John Hart Ely famously observed, “We were all brought up on sophisticated talk about the fluidity of the line between substance and procedure,” but for most of Erie’s history, the Supreme Court has answered the question “Does this state law govern in federal court?” with a “yes” or a “no.” Beginning, however, with Gasperini v. Center for Humanities, and continuing with Semtek v. Lockheed and Shady Grove v. Allstate, a shifting coalition of justices has pursued a third path. Instead of declaring state law applicable or inapplicable, they have claimed for themselves the prerogative to fashion law that purportedly accommodates the interests of both sovereigns. With the cover of an intellectual critique of the substance/procedure dichotomy, the...