Nicole Oresme on the Nature, Reflection, and Speed of Light

Abstract

L ATE MEDIEVAL VISUAL THEORY presents an untidy amalgam of Aristotelian natural philosophy, geometric optics, scholastic metaphysics, and even theology. Modern scholarship has treated each of these medieval categories of thought separately, and it has therefore proved very difficult for the student of medieval visual theory to comprehend his subject fully-that is, both to analyze what the medieval authors who wrote about vision were trying to accomplish, and to discern what sources they drew upon. Keeping these difficulties in view, the present paper unravels a hitherto unexamined instance of late medieval visual theory: Nicole Oresme's discussion of light in Questiones super libros Aristotelis de anima, based on Aristotle's On the Soul. I The details of Oresme's argument on the properties of light cannot be fully elucidated without reference to Book II of De anima, in which Aristotle suggested that a medium capable of bearing light could become light-bearing instantaneously.2 This passage forms the basis for Oresme's discussion. Being a careful scholastic, however, Oresme did not simply record his agreement with Aristotle. Rather, he set out first to explain what light was and what it meant for the medium to become instantaneously light-bearing. Only then did he attempt to discuss the possible immediacy of illumination. If we treat Oresme's argument under two headings-his preliminary explanations of the nature of light and the medium, and his specific arguments about instantaneous propagation-it appears that each part has a different relation to his sources. The specific arguments about instantaneous propagation may be unique to Oresme; at least I have not uncovered any precedents for them, although many comparable late medieval texts remain unedited. His preliminary considerations, however, have definite precedents in the many diverse elements of medieval visual theory, as comparison of those ideas of Oresme's with similar arguments in earlier texts will show. Oresme's preliminary arguments are conveniently treated before his special arguments about instantaneous propagation. In considering them, it is possible to test David Lindberg's recent division of late medieval visual theory into three