A Theory of Customary International Law Jackl. Goldsmithand eric A. Posner*e Customary international law ("CIL")is one of two primary forms of international law, the other being the treaty. CIL is typically defined as a"customary practice of states followed from a sense of legal obligation. "1 Conventional wisdom views CiL as a unitary phenomenon that pervades international law and international relations. Governments take care to comply with CiL and often incorporate its norms into domestic statutes. National courts apply CIL as a rule of decision, or a defense, or a canon of statutory construction. Nations argue about whether certain acts violate CIL. Violations of CIl are grounds for war or an international claim. Legal commentators view cil to be at the core of the study of international law And yet CIL remains an enigma. 2 It lacks a centrali lawmaker. a centralized executive enforcer. and a centralized authoritative decision-maker. The content of cil seems to track the nterests of powerful nations. The origin of CIl rules is not understood. We do not know why nations comply with CIL,or even what it means for a nation to comply with CIL. and we lack an explanation for the many changes in CIL rules over time. Both parts Associate Professor of Law, University of Chicago Professor of Law, University of Chicago. Thanks to Jaqueline Bhabha, Richard Epstein, Tracey Meares, Richard Ross, Cass Sunstein, Doug Sylvester, Adrian Vermeule, and participants at a workshop at the University of Chicago Law School for comments, and to Christopher Chow, Kyle gehrmann and Kathryn Walsh for research assistance I Restatement(Third)of the Foreign Relations Law of the United States S 102(2) (1986) 2 See G.J. H. van Hoof, Rethinking the Sources of International Law 176-178 (1983)("confusion and divergence of opinion... reign supreme as far as [Cil] is concerned"); David P. Fidler, Challenging the Classical Concept of Custom Perspectives on the Future of Customary International Law, 39 Ger. Y B. Int'l L. 198, 198(1997)("CIL stands at the heart of modern international law while generating frustration and frictions in its identification and application. CIL appears indispensable and incomprehensible. "A Theory of Customary International Law Jack L. Goldsmith* and Eric A. Posner** Customary international law (“CIL”) is one of two primary forms of international law, the other being the treaty. CIL is typically defined as a “customary practice of states followed from a sense of legal obligation.”1 Conventional wisdom views CIL as a unitary phenomenon that pervades international law and international relations. Governments take care to comply with CIL, and often incorporate its norms into domestic statutes. National courts apply CIL as a rule of decision, or a defense, or a canon of statutory construction. Nations argue about whether certain acts violate CIL. Violations of CIL are grounds for war or an international claim. Legal commentators view CIL to be at the core of the study of international law. And yet CIL remains an enigma.2 It lacks a centralized lawmaker, a centralized executive enforcer, and a centralized, authoritative decision-maker. The content of CIL seems to track the interests of powerful nations. The origin of CIL rules is not understood. We do not know why nations comply with CIL, or even what it means for a nation to comply with CIL. And we lack an explanation for the many changes in CIL rules over time. Both parts * Associate Professor of Law, University of Chicago. ** Professor of Law, University of Chicago. Thanks to Jaqueline Bhabha, Richard Epstein, Tracey Meares, Richard Ross, Cass Sunstein, Doug Sylvester, Adrian Vermeule, and participants at a workshop at the University of Chicago Law School for comments, and to Christopher Chow, Kyle Gehrmann and Kathryn Walsh for research assistance. 1 Restatement (Third) of the Foreign Relations Law of the United States § 102(2) (1986). 2 See G.J.H. van Hoof, Rethinking the Sources of International Law 176-178 (1983) (“confusion and divergence of opinion . . . reign supreme as far as [CIL] is concerned”); David P. Fidler, Challenging the Classical Concept of Custom: Perspectives on the Future of Customary International Law, 39 Ger. Y.B. Int’l L. 198, 198 (1997) (“CIL stands at the heart of modern international law while generating frustration and frictions in its identification and application. CIL appears indispensable and incomprehensible.”)