Sienho Yee,	Towards an International Law of Co-progressiveness (2004), ch. 2, pp. 27-39
The News that Opinio Juris “Is Not a Necessary Element of Customary [International] Law” Is Greatly Exaggerated

[Full text first published in German Yearbook of International Law (2000), 227.]

Conclusion:

In summary, the weak thesis that opinio juris plays no role during the process of the formation of customary international law assumes that during this period there is a fully-formed opinio juris, which is incorrect. During this period, it is difficult to speak of the role of opinio juris, although one can speak of a “candidate” opinio juris, pointing to a direction into which the law may develop. However, opinio juris is necessary for the ripening of a rule of customary international law at that big bang moment of resonance between practice, where required, and opinio, as well as for the maintenance of the rule afterwards. The strong thesis that opinio juris is not a necessary element of customary international law results from the mistake of equating conduct with law, which in turn stems from the mistake of viewing law purely from an external perspective as an observer, and from not being able to see the internal aspect of law. The news about the validity of the strong thesis is greatly exaggerated.