The sources of international law may be generally classified as primary or secondary. The primary or direct sources are treaties or conventions, customs, and the general principles of law. The secondary or indirect sources are the decisions of courts and the writings of publicists.
Not every treaty can be considered a direct source of international law. It is not always concluded by the great body of states. However bilateral treaties may become primary sources of international law if they are the same nature contain practically uniform provisions and are concluded by a substantial number of states. Examples are the “standard extradition treaties”.
The treaty, to be considered a direct source of international law, the general rule is that, it must be concluded by a sizable number of states and thus reflect the will or at least the consensus of the family of nations. Examples of these are “lawmaking treaties”.
A custom is a practice which has grown up between states and has come to be accepted as binding by the mere fact of persistent usage over a long period of time.
Most of the customary rules of law have been expressly affirmed and embodied in treaties and conventions like the Hague Conventions of 1899 and 1907. Significantly, these rules, by virtue of their force as international customs and their express recognition as generally accepted principles of international law, bind even those states which have not signed these conventions.
General Principles of Law
The general principles of law are mostly derived from the law of nature and are observed by the majority of states because they are believed to be good and just.
Although no international convention was necessary to bring them into existence, the general principles of law have nevertheless become universal application because of the unilateral decision of a considerable number of states to adopt and observe them in recognition of their intrinsic merit.
The doctrine of stare decisis is not applicable in international law, and so the decision of a court in one case will have only persuasive value in the decision of a subsequent case. In considering decisions of courts as subsidiary sources of international law, these decisions should carry correct application and interpretation of the law of nations or should undertake to establish the true rule of international law.
The second subsidiary source of international law like the writings of publicists, in order to qualify as such, must also be fair and presents unbiased representation of international law, and by an acknowledged authority in the field. Mere credentials are not enough. The jurist may be motivated by national pride or interest, or error in interpreting a rule of international law.