Much like today's young humanities PhDs, university graduates in the fourteenth century faced a landscape in which the traditional trajectory of the professional scholar began to shift, and many sought to re-contextualize the status and utility of their training within new environments. Unable to procure a shrinking number of benefices, clerks flooded into secular communities and found new occupations. London, which had developed institutions of higher learning with their own libraries and cultures, became a melting pot for the intellectually curious, and the definitions of knowledge and learning began to change. Aristotelian sources and concepts, which had reached Western Europe through the Arabic tradition beginning in the eleventh century, became more widely available. Clerks and lay authors experimented with these ideas to express new modes of authority in vernacular literature. This dissertation argues that four Middle English authors—William Langland, Geoffrey Chaucer, Thomas Hoccleve, and Johannes de Caritate—engaged in a new tradition of "vernacular Aristotelianism" in which they used sources and concepts associated with the Philosopher, many of which had Arabic origins, to articulate a new Middle English literary theory.The authors considered here capitalize on Aristotle's legacy in various ways. Chaucer, for instance, uses the language and methods of the alchemical tradition, which was steeped in Arabic sources, to undermine the distinction between the learned and the "lewed." Johannes de Caritate exploits the voice of the Philosopher in the pseudo-Aristotelian Secretum secretorum (originally the Kitāb sirr al-asrār) to bolster his authority as a medical expert. Langland challenges the value of established authorities by putting the wise words of Aristotle and Solomon at odds with their flawed lives, and Hoccleve incorporates Langland's Aristotelian treatment of the imaginative process into his own assertions about how one should interpret temporal evidence. As these four authors adapt Aristotelian ideas, each explores the role that knowledge plays in lived experience. Whether it be the obligations and privileges that come with learning, the utility of knowledge gained from morally dubious teachers, or the availability of spiritual truths in a flawed society, the power of knowledge is central to their arguments.