The third source of international law as enumerated in Article 38 are "general principles of law" recognized by "civilized" nations. The Guide to International Legal Research states that "this traditional naturalist approach provides a basis for decision when other sources offer no guidance, yet it is unclear what these general principles of law are. Thus, locating these general principles in the course of legal research is extremely difficult. They could be general principles of justice, natural law, analogies to private law, principles of comparative law, or general conceptions of international law."
The Guide suggests that the "best documentation of these legal principles is found in textbooks, general surveys or manuals, treatises, classics, and encyclopedias."
General principles of law are used primarily as "gap fillers" when treaties or customary international law do not provide a rule of decision. It has been suggested by scholars that as new treaties and customary law develop to address areas of international concern not previously covered, the significance of general principles will fade as these gaps in international law are filled.