In his prodigious philosophical work Periphyseon, the foremost intellectual of the ninth century, John Scottus Eriugena (ca. 800-877), defined musica broadly and in a way that solicits interdisciplinary applications: "Music is the discipline discerning by the light of reason the harmony of all things in natural proportions which are either in motion or at rest." In this dissertation, I trace resonances of the ars musica in Eriugena's writings using selections from his three greatest works: Periphyseon, his glosses on Martianus Capella's textbook De Nuptiis, and his commentary accompanying Pseudo-Dionsyius's treatise on the Celestial Hierarchy of Angels. Beginning with his comments on Capella, I present ways in which Eriugena's reflections on music as a liberal art intersect with his discussions of the cosmos and the human soul. For Eriugena and earlier Neo-Platonists, the consideration of quantity related to quantity in ratio was the proper province of musica, and this natural ordering, expounded by musicians, corresponded to the overall coherence observed throughout the cosmos. That is, the "natural proportions" in Eriugena's definition of music included all things that can be studied, visible or invisible. Although some previous musicological considerations of Eriugena's writings have sought insights on performance practices of the ninth century (e.g., the organicum melos question), most have dealt almost exclusively with his description of the harmony of the spheres; this project extends these discussions and explores a fundamental element in Platonic thought neglected in previous studies, i.e., music related to the spheres and the human soul. Eriugena's writings provide a perfect opportunity for such a study. Using my own translation of Eriugena's glosses on Martianus Capella's De Nuptiis, I demonstrate how Eriugena's short treatise on the harmony of the spheres incorporates a discussion on the motions of human souls superimposed upon the planetary system. Furthermore, the ordering of the celestial hierarchy of angels emanating from God is itself proportionally organized, in terms of the nature of each angelic hierarchy and how they interact while relaying the divine oracles. In the end, I demonstrate that a unifying theme in Eriugena's philosophical writings is the need for central, proportionally defined mediators, whether the sun, which modulates the celestial spheres, the mese in the Immutable System of tetrachords, or even specific ranks within the hierarchy of angels.
