L. Oppenheim, International Law. A Treatise. Volume 1…
“ Those who define law as rules set and enforced by a sovereign political authority do not deny the existence of customary law. But they maintain that the customary law has the character of law only through the indirect recognition on the part of the State which is to be found in the fact that courts of justice apply the customary in the same way as the written law, and that the State does not prevent them from doing so. ”
