This Article explores the Supreme Court’s inconsistent use of prescriptive inferences to justify the creation of federal common law in areas regulated by the law of nations. On the one hand, the court has employed an inference of lawmaking power from an unadorned grant of jurisdiction in the case of admiralty suits and tort claims brought by aliens and based on the law of nations. It has not done so with respect to the law merchant and private international law, even though jurisdictional grants to the federal courts exist. The Article shows that the modern development of the law merchant and private international law as State law, subject to federal legislative overrides as needed, demonstrates why prescriptive inferences in the field of foreign relations are unnecessary and perhaps even harmful.