§ 23 The Law of Nations as Customary Law
Even those writers who agree in the opinion that international law is or at least includes a system of positive law, differ among themselves as to the essential nature and source of these rules; and we proceed to consider the various answers that have been given to this question. Some writers consider that international law has its source in international custom—that it consists essentially and exclusively of the rules which are actually observed by sovereign states in their dealings with one another.77 This view, however, is not prevalent, and is, it is believed, unsound. International custom is not in itself international law; it is nothing more than one kind of evidence of the international agreement in which all such law has its source. There are many customs which, because they are based on no such underlying agreement, have not the force of law, states being at liberty to depart from them when they please. Conversely there is much law which is not based on custom at all, but on express international conventions. These conventions, if observed, will of course create a custom in conformity with the law; but they constitute law themselves from the time of their first making, and do not wait to become law until they have been embodied in actual practice. New rules of warfare established by convention in time of peace are law already in time of peace.