2 The Enabling Constraint

This chapter recounts the rise, consolidation, and fluctuation of the enabling constraint of the discourse on customary international law. It sets the stage for the discussion of the discursive performances that enabling constraint has prompted in international legal thought and practice. It also talks about the distinction between the two elements of customary international law: practice and opinio juris, which corresponds to a form of dualist thinking that came to thrive with modernity. The chapter describes dualist thinking about customary international law as a constraint that was formalized in the middle of the 20th century and that does not necessarily correspond to any ancient mode of thinking about customary international law. It shows that the dualist approach to the identification of customary international law has undergone a few variations over the last hundred years but uncontested in international legal thought and practice.