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.
§ 24. The Law of Nations as Imperative Law.
By some writers international law is regarded as a form of imperative law; it consists, they say, of rules enforced upon states by the general opinion of the society of states, and also in extreme cases by war waged against the offender by the state injured or by its allies. Thus Austin says:[[78]] “Laws or rules of this species, which are imposed upon nations or sovereigns by opinions current among nations, are usually styled the law of nations or international law.” In considering this view it is to be admitted that in many cases the rules of the law of nations are thus sanctioned and enforced by international opinion and force. But the question to be answered is whether this sanction is of the essence of the matter; because, if it is so, all rules so sanctioned must be, and no others can be, rules of international law. It is clear, however, that the sanction of war cannot be the essential test; for in the first place this sanction is but seldom applied even to undoubted violations of international law, and in the second place it is at least as often resorted to when there is no violation of such law at all. What then shall be said of the alternative sanction of international opinion? Is this the test and essence of a rule of international law? For the following reasons it is submitted that it is not:—
(1) Many forms of state action are censured by public opinion, which are admittedly no violation of the law of nations. A state may act within its legal rights, and yet so oppressively or unjustly as to excite the adverse opinion of other nations.
(2) There may be violations of international law which are in the particular circumstances regarded as excusable, and approved by international opinion.
(3) Public opinion is variable from day to day—dependent on the special circumstances of the individual case—not uniform as we pass from state to state—not uniform even throughout the population of the same state. International law, on the other hand, is a permanent, uniform system of settled rules, independent of the fickle breath of public approbation or censure—made and unmade by the express or implied agreements of sovereign governments, and not by the mere opinions and prejudices which for the moment are in public favour. International law is one thing, international positive morality is another thing; but the doctrine here criticised identifies and confounds them as one. International law is made, as has been said, by the acts and contracts of governments; international opinion is made chiefly by journalists and the writers of books. Opinion, if sufficiently uniform and sufficiently permanent, will doubtless in time constrain the law into conformity with it; but it is not the same thing.
(4) Public opinion cannot be made the basis of any rational or scientific body of rules or legal doctrines. For such opinion is simply the belief of the public that certain forms of conduct are in conformity with natural justice. So far as this belief is well founded, the law based upon it is simply the law of nature; so far as it is erroneous, the law based on it is simply a mistake which disappears ipso facto on being recognised as such. It is impossible to recognise as a subject of scientific interpretation and investigation any international law based on erroneous public opinion; and if based on true opinion, it is nothing save the principles of natural justice.
Certain writers seek to avoid the first of these objections by so defining international law as to include only one portion of the body of rules approved and sanctioned by international opinion, the remaining portion constituting international positive morality. According to this opinion international law consists of those rules which international opinion not merely approves, but also regards as rightly enforceable by way of war. International positive morality, on the other hand, consists of those rules of which opinion approves, but of the enforcement of which by way of war it would not approve. That is to say, international law is distinguished from international morality by an application of the distinction familiar to the older moralists between duties of perfect and duties of imperfect obligation.[[79]]
This view would seem to be exposed to all the objections already made to the cruder theory which we have just considered, with the exception of the first; and it is also exposed to this further criticism, that it is impossible thus to divide public opinion sharply into two parts by reference to the justification of war or any other kind of forcible compulsion. Whether such compulsion is right is a matter to be determined not by the application of any fixed or predetermined rules, but by a consideration of all the circumstances of the individual instance; and even then opinion will in most cases be hopelessly discordant. Moreover, there are forms of state action which are not the violation of any established rule of international law, and which nevertheless are so contrary to the rightful interests of another state that they would be held to be rightly prevented or redressed by way of war. Conversely there are rules of undoubted law which are of such minor importance, that a war for the vindication of them would be viewed by international opinion as a folly and a crime.