The theories of light that were in circulation in the European universities of the thirteenth century are currently not well understood, partly because of the tremendous diversity among these theories and the wide range of sources that were drawn upon (for instance, Albert the Great, writing in 1242, identifies eleven different sources and obliquely references many more). Indeed, in the mid-thirteenth century in Europe, theories of the natural world were in a state of accelerated turbulence and change, as European thinkers were exposed to a torrent of new debates and new theories about the physical realm, arriving from Greco-Arabic “natural philosophy.” In developing new visions of the physical world at this time, an important puzzle arose for thirteenth-century thinkers, concerning where light fits into the picture. Is light situated within the world of matter and motion (i.e., having “natural being”), or is it something immaterial? In this paper, building on previous work on medieval Arabic theories of light, I examine a group of three theories from Paris in the 1240s-50s that came down firmly on the physicality of light: the theories of Albert the Great, the early Thomas Aquinas, and Bonaventure. For all three, light is a property of bodies. Nonetheless, they have trouble explaining why light seems unlike other physical states (e.g., existing in different ways in solid bodies and translucent bodies). I will discuss how they resolved these difficulties, and what their resolution reveals about how they integrated new scientific knowledge
