Addressing the Chronological Paradox of CIL: From Good Faith to Opinio Juris, and Opinio Juris to New Customary Rules

This chapter addresses the chronological paradox of customary international law (CIL). The paradox is that for a new customary rule to be created states must believe that the law already obligates the behaviour specified in that rule (opinio juris). However, the behaviour in question can only be legally required once that rule has been created. As a result, creating a new customary rule would be impossible, or at the very least an incoherent process. This chapter challenges this conclusion. In addressing the chronological paradox, it provides a coherent interpretation of the creation of new customary rules. It argues that the sense of legal obligation (opinio juris) emerges from the general principle of good faith. Good faith leads to legal obligations, which compel a subgroup of states to engage in specific behaviour. Then, as a result of this subgroup’s repeated behaviour, a new customary rule emerges, obligating the entire community of states to act accordingly. To explain the shift from good faith to legal obligations and from legal obligations to customary rules, the chapter draws on interpretivism, social ontology and contemporary research on constitutive rules.