Ibn al-Haytham (Alhazen)

Works

• c. 1028Doubts concerning Ptolemy

  Critique of Ptolemy’s Almagest, Planetary Hypotheses and Optics: some of Ptolemy’s models, such as the equant, contradict his own principle that the heavens move in uniform circles; the true arrangement remains to be found. The treatise opened a long tradition of non-Ptolemaic astronomy in the Islamic world.

• c. 1030Book of Optics

  Seven books on vision: light and colour radiate from every point of an object in all directions, and the eye sees by receiving them; Ibn al-Haytham supports this with experiments, treats reflection and refraction, and analyses the inferences by which the mind recognises what it sees. Translated into Latin around 1200 as De aspectibus.

Position

Ibn al-Haytham is on the map for his theory of vision and for a method that joined mathematical demonstration, experiment and an account of perception. Earlier theories held either that the eye sends out visual rays, as Euclid, Ptolemy and al-Kindi taught, or that objects send forms or images to the eye through a medium, as the atomists and, in his own way, Aristotle held. In the Book of Optics Ibn al-Haytham argued that light and colour radiate from every point of a lit object in straight lines in all directions, and that the eye sees by receiving the rays that fall perpendicularly on it, so that each point of the object corresponds to a point in the eye. He supported this with long series of experiments with dark chambers, tubes and screens – a procedure he called iʿtibār, “testing” – and treated reflection and refraction mathematically. Perception, he held, is not only the reception of light: the mind recognises objects, their size, distance and shape by judgement and inference, often so quickly that we do not notice, and errors of sight can be explained as failures of these inferences. In the Doubts concerning Ptolemy he applied the same critical attitude to astronomy: some of Ptolemy’s models break his own principle of uniform circular motion, so that the true configuration of the heavens remains to be found. The Optics was translated into Latin around 1200 as De aspectibus; Roger Bacon, John Pecham and Witelo built on it, Kepler’s optics answered it, and Friedrich Risner printed it in 1572. The Doubts opened a tradition of non-Ptolemaic astronomy that continued in Maragha with Nasir al-Din al-Tusi. A. I. Sabra’s edition and translation of the Optics (1983, 1989) made it a subject of the modern philosophy of perception as well as of the history of science.

Life & context

Abu ʿAli al-Hasan ibn al-Hasan ibn al-Haytham was born in Basra around 965. According to later biographers he held an administrative post there before turning to science, and went to Egypt under the Fatimid caliph al-Hakim, reportedly with a plan to regulate the Nile flood, which he abandoned when he saw the river. A widely repeated story says that he then feigned madness to escape the caliph’s anger until al-Hakim’s death in 1021; it comes from sources written long afterwards. In Cairo he lived by copying and teaching mathematical works and wrote most of his more than ninety treatises on mathematics, optics, astronomy and natural philosophy. Lists of his writings, one of them in his own words, survive in the biographical dictionary of Ibn Abi Usaybiʿa. He died in Cairo around 1040.

Further main works

• On the Light of the Moon
• On the Shape of the Eclipse
• On the Configuration of the World (Fī hayʾat al-ʿālam; attribution debated)
• Commentary on the Premises of Euclid, with an attempt to prove the parallel postulate