Writers are, however, as we have already indicated, far from being unanimous in their analysis of the essential nature of the law of nations, and the various competing theories may be classified as follows:—
(1) That the law of nations is, or at least includes, a branch of natural law, namely, the rules of natural justice as applicable to the relations of states inter se.
(2) That it is a kind of customary law, namely the rules actually observed by states in their relations to each other.
(3) That it is a kind of imperative law, namely the rules enforced upon states by international opinion or by the threat or fear of war.
(4) That it is a kind of conventional law, as already explained. Having accepted the last of these theories as correct, let us shortly consider the nature and claims of the three others.
§ 22. The Law of Nations as Natural Law.
All writers on international law may be divided into three classes by reference to their opinions as to the relation between this law and the principles of natural justice. The first class consists of those who hold that the law of nations is wholly included within the law of nature—that it consists merely of the principles of natural justice so far as applicable to sovereign states in their relations and conduct towards each other—that the study of international law is simply a branch of moral philosophy—and that there is no such thing as a positive law of nations, consisting of a body of artificial rules established by states themselves. Thus Hobbes says:[[72]] “As for the law of nations, it is the same with the law of nature. For that which is the law of nature between man and man, before the constitution of commonwealth, is the law of nations between sovereign and sovereign after.” The same opinion is expressed by Christian Thomasius,[[73]] Pufendorf,[[74]] Burlamaqui,[[75]] and others, but is now generally discredited, though it is not destitute of support even yet.
A second opinion is that international law is both natural and positive—that it is divisible into two parts, distinguished as the natural law of nations, which consists of the rules of natural justice as between states, and the positive law of nations, consisting of rules established by states by agreement, custom, or in some other manner, for the government of their conduct towards each other. The natural law of nations is supplementary or subsidiary to the positive law, being applicable only when no positive rule has been established on the point. Representatives of this opinion are Grotius, Wolf, Vattel, Blackstone, Halleck, Wheaton, Phillimore, Fiore, Twiss, and others. The third opinion is that international law is wholly positive—that it consists exclusively of a set of rules actually established in some way by the action of sovereign states themselves—and that the rules of natural justice are not in themselves rules of international law at all, but pertain to that law only if, and only so far as, they have been actually incorporated into the established system of positive law. This is now the prevalent opinion, and we have here accepted it as the correct one.[[76]] By those who maintain it the rules of natural justice as between states are called international morality, and are distinguished by this name from international law. These two bodies of rules are partly coincident and partly discordant. The conduct of a state may be a breach of international morality but not of international law, or a breach of law though in accordance with morality, or it may be both immoral and illegal.
The question whether rules of natural justice are to be included as a part of international law is, indeed, in one aspect, a mere question of words. For these rules exist, and states are in honour bound by them, and the question is merely as to the name to be given to them. Nevertheless, questions of words are often questions of practical importance, and it is of undoubted importance to emphasise by a difference of nomenclature the difference between rules of international morality, by which, indeed, states are bound whether they have agreed to them or not, but which are uncertain and subject to endless dispute, and those rules of international law, which by means of international agreement have been defined and established and removed from the sphere of the discussions and insoluble doubts of moral casuistry.