The English translation by Claire C. Reidl of Robert of Grosseteste’s De Luce (on Light). The Author (1175-1253), appointed Bishop of Lincoln in 1235, was one of the most acknowledged masters of Oxford. He maintained the necessity of studying physical nature through mathematics; his conceptions were influenced by Neoplatonism and Arabian treatises on Optics. Here Grosseteste explains the “atemporal” instant of the origin of the universe maintaining that light is the first corporeal form, that makes the first matter (without any form) expand in three dimensions. Light spreads carrying (informed) matter along with it, and multiplying infinitely it originates finite quantity, so structuring the universe, conceived as series of spheres. This text is divided into two parts, the first describing the metaphysics of light, the second his cosmogony. It begins by developing metaphysical notions, and then the mathematical formulas that describe the formation of the 13 spheres of the universe (9 heavenly unchangeable spheres, 4 – the 4 elements – belonging to the lower changeable world); finally, the text discusses the theme of the perfect number. The novelty lies in the conscious synthesis between the biblical Genesis and the cosmogony of the Aristotelian De caelo, though reality has here a mathematical structure. This text is an example of the philosophical-scientific synthesis which characterized the Oxford school, and an important source of the “Metaphysics of Light” conceived in the Middle Ages. (Interdisciplinary Encyclopedia of Religion & Science Online)