"In the study of light, he said, the argument proceeded both from the effect to the cause and from the cause to the effect, and he prayed God, the light of all, to help him. He arranged his work as a structure of theory built up from a set of empirical facts and the rules of geometry, with the argument sometimes ascending inductively to a 'common nature' and the grasping of a universal, and sometimes descending deductively by 'composition' to consequences by means of which a theory could be verified or falsified by experiment."