How was music theorized as a spiritual practice in the middle ages? Through cross-disciplinary analyses of music-theoretical, theological, and philosophical texts, this dissertation explores webs of principles, deductions, inferences, and experiments through which medieval thinkers understood and regulated musica in spiritual practice. In so doing, it challenges the practical-speculative dichotomy that has been foundational to most narratives of the history of music theory by redefining the scope and purpose of music theory and placing it in dialogue with medieval theories of listening, performance, and communication with the divine. In the first chapter, “The via scientiae,” I argue that Augustine’s treatise De musica presents a carefully constructed ethical pedagogy aimed at right belief and good habit and built upon an experiment-driven “scientific method.” The second chapter, “The via cultus,” takes up the regulation of the use of music in public devotion in Augustine’s Confessions. While Augustine used a mathematical model for probing the ethics of music in the De mus., here he emphasizes a rhetorical model. My third chapter acts as a bridge between the early and later middle ages by examining Aquinas’s treatment of song in the liturgy in his Summa theologiae. Aquinas broadens the varieties of licit spiritual experience from that of Augustine by introducing an ends- and intention-based model and sets up the foundations for distinguishing sacramental validity from spiritual refreshment. The fourth chapter, “The via mystica,” unpacks a site of synthesis between speculative music theory, psychology, and mystical theology as found in the Tractatus de canticis of fifteenth-century theologian Jean Gerson. Here I show how Gerson’s meditations on the canticordum (song of the heart) reframe the performance the divine office as an expression of the vocation of contemplatives rather than a distraction from the supposed “real” (silent and private) work of contemplatio. My last chapter focuses on the writings of music theorist Johannes Tinctoris. Here I demonstrate Tinctoris’s interest and reliance upon scholastic theological argument in his protracted investigations across his corpus of the spiritual role of musicians.