The mathematician and astronomer of Basra and Cairo, Latinized as Alhazen, who helped found the medieval science of mathematical and experimental optics. His Book of Optics (Kitab al-Manazir), written under the Fatimid caliphate around the 1010s, argued that vision works by light entering the eye from objects rather than by rays the eye sends out, and built that argument on systematic experiment and geometry. Translated into Latin as Perspectiva, the work shaped Roger Bacon, Witelo, and eventually Kepler, and his insistence on testing claims against controlled observation has led many modern writers to call him an early pioneer of the experimental method.
By a later and much-repeated story, a man sits under house arrest in Cairo, having talked his way out of a death sentence by pretending to lose his mind. He had promised a volatile caliph that he could tame the Nile's floods, then realized the river would not obey him, and feigned madness to survive. According to the traditional account, it was in those shut-in years that Ibn al-Haytham did much of his real work: passing light through small holes, studying how it bent and bounced, and asking a question older science had answered backwards. Does the eye send rays out to touch the world, as influential Greek writers had taught? Or does light travel from the world into the eye? He took the second view and set out to prove it, not by argument alone, but by experiment.