In his comment on Sosa v. Alvarez-Machain, Professor Edward Purcell characterizes Erie Railroad Co. v. Tompkins as “a kind of jurisprudential Rorschach test, a device that reveals the political goals and values of those who seek to use it.” The same can be said of Sosa itself.

When Professor John McGinnis looks at Sosa, he sees an “astringently positivist” approach to the identification of customary international law. In his view, Sosa confirms the errancy of scholars who find customary international law status for “substantial human rights norms” that can then be applied in U.S. domestic law. These scholars err because “instead of requiring that nation-states actually engage in a practice, they substitute statements by nation-states that give the norms verbal endorsement – often at high levels of generality,” such as UN General Assembly Resolutions. Although Professor McGinnis does not specifically mention treaties, others who share his general view of Sosa have – Professors Curtis Bradley, Jack Goldsmith, and David Moore, for example.

According to this “positivist” critique, “publicists' theories and international court judgments are useful [for identifying the content of customary international law] only if they provide evidence of a norm from actual state practice.” Proponents of this critique define “actual state practice” narrowly to include only what states do, not what they say. In this perspective, all talk is “cheap talk,” and the fact that such talk is engaged in by sovereigns does not make it a “sovereign act[]” capable of “ground[ing]” a customary international law norm that can be applied in U.S. law.