We know that Lorenzo Ghiberti^§MS2.2b had read in translation Alhacen’s great work on the science of optics,¹⁴⁴ because he summarized it at length in the Commentarii.

Born in Basra in 965, Al-Hasan ibn al-Haytham, or Alhacen as Ghilberti and his Florentine contemporaries called him, came to Cairo in 1021 to work in the court of the Caliph. A mathematician, logician, and scientist whose genius was widely known, his task was to design a way to control the flow of the Nile. When this proved impossible, several biographers report, he was imprisoned, feigned madness, and on release lived the rest of his life in the spartan conditions of a tent in the grounds of the Azhar mosque. There, he wrote and tutored.

Among Alhacen’s many books were works on logic, metaphysics, and theology, but he is best known for his Kitāb al-Manāẓir or Book of Optics – over 200,000 laboriously expounded words, completed in about 1030.¹⁴⁵ In 1200, it was translated into Latin as De Aspectibus and the author’s name Latinized to Alhacen, then translated into Italian in the fourteenth century. Ghiberti had read the Italian edition.¹⁴⁶

Alberti’s centric ray Alhacen had already identified as the central ray in a “cone of vision.” Credited by some to be the inventor of the camera obscura, a box with a pinhole at one end in which an inverted image is projected at the other, Alhacen conducted a series of experiments proving the physical existence of light. He showed that it traveled in straight lines along pathways, and that it follows a geometrical course when reflected or refracted. This was a radical reversal of the ancient Western idea that the eyes emitted visual rays and these were the source of our seeing. In Alhacen’s theory, eyes are a site of reception rather than emission.¹⁴⁷ However, he added that “vision is not achieved by pure sensation alone, that it is accomplished only by means of discernment and prior knowledge.”¹⁴⁸

Alhacen made several major methodological innovations, extending and transforming the theories of optics of the Greeks, principally those of Galen and Ptolemy. He was the first to offer an image or picture-oriented theory of vision based on mathematical principles, and to achieve this by combining experimental methods of empirical observation with geometry and mathematics. He was also the first to propose a sophisticated psychology of vision, where what is seen is as much a cognitive matter as one of empirical sensation, requiring, in addition to vision itself, inference and interpretation.

The ancient visual ray theory had placed the viewer as an unmediated agent in seeing – this was a seemingly straightforward way to explain the eye’s capacity to detect distance. What Alhacen proposed aligns more closely with modern theories of vision. For him the properties of things were at play in the detection of distance: the size of the object, the angle of its edges in the visual field, and its distance. But these mathematically determinable variables in vision were not enough. In addition to the light the eye sees, he says that an interpretation of the meanings of objects and distances must be based on learned experiences of things and spaces.

So Alhacen’s theory is at once objectivist, at least compared to the anthropocentrism of the visual ray theory, and subjectivist, where the viewer is positioned as an interpreter who makes meaning by adding meaning to what they see.

It was not just the centric ray that the fourteenth-century Florentine image theorists took from Alhacen. It was also this dialectic of objectivism and subjectivism: an outside world that acts in ways which are apprehensible (objects, spaces, light), and the reciprocal relation of our making sense of that world in a culture of imaging.^§MS2.2c

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¹⁴⁴ A. Mark Smith, 2001a, “The Latin Source of the Fourteenth-Century Italian Translation of Alhacen’s ‘De Aspectibus’,” Arabic Sciences and Philosophy 11(1):27–43.

¹⁴⁵ Al-Hasan ibn al-Haytham, c.1030 [1983], Kitāb Al-Manāẓir, Books I–II–III, “on Direct Vision”, Kuwait: National Council for Culture, Arts and Letters.

¹⁴⁶ Al-Hasan ibn al-Haytham, c.1030 [1989], The Optics of Ibn Al-Haytham, Books I–II–III, “on Direct Vision”, Volume 2: Introduction, Commentary, Glossary, Concordance, Indices, translated by A.I. Sabra, London, UK: The Warburg Institute, pp. xix–xxiv; A. Mark Smith, 2001b; Alhacen’s Theory of Visual Perception: A Critical Edition, with English Translation and Commentary, of the First Three Books of Alhacen’s De Aspectibus, the Medieval Latin Version of Ibn Al-Haytham’s Kitāb Al-Manāẓir, Volume 91, Philadelphia, PA: American Philosophical Society, pp. ix–xvi, cvii; Jim Al-Khalili, 2015, “Book of Optics,” Nature 518:164–65.

¹⁴⁷ David G. Lindberg, 1976, Theories of Vision from Al-Kindi to Kepler, Chicago, IL: University of Chicago Press, pp. 61–66.

¹⁴⁸ Al-Hasan ibn al-Haytham, c.1030 [1989], The Optics of Ibn Al-Haytham, Books I–II–III, “on Direct Vision”, Volume 2: Introduction, Commentary, Glossary, Concordance, Indices, translated by A.I. Sabra, London, UK: The Warburg Institute, pp. liv–lv; Abdelhamid I Sabra, 2003, “Ibn Al-Haytham’s Revolutionary Project of Optics: The Achievement and Obstacles,” pp. 85–118 in The Enterprise of Science in Islam: New Perspectives, edited by J.P. Hogendijk and A.I. Sabra, Cambridge, MA: MIT Press, p. 91.

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Cope, Bill, and Mary Kalantzis. 2020. Making Sense: Reference, Agency and Structure in a Grammar of Multimodal Meaning. Cambridge UK: Cambridge University Press, pp. 222–224. https://doi.org/10.1017/9781316459645

Al-Hasan ibn al-Haytham’s "Book of Optics" §MS2.2.1a