One of the primary points of distinction between international law and municipal law may be drawn from the nature of their sources. International law has been described by authors as a decentralized system which arguably lacks a predefined and concrete structure which discriminates between potential sources as to their importance and value in the resolution of issues  . The international judge does not have reference to a constitution, codes of law or statutes as in the municipal scenario. This is due to the fact that in international law there is no compulsory jurisdiction and any rules which a state must abide to are the same rules which it has expressly consented to, either by way of its consistent practice in the formation of custom or else expressly in the formation of treaties. Technically speaking, such ‘contractual’ arrangement usually creating mutual obligations by way of treaty are the only source of international which is by its very nature objectively binding upon states, a reflection of the pacta sunt servanda principle in domestic Civil Law. It is for this reason that the law of treaties has been held to be a primary principle for centuries. Custom, on the other hand is primarily a matter of evidence. Brownlie  speaks of a categorization between the formal and the material sources of law. The formal sources are the ‘legal procedures and methods for the creation of rules of general application which are legally binding on the addressee’ such as international custom created by constant state practice and the law contained in treaties. The material sources, on the other hand, ‘provide evidence of the existence of rules which, when proved, have the status of legally binding rules of general application’, here one can include decisions of international courts or tribunals, resolutions of the General Assembly of the United Nations and possibly Doctrinal sources. Brownlie continues to argue that in reality, save the usefulness in academia, the so-called elevation of ‘formal sources’ do not really exist in international law  as this would create a hierarchy which is not actually present. The Statute of the ICJ  constitutes a generally accepted list of the sources of international law ascribed to by the court  :
38 (1)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.