By this lux as the first corporeal form Grosseteste did not, of course, mean simply visible light. As an emanation or propagation of substance and power lux was the basis of all bodily magnitude and of all natural operations, of which the manifestation of visible light was only one. One of the most important functions of lux was to be the intermediary between spirit and matter. It was the instrument by which God produced the macrocosm of the universe, and the instrument by which the soul made contact with the physical body and the things of sense in the microcosm of man.

Robert Grosseteste and the Origins of Experimental Science 1100-1700 (1953)