III SOURCES OF THE LAW OF NATIONS
Hall, pp. 5-14—Maine, pp. 1-25—Lawrence, §§ 61-66—Phillimore, I. §§ 17-33—Twiss, I. §§ 82-103—Taylor, §§ 30-36—Westlake, I. pp. 14-19—Wheaton, § 15—Halleck, I. pp. 55-64—Ullmann, §§ 8-9—Heffter, § 3—Holtzendorff in Holtzendorff, I. pp. 79-158—Rivier, I. § 2—Nys, I. pp. 144-165—Bonfils, Nos. 45-63—Despagnet, Nos. 58-63—Pradier-Fodéré, I. Nos. 24-35—Mérignhac, I. pp. 79-113—Martens, I. § 43—Fiore, I. Nos. 224-238—Calvo, I. §§ 27-38—Bergbohm, "Staatsverträge und Gesetze als Quellen des Völkerrechts" (1877)—Jellinek, "Die rechtliche Natur der Staatsverträge" (1880)—Cavaglieri, "La consuetudine giuridica internazionale" (1907).
Source in Contradistinction to Cause.
§ 15. The different writers on the Law of Nations disagree widely with regard to kinds and numbers of sources of this law. The fact is that the term "source of law" is made use of in different meanings by the different writers on International Law, as on law in general. It seems to me that most writers confound the conception of "source" with that of "cause," and through this mistake come to a standpoint from which certain factors which influence the growth of International Law appear as sources of rules of the Law of Nations. This mistake can be avoided by going back to the meaning of the term "source" in general. Source means a spring or well, and has to be defined as the rising from the ground of a stream of water. When we see a stream of water and want to know whence it comes, we follow the stream upwards until we come to the spot where it rises naturally from the ground. On that spot, we say, is the source of the stream of water. We know very well that this source is not the cause of the existence of the stream of water. Source signifies only the natural rising of water from a certain spot of the ground, whatever natural causes there may be for that rising. If we apply the conception of source in this meaning to the term "source of law," the confusion of source with cause cannot arise. Just as we see streams of water running over the surface of the earth, so we see, as it were, streams of rules running over the area of law. And if we want to know whence these rules come, we have to follow these streams upwards until we come to their beginning. Where we find that such rules rise into existence, there is the source of them. Of course, rules of law do not rise from a spot on the ground as water does; they rise from facts in the historical development of a community. Thus in Great Britain a good many rules of law rise every year from Acts of Parliament. "Source of Law" is therefore the name for an historical fact out of which rules of conduct rise into existence and legal force.
The two Sources of International Law.
§ 16. As the basis of the Law of Nations is the common consent of the member States of the Family of Nations, it is evident that there must exist, and can only exist, as many sources of International Law as there are facts through which such common consent can possibly come into existence. Of such facts there are only two. A State, just as an individual, may give its consent either directly by an express declaration or tacitly by conduct which it would not follow in case it did not consent. The sources of International Law are therefore twofold—namely: (1) express consent, which is given when States conclude a treaty stipulating certain rules for the future international conduct of the parties; (2) tacit consent, which is given through States having adopted the custom of submitting to certain rules of international conduct. Treaties and custom are, therefore, exclusively the sources of the Law of Nations.
Custom in Contradistinction to Usage.
§ 17. Custom is the older and the original source of International Law in particular as well as of law in general. Custom must not be confounded with usage. In everyday life and language both terms are used synonymously, but in the language of the jurist they have two distinctly different meanings. Jurists speak of a custom, when a clear and continuous habit of doing certain actions has grown up under the ægis of the conviction that these actions are legally necessary or legally right. On the other hand, jurists speak of a usage, when a habit of doing certain actions has grown up without there being the conviction of their legal character. Thus the term "custom" is in juristic language a narrower conception than the term "usage," as a given course of conduct may be usual without being customary. Certain conduct of States concerning their international relations may therefore be usual without being the outcome of customary International Law.
As usages have a tendency to become custom, the question presents itself, at what time a usage turns into a custom. This question is one of fact, not of theory. All that theory can point out is this: Wherever and as soon as a frequently adopted international conduct of States is considered legally necessary or legally right, the rule which may be abstracted from such conduct, is a rule of customary International Law.
Treaties as Source of International Law.