When Don Carlos premiered at the Paris Opéra in March 1867, there was considerable excitement among critics about the prospect of a new work from one of Europe's most famous and popular living composers. In the event, the opera's reception was riddled with inconsistencies and contradictions. The fundamental problem was the work's ambiguous position: as a new grand opera appearing at a time when Parisian operatic culture was centered ever more on old masterpieces. Moreover, the new work's length (although characteristic of its genre) seemed ill suited to performance in Second Empire Paris, where the pace of life was felt to be constantly accelerating. In this article I ask how and why Don Carlos—a work judged by many critics to be the epitome of “modern” Verdi—was so at odds with broader conceptions of Parisian modernity. Focusing particularly on the Act IV Duo between Philip II and the Grand Inquisitor, I explore how aspects of the scene's musical unfolding foreground tensions between an increasingly prominent operatic past and an imagined operatic future. Ultimately, I argue that the opera's reception was saturated with concerns about an emerging phenomenon of “canonic listening”: an ideal encounter with music extending over countless repeated hearings and predicated on the value of sustained, concentrated engagement with a complex musical surface.