The great composer of the French Baroque, Jean-Philippe Rameau, was also the most influential music theorist since the Middle Ages: the inaugurator, in his epochal Traité de l’harmonie (1722) and in many subsequent writings, of the modern theory of harmony. This monograph proposes a revision of the widely accepted view of Rameau’s harmonic theory as a product of Enlightenment thought. In it I identify the historical roots of Rameau’s determinative ideas regarding methodology and the fundamental concepts of harmony in systems of thought—Neoplatonism, Neopythagoreanism, and Aristotelian scholasticism—that long predated the regime of the eighteenth-century philosophes and their seventeenth-century precursors, yet were still current during Rameau’s formative years; ideas which demonstrably exercised a profound influence on Rameau’s thinking prior to his learning of the corps sonore and other contemporaneous developments in science and philosophy. The book further aims to reveal how the fundamental concepts of harmony coexist in Rameau’s theory in a state of uneasy tension and mutual conflict: a condition which, owing to the genetic relation of Rameau’s ideas to modern harmonic theory, continues to affect the latter down to the present day.