Grosseteste also employs this theory of light as part of his explanation of the nature of the heavenly movements. He explains that the heavenly bodies can only move with a circular movement because the lumen in them is incapable of rarefaction or condensation, and as a result cannot incline the parts of their matter upward (to rarefy them) or downward (so as to condense them). Indeed, the heavenly spheres can only receive movement “from an intellective motive power,” that is, an intelligence or angel, “that in incorporeally casting its mental gaze back over itself rotates the spheres with a corporeal revolution” (Lewis 2013, 246).  Because the elements, on the other hand, can be rarefied and condensed, they can incline the lumen in themselves away from the center of the universe, so as to rarefy it, or toward the center so as to condense it, and this accounts for their natural capacity to move up and down.