With Robert Grosseteste light is made to play the leading part in the entire process. In his unpublished _Hexaemeron_[613] and in the _De luce_[614] he sets forth a theory of cosmogony which was derived in part from the Moslems but in essentials was original.[615] We trust that the following brief statement of the theory does not do violence to the thought of Grosseteste as expressed in the _De luce_. He conceived of light as the first corporeal form and also as giving form to the _materia prima_ of the universe. By radiating through the unformed _materia prima_ the light converted it into a sphere. Thereupon the light made its way from the outer edge of the sphere towards the center. As it passed through the various realms of the universe it diffused, rarified, and purified the _materia_ of each, but with each stage of its advance its powers were diminished and correspondingly the potentiality of each successive realm of being purified was diminished. Thus thirteen concentric spheres were produced, nine celestial spheres and four spheres of the elements, and each of these was more complex, dense, and impure than its neighbor above. THE NATURE OF THE SIX DAYS (4) Were the Six Days described in the book of Genesis actual divisions of time? The words of the Bible seemed to be contradictory on this point. From the words of Genesis alone one would gather that the completion of the universe was accomplished in six days. On the other hand, we read in Ecclesiasticus (xviii, 1), “He that liveth forever created all things together” (Qui vivit in aeternum, creavit omnia simul). According to Theodoric of Chartres[616] these two statements referred to different events. The passage in Ecclesiasticus applied only to the creation of the _materia primordialis_ on the first day. The works of the succeeding days were the result of the automatic development of natural processes by which the universe became as we now know it. Belief in the reality of the duration of the Six Days was shared with Theodoric by most commentators, such as William of Conches, Hugh of St. Victor,[617] Peter Lombard, and Peter Comestor. Augustine, however, had argued in a more abstruse vein that the “days” were not actual units of time but that they represented merely so many distinct operations in the work of creation.[618] And in our period Arnold of Chartres urged that the Creation was carried out in one day and all at once (_uno die et semel_).[619] ETERNITY OF THE UNIVERSE