Like all law, international law is a practice of reason-giving, one in which agents invoke legal norms to justify their conduct. Practitioners of inter- national law generally proceed on the assumption that those norms do, in fact, justify the conduct they sanction. Theorists, in contrast, tend to take a more critical stance towards the practice of international law, including the assumption that the law succeeds in providing a justification for its subjects’ conduct. Why treat the claim that international law prohibits Φ-ing as in itself a reason not to Φ? Or using the terminology I will employ in this chapter, what makes an international legal norm prohibiting Φ-ing legitimate? In the modern era, the consent of States to be bound by international law has played an increasingly prominent role in addressing these questions, with some theorists going so far as to maintain that only State consent can explain why international law binds; that is, why it provides States (and their subjects) with reasons for action. Though not uncontested, the belief in the centrality of State consent to international law’s legitimacy remains widespread today.

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