Along with international conventions and customary law, Article 38 of the ICJ Statute recognizes "the general principles of law recognized by civilized nations," as major sources of international law.
Examples of general principles of law include:
Cheng, Bin. General Principles of Law, As Applied by International Courts and Tribunals. Cambridge ; New York : Cambridge University Press, 2006..
Raimondo, Fabián. General Principles of Law in the Decisions of International Criminal Courts and Tribunals. Leiden: M. Nijhoff Pub, 2008.