Ibn al-Haitham used experimental evidence to check his theories, which was unusual for his time because physics before him was more like philosophy, without experiment. He was the first to introduce experimental evidence as a requirement for accepting a theory, and his Book of Optics was actually a critique of Ptole my's book Almagest. Athousand years on, this optics book is still quoted by professor training research students to be factual and not be swayed by options or prejudice. Some science historians believe that Snell's Law, in optic, actually resides in the work of Ibn Sahl and abn al-Haitam.