Grosseteste employs the notion of light to explain the genesis of the Aristotelian cosmos as a system of nested celestial spheres surrounding the four sublunary or elemental spheres. According to Grosseteste, the infinite self-multiplication of the initial point of light extended the first matter it informed into a spherical form, since light diffuses itself spherically. The outermost parts of the matter of the sphere thereby generated, unlike the parts of the matter below them, were maximally extended and rarefied and formed the outermost (or first) sphere, the firmament. Yet because light is essentially self-multiplicative, the light in this outermost sphere continued to multiply itself, only now it did so back inwards toward the center from all parts of the outermost sphere, since it had already diffused itself outward as far as it possibly could. And since light, being a substantial form, cannot exist apart from matter, this inwardly directed light drew with itself what Grosseteste calls the spirituality of the matter of the outermost sphere, and thus lumen, a body comprised of light and the spirituality of this matter, proceeded inwards. Moving inwards, lumen concentrated the matter existing below the outermost sphere, leaving in its wake below the outermost sphere a second sphere comprised of matter whose parts were rarefied as much as they could be. This sphere in turn generated lumen, which moving inwards further concentrated the matter below it and rarefied the outermost parts of this matter so as to produce the third sphere. The repetition of this process gave rise to the nine celestial spheres, each sphere being comprised of matter whose parts were incapable of further rarefaction.