The determination of rules of customary international law has been typically inserted in the methodological dualism between induction and deduction. By induction, interpreters find existing legal rules on the basis of the empirical material – state practice and opinio juris. By deduction, interpreters find legal rules by deducing them from existing principles and rules of international law. Notably, while induction is portrayed as empirically-grounded – therefore arguably objective – deduction is presented as a logical exercise, thus disguising the margin of manoeuvre that interpreters enjoy in ascertaining rules of customary international law. The chapter contends that the methodological dualism informing the discourse on the determination of rules of customary international law shall be revisited to reflect the argumentative nature of such a determination. This twist is conducive to unveil the role that discretion plays in the ascertainment of rules of customary international law, instead of embracing the methodological lens which rather mystifies it. Accordingly, interpreters operate within an argumentative framework in that they necessarily select and appreciate evidence of practice and opinio juris, which is far from being incontrovertible, let alone fully representative of the majority of states.