"In reforming optics he as it were adopted ‘‘positivism’’ (before the term was invented): we do not go beyond experience, and we cannot be content to use pure concepts in investigating natural phenomena. Understanding of these cannot be acquired without mathematics. Thus, once he has assumed that light is a material substance, Ibn al-Haytham avoids discussing its nature further, confining himself to considering its propagation and diffusion. In his optics ‘‘the smallest parts of light," as he calls them, retain only properties that can be treated by geometry and verified by experiment; they lack all sensible qualities except energy."