At the end of the eighteenth century, Romantics began to take instrumental music seriously as an art form with its own special nature. But they soon found it impossible to say what makes “music itself” specifically musical, except by rejecting what they perceived as extra-musical. The dichotomy stuck, fueling the rise of musical formalism in the nineteenth century, shaping the reception of bebop in the twentieth century, and stoking feuds between music historians and theorists along the way. By tracing this intellectual history from its antecedents in ancient Greece to its consequences in recent “topic theory,” Dylan J. Principi argues that the idea of the extramusical continues to hold sway over how we interpret what music means.