San Diego Law Review

Document Type



This Article does not contend that arguments for extension of custom are illegitimate. Instead, it makes two more limited claims. First, there is an important difference between arguments from pure custom and arguments for the extension of custom, with the latter being more properly called common law arguments. Second, the legitimacy of common law arguments in some fields, especially constitutional law and international law, is substantially more problematic than the legitimacy of arguments from pure custom. The Article develops as follows. Part II sets out in greater detail the proposed distinction between arguments from pure custom and arguments for extension of custom. Part III illustrates the distinction by reference to the constitutional debate over the President’s military intervention in Libya and to the customary international law debate over the secondary liability of multinational corporations. These Parts are intended to be only descriptive, to illustrate the need to reconsider the general category of arguments from custom. Part IV then turns to normative considerations, and in particular argues that questions of legitimacy are substantially different for the two types of arguments described in the prior Parts: arguments from pure custom have more secure legitimacy than arguments for the extension of custom because the former but not the latter can be said to rest on general consent. This Part further argues that the concern is especially acute for customary international law because traditionally, customary international law grounds its legitimacy in consent. Arguments not grounded in consent require a complete reformulation of the authority of customary international law. In contrast, the message for constitutional law is less certain because constitutional law is more conflicted regarding the theoretical source of its legitimacy.