French polymath and savant Marin Mersenne played a pivotal role in the evolution of seventeenth-century musical thought, serving as the central hub of a vast correspondence network and writing prolifically on theology, mathematics, natural philosophy, and music. Yet when it comes to one of the most commonplace early modern assumptions about music--its kinship with rhetoric--Mersenne has received little attention from music scholars, for whom the history of the topic has long centered on figures from the German-speaking world. Challenging this received wisdom, Universal Harmony in the Age of Eloquence uses Mersenne as a case study to broaden our understanding of rhetoric's place in the early modern musical imagination and, conversely, of music's place in the early modern rhetorical imagination. Author André de Oliveira Redwood invites readers to reframe the music-rhetoric relationship from the perspective of Mersenne's world--a world of preachers and scholars, Jesuit classrooms and Minim cloisters, epistolary exchange and learned tomes. Turning to Mersenne's writings, including his ambitious Harmonie universelle , Redwood shows that Mersenne understood music and rhetoric to be mutually informative disciplines joined in the service of instruction, persuasion, and devotion. An important contribution to our understanding of seventeenth-century French musical thought, Universal Harmony in the Age of Eloquence showcases Mersenne's rhetorical thought and offers a new model for interdisciplinary work between music theory, theology, the history of science, and rhetorical studies.