The conception of those rules and principles of which international law treats has varied greatly with periods, with conditions, and with writers.
The early terminology indicates the vagueness of the conceptions of the principles governing conduct of man toward his fellows.
(a) Jus naturale is defined broadly by Ulpian[4] as "the law which nature has taught all living creatures, so as to be common to men and beasts." Grotius also uses this term, defining it as "the dictate of right reason, indicating that any act from its agreement or disagreement with rational nature has in it moral turpitude or moral necessity, and consequently such act is either forbidden or enjoined by God, the author of nature."[5] Lieber says, "The law of nature, or natural law ... is the law, the body of rights, which we deduce from the essential nature of man."[6] The discussion of jus naturale has been carried on from an early period,[7] covering many portions of the field of modern international law, and making possible the broadening and strengthening of its foundation.
(b) Jus gentium, according to Justinian, is "that which natural reason has established among all men, that which all peoples uniformly regard."[8] "Jus gentium is common to the whole human kind."[9] This idea of a body of law common to all men assumed a different meaning when states multiplied and writer after writer redefined and qualified its meaning. Jus gentium became the subject of many controversies.[10] Among the qualifying terms were "internal," "necessary," "natural," "positive."
(c) Other terms were used to name the field or portions of the field of modern international law. Jus fetiale applied particularly to the declaration of war and sanction of treaties.[11] Jus inter gentes was used by Zouch in 1650 to name the real field of international law. Law of nations was the term commonly used in England till the days of Bentham; since that time the term international law, which he adopted, has steadily grown in favor, till almost universal in the English language.[12]
The change in terminology shows in a measure the growth in demarking the field of international law.
[§ 5. Historical Bases]
International law in its beginning may have been largely determined by abstract reasoning upon what ought to be the principles and rules governing interstate relations; but in its later development, as it has become more and more recognized as a safe guide for the conduct of states in their relations with other states, not abstract reasoning as to what ought to be, but direct investigation of what is, has determined the character of the rules and principles. What is state practice in a given case can only be determined by reference to history. From the history of cases and practice, the general rule and principle is derived, and modern international law thus comes to rest largely upon historical bases.
[§ 6. Ethical Bases]
While international law now looks to history as one of its most important bases, it must nevertheless accord somewhat closely with the ethical standards of the time, and will tend to approximate to them. The growth of the body of law upon slavery has rested on both ethical and historical bases. International law is principally an output of civilized nations having certain ethical standards. Such ancient practices as the giving of hostages for the fulfillment of treaty stipulations have disappeared, and ethical bases are generally recognized in determining practice.[13] While these ethical bases should be recognized, international law cannot be deduced from the subtle reasoning upon the abstract ideas of what it ought to be. Modern international law treats mainly of what is, but what is in international relations is always conditioned by a recognition of what ought to be.