Traditionally, historians of science have only been interested in Islamic science because of its relationship to Greek science, and in the ways in which it was instrumental in transporting Aristotle, Ptolemy and Galen to the West. Moreover, the successes and failures of Islamic science have been judged according to the metric of the Scientific Revolution. As such, the actual context of the works of Islamic scientists and physicians has been overlooked, thereby producing a skewed picture of science in Islamic societies. This dissertation seeks to correct this picture by placing Islamic medicine firmly within its context. In the process, it provides a new framework with which to understand the relationship between reason and revelation in Islamic societies, and suggests new ways to revisit the entire problem of the decline of Islamic science. The dissertation specifically examines the corpus of writings of an Egyptian physician-jurist, Ibn al-Nafīs (d. 1288), best known to Western historians of science as the discoverer of the pulmonary circulation of blood. Although his discovery has been known for a century, there has been no study that situates Ibn al-Nafīs's discovery within the context of his time. This dissertation seeks to fill that lacuna. Focusing on his views on the soul (nafs) and spirit (rūh)--two concepts central to theological discussions about the afterlife and medical physiology, this dissertation positions Ibn al-Nafis's philosophical and physiological discussions against the background of earlier and contemporary philosophical, medical and theological works. Through this contextualization, this study reveals that Ibn al-Nafīs's new theory of the pulmonary transit of blood is the offspring of his new psychology and physiology. It also reveals that on the basis of his new physiology, Ibn al-Nafīs rejected the Galenic theory of pulsation and posited a new theory in its stead" an important point that has hitherto been missed by historians. Moreover, this work reveals that Ibn al-Nafīs's new physiological and psychological theories are themselves the direct result of his solution to the thirteenth century debates over reason and revelation. Consequently, the dissertation problematizes existing historical accounts, and seeks to replace historical models that posit an antagonistic and destructive relationship between reason and revelation during this period.