[2] The best example of universal International Law is the law connected with legation.
International Law in the meaning of the term as used in modern times did not exist during antiquity and the first part of the Middle Ages. It is in its origin essentially a product of Christian civilisation, and began gradually to grow from the second half of the Middle Ages. But it owes its existence as a systematised body of rules to the Dutch jurist and statesman Hugo Grotius, whose work, "De Jure Belli ac Pacis libri III.," appeared in 1625 and became the foundation of all later development.
The Law of Nations is a law for the intercourse of States with one another, not a law for individuals. As, however, there cannot be a sovereign authority above the several sovereign States, the Law of Nations is a law between, not above, the several States, and is, therefore, since Bentham, also called "International Law."
Since the distinction of Bentham between International Law public and private has been generally accepted, it is necessary to emphasise that only the so-called public International Law, which is identical with the Law of Nations, is International Law, whereas the so-called private International Law is not. The latter concerns such matters as fall at the same time under the jurisdiction of two or more different States. And as the Municipal Laws of different States are frequently in conflict with each other respecting such matters, jurists belonging to different countries endeavour to find a body of principles according to which such conflicts can be avoided.
Legal Force of the Law of Nations contested.
§ 2. Almost from the beginning of the science of the Law of Nations the question has been discussed whether the rules of International Law are legally binding. Hobbes[3] already and Pufendorf[4] had answered the question in the negative. And during the nineteenth century Austin[5] and his followers take up the same attitude. They define law as a body of rules for human conduct set and enforced by a sovereign political authority. If indeed this definition of law be correct, the Law of Nations cannot be called law. For International Law is a body of rules governing the relations of Sovereign States between one another. And there is not and cannot be a sovereign political authority above the Sovereign States which could enforce such rules. However, this definition of law is not correct. It covers only the written or statute law within a State, that part of the Municipal Law which is expressly made by statutes of Parliament in a constitutional State or by some other sovereign authority in a non-constitutional State. It does not cover that part of Municipal Law which is termed unwritten or customary law. There is, in fact, no community and no State in the world which could exist with written law only. Everywhere there is customary law in existence besides the written law. This customary law was never expressly enacted by any law-giving body, or it would not be merely customary law. Those who define law as rules set and enforced by a sovereign political authority do not deny the existence of customary law. But they maintain that the customary law has the character of law only through the indirect recognition on the part of the State which is to be found in the fact that courts of justice apply the customary in the same way as the written law, and that the State does not prevent them from doing so. This is, however, nothing else than a fiction. Courts of justice having no law-giving power could not recognise unwritten rules as law if these rules were not law before that recognition, and States recognise unwritten rules as law only because courts of justice do so.
[3] De Cive, XIV. 4.
[4] De Jure Naturæ et Gentium, II. c. iii. § 22.
[5] Lectures on Jurisprudence, VI.