Ibn al-Haytham (about 965-1040) wrote the Book of Optics (Kitab al-Manazir). He argued that we see because light enters the eye, not because rays leave it, and he tested his ideas with experiments, including with a dark chamber. He is often cited as a pioneer of the experimental method, although the method grew over time and many scholars contributed. Places of learning also spread: the University of al-Qarawiyyin in Fez was founded in 859 by Fatima al-Fihri, a woman from Kairouan, and is recognised as one of the oldest continuously operating institutions of learning; al-Azhar in Cairo began teaching from 970. Women and men of many regions took part in study, and the Muslim tradition of ijaza, a certificate of authorisation to transmit a book or a science, linked students to their teachers by name.