This study addresses the general question of how medieval music theory participated in the discourse of the related disciplines of philosophy, natural science and theology. I focus on a specific instance of scientific inquiry: the fourteenth-century music treatise Speculum musicae , written by an author known to us as Jacobus. A detailed analysis of Speculum musicae reveals an aesthetic system whose elements are assigned meaning and value through the anagogical relationships that the author posits (either explicitly or implicitly) with systems articulated in philosophical and theological treatises at the turn of the fourteenth century. My central concerns are uncovering the impetus behind the production of this treatise, determining where Jacobus's philosophies fit within particular schools of medieval thought, as revealed through his vocabulary choices, supporting sources, and methods of reasoning, and then extrapolating from these philosophies which rationale (ratio ) most informs his positions on particular issues, such as his classification of music, or his defense of the ancient art of singing against the modern art. I hope to present a fresh perspective on one of the most important yet one of the most mysterious ages in the history of music. The turn of the fourteenth century was a fascinating time for music: we find musical systems in a pronounced state of flux with various theoretical solutions proposed in response to the problems of notating this increasingly complex music. Analyzing the background of these theoretical formulations, and assessing the various judgments of "good" practice, and the kinds of arguments used to bolster these judgments, will uncover reasons for the overturning of musical systems and go some way toward explaining the nature of musical change.