflect themselves in the law which it administers. This influence of custom upon law, however, is characteristic rather of the beginnings of the legal system than of its mature growth. When the state has grown to its full strength and stature, it acquires more self-confidence, and seeks to conform national usage to the law, rather than the law to national usage. Its ambition is then to be the source not merely of the form, but of the matter of the law also. But in earlier times it has perforce to content itself with conferring the form and nature of law upon the material contents supplied to it by custom.
A second ground of the law-creative efficacy of custom is to be found in the fact that the existence of an established usage is the basis of a rational expectation of its continuance in the future. Justice demands that, unless there is good reason to the contrary, men’s rational expectations shall, so far as possible, be fulfilled rather than frustrated. Even if customs are not ideally just and reasonable, even if it can be shown that the national conscience has gone astray in establishing them, even if better rules might be formulated and enforced by the wisdom of the judicature, it may yet be wise to accept them as they are, rather than to overturn all those expectations which are based upon established practice.