Prologue
At its most abstract level, international law concerns norms that govern the conduct of States and the relations between them. That isn't to say that international law's reach does not go beyond the rights and obligations of States. It is recognised that individuals may incur responsibility under international law, while corporations have not been left untouched either.

Additionally, over the last century, international organisations have played an increasingly important role in the development of international law. Nevertheless, States continue to be the main subjects of international law. Any system of law must have sources. For instance, in Israeli law, Knesset legislation, government regulations and judicial precedents, among others, serve as binding sources of law.

As for international law, while its main sources are clear, as shall be demonstrated below, the process of identifying them is sometimes more complex. The most accepted statement of the sources of international law may be found in Article 38(1) to the Statute of the International Court of Justice (ICJ). The ICJ Statute is to a large degree reproduction of the Statute of the Permanent Court of International Justice, the ICJ's predecessor. Article 38(1) to the ICJ Statute states: 

The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:

1. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
2. international custom, as evidence of a general practice accepted as law;
3. the general principles of law recognized by civilized nations;
4. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.

As can be seen, there are three primary sources in international law: treaty, custom and general principles of law. Additionally, there are two subsidiary sources that may assist one in identifying norms derived from the primary sources: judicial decisions and the teachings of the most highly qualified publicists i.e. leading international law scholars.


Oppenheim defines the term source of law as the name for a historical fact out of which rules of conduct come into existence. The term source refers to methods or procedures by which international law is created. The source may be formal (i.e. legal procedures and methods for the creation of general rules) or material (i.e. actual materials providing evidence of the existence of rules).

Oppenheim opined that there is only one source of international law viz. common consent. Lawrence said that there is only one source of law i.e. consent of nations. Consent may be either express or tacit (implied). Logically, therefore, there are two chief sources of international law; Treaties (express consent) and Customs (tacit consent).

Article 38 of the Statute of International Court of Justice:

1. The Court, whose function is to decide in accordance with international law, such disputes as are submitted to it, shall apply;
   
   1. International Conventions, whether general or particular, establishing rules expressly recognized by contesting States.
   2. International Custom as evidence of general practice accepted by law.
   3. General principles of law recognized by civilized nations.
   4. Subject to provisions of Article 59, judicial decisions and teachings of most highly qualified publicist of various nations, as subsidiary means for determining the rules of law.
       
2. This provision shall not prejudice the power of the court to decide a case ex aequo et bono (considerations of fair dealing and good faith), if the parties agree thereto (compromise, conciliation, etc.), which may be independent of or even contrary to law.

A source Decisions or determinations of the organs of International Institution does not find mention in Art. 38 but it has now become a well-recognized source. Some writers have mentioned additional sources of international law viz. International comity, State papers, State guidance for their officers, Reason equity and justice, Resolutions of international conferences, Decisions of municipal courts on matters touching foreign nationals or other States, etc.

Thus Article 38 is not exhaustive. If Art. 38 be simply declaratory, it clearly cannot inhibit the emergence of new source of law, brought into being by the development of the international community and its progressive organization. Sources enumerated in Art. 38 are hardly adequate to meet new challenges posed by today's world order. An authoritative order of the use of sources of international law is given in Art. 38. Thus, the Court is expected to apply the above sources in order in which they appear.

Thus, international conventions (i.e. treaties) shall be give preference by the court, and in those cases where conventions are not available, they would be decided in accordance with international custom. General principles of law shall find place only where conventions and customs are not available. Judicial decisions and teachings, etc. have been regarded as subsidiary means for the determination of rules of law.

1. International Customs

It is the oldest and most original source of international law. Customary rules of international law are the rules, which have been developed, in a long process of historical development. Development or Formation of a Custom
(a) Custom and Usage: Custom is such a usage as has the force of law. Usage is an international habit, which has yet not received the force of law. Usage represents the twilight zone of custom, custom begins where usage ends.

When States in their international relations start behaving in a particular way in certain circumstances, it is expected that in the similar circumstances they will behave in the same way (usage). But when this usage receives the general acceptance of recognition by the States in their relations with each other, there develops a conception that such a habit or behaviour has become right as well as obligation of the State and in this way usage becomes the custom.

Westlake defines custom as: that line of conduct which the society has consented to regard as obligatory.

A usage may or may not give rise to a custom. A usage crystallize into a custom through the conduct of the States at the diplomatic level, actions or statements of various States, foreign policy documents, speeches of delegates at international conventions and conferences, practice of international organs like the U.N., ICJ, etc. The decisions of municipal courts of States, military and administrative tribunals in their deliberations and proceedings, following a specific usage in practice, convert a usage into custom. The true test whether a usage has crystallized into a custom is that it must be approved by the common consent of civilized nations or general consensus of opinions.

Opiniojuris sine ncessitatis

According to Art. 38 of the Statute of ICJ, international custom should be the evidence of general practice accepted as law. This feeling on the part of States that acting as they do, they are fulfilling legal obligations is called opiniojuris sine ncessitatis.
(b) Generation of customary rules by treaty: A provision of a treaty may also generate a rule of customary international law, as held in North Sea Continental Shelf Case (1969) ICJ Rep.3.

In this case, the World Court observed:

1. A treaty provision can generate customary international law, but only when the provision concerned is of a fundamentally non-creating character such as could be regarded as the basis of a general rule of law.
    
2. The passage of only a short time period is not necessarily a bar to the formation of a new rule of customary international law, however, an indispensable requirement would be that within the period in question, State practice (including that of States whose interests are specially affected), should have been both extensive and virtually uniform in the sense of the provision invoked; and should moreover have occurred in such a way as to show a general recognition that a rule of law or legal obligation is involved. Thus, opiniojuris must be present before a practice/usage is to become a customary rule.

The Right of Passage Case (ICJ Rep. 1960) demonstrates that a treaty provision practiced for a long time may become a custom. The case illustrates that when in regard to any matter or practice, two States follow it repeatedly for a long time it becomes a binding customary rule. The case, like the Asylum case, made it clear that local or regional custom, which derogates from general customary law becomes binding only on States that support it.

Essential Ingredients of a Custom

According to Art. 38 of the Statute of ICJ, there are two requirements for the existence of international custom, namely: firstly, there should be a sufficiently uniform State practice; and secondly, the belief that such a practice is obligatory (i.e. the opiniojuris).

In Pacquete Habana Case (U.S. Supreme Court, 1900), the court examined State laws and practices, treaties, writings of publicists evidencing usage, and decisions of court, and held that these materials uniformly proved the existence of a valid customary rule giving immunity to small fishing vessels, honestly pursuing their calling, from belligerent capture in time of war.

The Asylum Case (ICJ Rep. (1950)) shows that international law recognized the existence of a special custom or local custom. The local or regional custom which derogates from general customary law becomes binding only on States that support it.

Generality of Practice

Although universality of practice is not necessary, the practice should have been generally observed or repeated by numerous States. If a State acts in a way prima facie incompatible with a recognized rule, but defends its conduct by appealing to exceptions or justifications contained within the rule itself, then, whether or not the State's conduct is in fact justifiable on that basis, the significance of that attitude is to confirm rather than to weaken the rule (Nicaragua Case, ICJ Reports, 1986).

Opiniojuris et necessitates: The S. S. Lotus Case (1927) demonstrates that the opiniojuris is essential to the creation of a new customary rule of international law.

Application of International Customs: Difficulties

The ICJ or any International Tribunal cannot suo-motu (i.e. on its own motion) presume a custom as a guiding factor. It is quite difficult to establish an international custom before such a tribunal because:

1. the very existence of a custom must be proved,
2. it has to be established that the custom in question has had a continuous practice among a large number of States, and that too consistently and without any break whatsoever or without any variation in its practice, and
3. it is very difficult to prove opiniojuris. For deducing this subjective element, treaties, resolutions of General Assembly of U.N., international conferences and statements of State representatives, etc. may be seen.

2. International Treaties In the modem period

International treaties are the most important source of international law. Article 38 of the Statute of ICJ lists international conventions whether general or particular, establishing rules expressly recognized by the contesting States as the first source of international law. The term convention implies any treaty, convention, protocol or agreement, etc. Treaties may be classified into following two categories:

1. Law-making treaties: These treaties have a large number of parties and create general or universal norms. These may be of two kinds: those enunciating universal rules e.g. United Nations Charter, and those laying down general rules e.g. 1958 Geneva Convention on the Law of the Sea, Vienna Convention on the Law of Treaties, 1969, etc.
2. Treaty-contracts: It is a treaty between two or only a few States dealing with a special matter concerning these States exclusively.

A series or a recurrence of treaties laying down a similar rule may produce a principle of customary international law. Further, subsequent independent acceptance may generalize a treaty originally concluded between a limited numbers of States. A treaty is therefore, of considerable evidentiary value as to the existence of a rule, which has crystallized into law by an independent process of development. International treaties are agreements of a contractual character between States or Organizations of States creating legal rights and duties (Oppenheim). According to Art. 2 of the Vienna Convention, 1969, A treaty is an agreement whereby two or more States establish or seek to establish relationship between them governed by international law. This does not mean that other subjects of international law such as international organizations cannot conclude treaties (Art. 3).

Pacta Sunt Servanda (Basis of the Binding Force of Treaties)

According to Starke:
In nearly all the cases the object of the treaty is to impose binding obligations on the States who are parties to it. Lauterpacht stresses: treaties are legally binding because there exists a customary rule of international law that treaties are binding.

Pacta sunt servanda means States are bound to fulfill in good faith the obligations assumed by them under treaties (sanctity of contracts).

Vienna Convention on the Law of Treaties provides:

Every treaty in force is binding upon the parties to it and must be performed by them in good faith (Art. 26). Art. 27 strengthen the rule by providing that no party to a treaty might attempt to justify its failure to perform any of its international treaty obligations by invocation of its internal law. Thus, perhaps the most fundamental principle of international law and surely the basic principle of treaties is pactasuntservanda.


Exceptions to the principle
When a new State comes into existence as a result of revolt, it does not consider itself to remain bound by the treaties concluded by the Predecessor State. The principle is also not applied when the obligations arising from the treaty is related to that part of the territory which has been ceded/merged with other States.

Rebus sic stantibus (an implied clause in every treaty that provides that the agreement is binding only so long as the material circumstances on which it rests remain unchanged) is another important exception. Finally, pacta suntser vanda is not an absolute principle for it fails to explain the binding force of customary rules of international law.

Custom and Treaties:

Inter-relationship Custom and treaties are the two methods for the creation of the legally binding rules ever since the beginning of the international community. In both, consent of the States is reflected, and therefore, they did not impose obligations on those States who did not wish to be bound by them. However, while in customary rules, consent of the States is implied, in the treaties, consent is express. Moreover, customary rules are often not precise and adequate, while treaty-rules are clear and precise. This has led to the transformation of many customary rules into treaty rules.

The customary rules of international law cannot be regarded as to have ceased to exist if they have been transformed into treaty rules. The court cannot be prevented to apply customary rules in such cases. Thus, principles such as those of the non-use, force, non-intervention, respect for the independence and territorial integrity of States, etc continue to be binding as part of customary international law, despite the operation of provisions of conventional law in which they have been incorporated (Nicaragua Case, ICJ. Rep. 1986).

The existence of identical rules in international treaty law and customary law has been clearly recognized in the North Sea Continental Shelf Cases. There is an instance where custom has repealed treaty - law of the sea has witnessed repeal of four Geneva Conventions of 1958 by the custom that emerged in the late 1960's and 1970's, which was finally codified into the U.N. Convention on the Law of Sea in 1982. The correct view is that treaty and custom have identical legal force and are mutually reinforcing. These do not oppose each other but interact and supplement each other. However, a treaty is superior to custom when there is an apparent conflict between the two (S.S. Wimbledon Case, ICJ, 1923).

3. General Principles of Law

Article 38 (1) (c) of the Statute of ICJ lists General principles of law recognized by civilized States' as the third source of international law independent of custom or treaty. The phrase means principles as general as to apply within all systems of law that have achieved a comparable state of development. The rationality for the inclusion of general principles of law as one of the sources of international law lies in the fact that a principle, which has been found to be generally accepted by certain civilized legal systems, may fairly presumed to be so reasonable as to be necessary to the maintenance of justice under any system.

Examples are:
the rule of pactasuntservanda, that contracts must be kept, the principle that no man may be a judge in his own cause, the right of self defence, etc. The principles of law recognized by many States do not become principles of international law automatically. They are required to be recognized by the World Court.

Before the court applies any such principle, certain considerations are taken into account.

1. Firstly, a rule is a general principle of law, i.e. it is not limited in scope.
2. Secondly, the rule is recognized by most of the States of the world community.
   The word recognized' presupposes the existence of the rule in the municipal law. It may be noted that general principles of law include substantive as well as procedural principles. Some general principles of law' common to municipal legal systems are:
   
   1. Res judicata (a thing or matter settled by judgment): A matter once judicially decided is finally decided, and there is an absolute bar to a subsequent action involving the same claims, demand or cause of action. This principle was taken into account in the U.N. Administrative Tribunal Case.
   2. Prescription (a claim to a right founded upon enjoyment): Eastern Greenland Case (PCIJ, 1933), Island of Palmas Case.
   3. Subrogation (substitution or stepping into shoes of another): Mavrommatis Palestine Concessions Case (PCIJ, 1925).
   4. Estoppel (preclusion): A State party to international obligation is bound by its previous acts or attitude when they are in contradiction with its claims in the litigation (Serbian and Brazilian Loan Case). The principle was also applied in Temple of PreahVihar Case.
   5. Equity (reasonableness, fairness): The ICJ has been increasingly referring to equity in its judgments in recent years viz. The Gulf of Maine Boundary Case, North Sea Continental Shelf Cases.
   6. Other principles: The principle that every violation of an engagement involves an obligation to make reparation (i.e. repair a wrong), is recognized in Chorzow Factory Case.

Territoriality of criminal law recognized in S. S. Lotus Case. Status of General Principles of Law Positivists claim that customs and treaties were the only sources of international law as according to them international law is a body of rules on which the States have given their consent.

However, the majority of international lawyers opine that the general principles of law are a primary source of international law because they are valid through all kinds of human societies (Judge Tanka, dissenting opinion, South West Africa case ICJ, Rep. 1966). Lauterpacht has rightly remarked that the main function of the general principles of law' has been that of a safety valve to be kept in reserve rather than a source of frequent application. Art. 38 (1) (c) describe the inexhaustible reservoir of legal principles from which the tribunals can enrich and develop international law.

4. Judicial Decisions and Juristic Works

Art. 38 (1) (d) of the Statute of ICJ states that the Court shall apply subject to the provisions of Art. 59, judicial decisions and teachings of the most highly qualified publicists of various nations, as subsidiary means for the determination of rules of law.

Thus, judicial decisions and juristic works are subsidiary and indirect sources of international law. The decisions of International Court of Justice do not create a binding general rule of international law, as Art. 59 of the Statute of ICJ make it clear that the decisions of the court will have no binding force except between the parties and in respect of the particular case. Although ICJ has not adopted the common law doctrine of precedent (i.e. stare decisis), it has largely adopted its substance.

So far as the advisory opinion of the ICJ is concerned, it is not binding at all. But it clarifies the rule of international law on a particular point or matter. The decisions of ICJ have enormously influenced States and codifying agencies. The rule of equitable principles enunciated in North Sea Continental Shelf Cases was adopted and developed in many cases and also incorporated in the 1982 U.N. Convention on Law of Sea. In practice, the decisions of ICJ have assumed such an importance that these constitute a source of international law and are not merely subsidiary means for determining the rules of law.

State judicial decisions

It is true that decisions of municipal courts of a State have little value in international law, but the uniform decisions of the courts of many States have a tendency to create evidence of international custom especially in those fields of international law which are interwoven between international law and municipal law such as nationality, extradition and diplomatic immunities.

Writings of Jurists

This source may be resorted to as final resort i.e. only when all the other sources listed in Art. 38 have failed to resolve the dispute before the Court. Sometimes juristic opinion does lead to the formation of international law. Calvo clause and Drago doctrine are the instances of such rules in international law. In Paquete Habana Case, Justice Gray observed: Where there is no treaty and no controlling executive or legislative act or judicial decision, resort must be had to the customs and usages, and, as evidence of these, to the works and commentators, who by years of labour, research and experience have made themselves peculiarly well acquainted with the subjects which they treat.

5. Resolutions of the General Assembly (Modern Sources of International Law)

After the establishment of U.N., most of the development of international law and its codification has taken place through the instrumentality of international organizations. General Assembly, one of the principal organs of U.N., has established International Law Commission. The decisions and determinations of organs are now recognized as an important source of international law (although they do not find mention in Art. 38 of the Statute of ICJ).

Resolutions of the General Assembly of the United Nations do not possess legal character, and as such are not binding on the States. They do not create any legal obligations on its members irrespective of the fact that they have been adopted unanimously or by overwhelming votes or even if the contents of it is a matter of common interest for all the States. A perusal of the cases like South West Africa Voting Procedure Case (ICJ, Rep. 1955), Anglo-Norwegian Fisheries Case (ICJ, Rep. 1951), ICJ's Advisory opinion on Namibia and the Western Sahara Case indicates that legal effect may be given to collective pronouncements of the General Assembly despite their non-binding character.

Conclusion
Since there is no world government, there is no world Congress or parliament to make international law the way domestic legislatures create laws for one country. As such, there can be significant difficulty in establishing exactly what international law is.

Various sources, however principally treaties between states are considered authoritative statements of international law. Treaties are the strongest and most binding type because they represent consensual agreements between the countries who sign them. At the same time, as stated in the statute of the International Court of Justice (ICJ), rules of international law can be found in customary state practice, general principles of law common to many countries, domestic judicial decisions, and the legal scholarship.