This book examines the dissolution, and at the same time some especially noteworthy ramifications, of speculative music theory in the Pythagorean style, the central component of what the Middle Ages and Renaissance called musica theorica. A substantial introductory section presents the Pythagorean-Platonic background, and the structure and content of musica theorica, distinguishing it from its complement, musica practica, and goes on to discuss a series of crucial moments foreshadowing and preparing the abandonment of the Pythagorean view. The main body of the book comprises a series of studies, each examining how Pythagorean musica theorica exercised a crucial yet heretofore unrecognized influence on an important moment in the history of western music theory, beginning with the conceptualization of the musical note as the “element” of music in the Carolingian period, and continuing with chapters on Ficino; Descartes; Rameau; and Schenker.