(_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.
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.
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_.