In his informative and timely book, Justice Breyer repeatedly emphasizes that the Supreme Court is a domestic court, not an international tribunal, and that there is no Supreme Court for the world.1 Against that backdrop, Justice Breyer suggests various functions that the Supreme Court can perform as it faces an increasingly international docket, such as promoting harmonization, fostering collaboration, and helping to promote the rule of law. I do not disagree that the Supreme Court can play these roles in appropriate cases. In this Essay, I will describe another important role that the Supreme Court does and should serve, which is to act as a filter between international law and the American constitutional system. Although not Justice Breyer’s focus, this filtering role is illustrated by a number of the cases that he discusses in his book. By filter, I do not mean that the Court is or should be some sort of impermeable barrier to international law. Rather, the idea is that the Court should ensure that when international law passes into the U.S. legal system, it does so in a manner consistent with domestic constitutional values. Although this filtering process might in some instances dilute or narrow international law as it is applied within the United States, this is not inevitably the result. Filters can operate to improve and refine something for a particular purpose—think of a filter for coffee, for example—and this is often what happens when U.S. courts attempt to tailor international law to the U.S. domestic system. This filtering role is needed because international law is generated through processes that often make it ill-suited for direct application in the U.S. legal system, and because it is frequently designed to perform functions different from those demanded of domestic law. The two principal sources of international law are customary international law and treaties. Customary international law arises out of the evolving practices and beliefs of nations.2 Although the United States contributes to its formation and change, this body of law does not require any specific approval process in the United States. Moreover, because customary international law is unwritten and evolving, its content is often uncertain and contested. Indeed, even the types of evidence that should count toward ascertaining its content are the subjects of substantial dispute.3 Because of its contested and evolutionary character, determinations of the content of customary international law implicate not only legal considerations but also considerations of U.S. foreign policy.4
Breyer suggests various functions that the Supreme Court can perform as it faces an increasingly international docket, such as promoting harmonization, fostering collaboration, and helping to promote the rule of law. another role that the Supreme Court does is to act as a filter between international law and the American constitutional system. the idea is that the Court should ensure that when international law passes into the U.S. legal system, it does so in a manner consistent with domestic constitutional values. Filters to improve and refine something for a particular purpose—think of a filter for coffee, for example—and this is often what happens when U.S. courts tailor international law to the U.S. domestic system. This filtering is needed because international law is generated through processes that make it ill-suited for direct application in the U.S. and because it is designed to perform functions different from those demanded of domestic law. Customary international law arises out of the evolving practices and beliefs of nations. Although the U S contributes to its formation and change, this law does not require specific approval in the U S because customary international law is unwritten and evolving, its content is uncertain and contested. Because of its contested and evolutionary character, determinations of the content of customary international law implicate not only legal considerations but also considerations of U.S. foreign policy.