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This Article addresses a longstanding puzzle about customary international law (CIL): How can it be, at once, so central to the practice of international law — routinely invoked and applied in a broad range of settings — and the source of such persistent confusion and derision? The centrality of CIL suggests that, for the many people who use it, it is not only comprehensible but worthwhile. They presumably use it for a reason. But then, what accounts for all the muddle and disdain?

The Article argues that the problem lies less in the everyday operation of CIL than in the conceptual baggage that is brought to bear on it. Most contemporary accounts of CIL reflect what can be called a “rulebook conception.” They presuppose that, in order for a given proposition to be CIL, it must apply more or less in the same way in all cases of a given type, rather than fluctuate without established criteria from one situation to the next. This rulebook conception is wrong. It does not accurately describe the range of normative material that global actors, in the ordinary course, use and treat as CIL. And because it is wrong, it systematically sows confusion and leads analysists to devalue CIL as a kind of international law. We should stop imagining that CIL operates like a rulebook and should recognize that it is an inherently contingent and variable kind of law.


International Law | Law