Article 38 of the International Court of Justice (ICJ) Statute enumerates the sources of "international law"...
(a) international conventions, whether general or particular, establishing rules expressly recognized by 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) … 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.