The mathematical, medical, and physical-philosophical traditions of medieval visual theory provide important insights and arguments in response to very specific questions defined by the respective interests of these traditions. Each of these three traditions also appears to have a larger framework of assumptions, within which it conceives, states, and solves its specific questions concerning vision. Within the medical tradition, the theory of Hunayn ibn Ishaq (A.D. 808-873) seems to have been the most important in the medieval Islamic world, and his theory involves a comprehensive set of assumptions which are only partially explicit in his Ten Treatises on the Eye. These assumptions are cosmological in nature, and they direct the thinking applied to specific questions at every stage of the theory's development. Hunayn's theory of vision is, in fact, a theory of the cosmological natures of the pathway from the brain to the perceived object. Contents of this study: Intro.; 1st tract: the nature and structure of the eye; 2nd tract: the nature and uses of the brain; 3rd tract: the working of the visual pneuma; The crystalline lens; Problem-solving and cosmology. Illus.