[Footnote 73: See details with regard to the controversy concerning the freedom of the open sea below, §§ 248-250. Grotius's treatise "Mare liberum" is--as we know now--the twelfth chapter of the work "De jure praedae," written in 1604 but never published by Grotius; it was not printed till 1868. See below, § 250.]
Grotius, as a child of his time, could not help starting from the Law of Nature, since his intention was to find such rules of a Law of Nations as were eternal, unchangeable, and independent of the special consent of the single States. Long before Grotius, the opinion was generally prevalent that above the positive law, which had grown up by custom or by legislation of a State, there was in existence another law which had its roots in human reason and which could therefore be discovered without any knowledge of positive law. This law of reason was called Law of Nature or Natural Law. But the system of the Law of Nature which Grotius built up and from which he started when he commenced to build up the Law of Nations, became the most important and gained the greatest influence, so that Grotius appeared to posterity as the Father of the Law of Nature as well as that of the Law of Nations.
Whatever we may nowadays think of this Law of Nature, the fact remains unshaken that for more than two hundred years after Grotius jurists, philosophers, and theologians firmly believed in it. And there is no doubt that, but for the systems of the Law of Nature and the doctrines of its prophets, the modern Constitutional Law and the modern Law of Nations would not be what they actually are. The Law of Nature supplied the crutches with whose help history has taught mankind to walk out of the institutions of the Middle Ages into those of modern times. The modern Law of Nations in especial owes its very existence[74] to the theory of the Law of Nature. Grotius did not deny that there existed in his time already a good many customary rules for the international conduct of the States, but he expressly kept them apart from those rules which he considered the outcome of the Law of Nature. He distinguishes, therefore, between the _natural_ Law of Nations on the one hand, and, on the other hand, the _customary_ Law of Nations, which he calls the _voluntary_ Law of Nations. The bulk of Grotius's interest is concentrated upon the natural Law of Nations, since he considered the voluntary of minor importance. But nevertheless he does not quite neglect the voluntary Law of Nations. Although he mainly and chiefly lays down the rules of the natural Law of Nations, he always mentions also voluntary rules concerning the different matters.