Compare the major sources of international law, addressing: (1) the nature of each, (2) any problems to which each gives rise, (3) how important a source each is, and (4) any particular use each has, if any.

There are many sources of International law and they are authoritatively listed in Article 38(1)¹  of the Statute of the International Court of Justice. Amongst the others, the first source of international law is international conventions. Typically, Conventions or treaties are agreements between States. And Pacta sunt servanda² is a foundational principle of international law. It is a principle of international law that whenever States ratify treaties, they bind themselves to act in conformity with them. After looking at Article 38(1), it can be established that treaties are express sources of international law as between the States that ascent to them. However, it is to be pointed that a treaty obligation cannot bind parties who have not signed to the same. A similar analogy can be drawn from contracts; only the rights and obligations of the signing parties are affected and furthermore a State isn’t required to abide by a treaty in its dealings with a state that is not Party to that treaty. Also, it has to be pointed that treaties and conventions are the fastest method by which states can modify international law between themselves as they typically prescribe when they will come into force. This also means that they can be used to quickly deal with events taking place in the world such as terrorism or cyber-crime. Additionally, they also allow States, which do not agree with the content of particular treaties, not to become bound by it.³ In the contemporary era, treaties are undeniably the paramount source of international law and some such as the Charter of the United Nations enjoy almost universal acceptance. 

The next in the line in terms of source of international law is custom. By custom, it is meant the conduct of States in a historic context. International law is law for states and by States, and where States have acted in a uniform manner over a sustained period of time, a custom is created that gives legal force to what was previously a non-binding tradition. Worded differently, there are two essentials to finding that a custom has indeed come into existence on a particular topic; there must be consistent State practice⁴ and there must be opinio juris.⁵ It is a fact that customary law takes years or even centuries to develop and its precise scope is often unclear as it is not an express source of law. Furthermore, it is also subject to continuous change as the underlying practices change. Also, it is pertinent to note that customary law binds all States and it can become altered by being broken consistently. Finally, custom remains an important source of law although it has been overtaken in recent years by treaty law. However, at times it appears to be very vague and uncertain. And this is often considered to be the source of the flexibility of international law.

The other sources of International law would include general principles of law recognized by civilized States, as well as writings by pre-eminent scholars and domestic judicial decisions. It is to be stressed that the former is a secondary source of law, whose effect is merely to provide the procedural rules that ensure fairness, such as by dictating that every party to a dispute must be heard. It is safe to conclude that these are relatively less important sources of law as against both customs and treaties. Moreover, the writings of scholars too merely serve to guide international courts and in present day they do not have the importance that they once did.⁶

¹ Article 38@(1) of ICJ statute:
“The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. 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”.
² Pacts must be honoured 
³ Or to become only bound by the parts to which they agree by making reservations upon acceptance.
⁴ State practice is the practice in a particular area or topic, of States and maybe evidenced by their declarations on international forums or conduct with each other.
⁵ Opinio juris is the belief by States that they are required to act in that manner as if by law. This may be found in their submissions to international courts or tribunals or in the judgements of their own tribunals or other declarations as to the reasons behind their practices.
⁶ They now serve only as guides to what the law is rather than what the law ought to be. Domestic decisions also serve this purpose but in addition may also be sources of opinio juris, but care must be taken when using domestic decisions or academic writings as both sometimes get the law wrong.

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