Medieval Optics

Medieval optics, also known as perspectivist optics from the mid-13th century on, offered a complete theory of human cognition. Whereas modern optics limits itself to the study of the behavior and properties of light, perspectivist optics sought to explain how human beings perceive and then understand the world around them. The perspectivists contended that vision proceeds through a process of intromission, in other words, we see objects in the world because information from those objects, called “species,” reach and then reproduce themselves within the eye and then throughout the various faculties of human the brain. Prior to the 13th-century popularization of perspectivist optics, most European and Christian thinkers believed vision occurred through a process of extramission, in which vision depended on visual rays extending out from the eyes to the things in the world. Medieval optics derives largely from the work of Alhacen (b. c. 965–d. c. 1040), the 12th-century Arabic thinker. In The Book of Optics, translated into Latin in 1200 as De aspectibus, Alhacen (Ibn al-Haytham) wove together the mathematical and geometrical aspects of Ptolemy’s extramission account of vision, Galen’s physiological account of the eye, and ideas about light from such Arabic thinkers as Alkindi (b. c. 801–d. c. 873), to create a sophisticated intromissionist account of vision. The Franciscan Roger Bacon (b. c. 1219–d. c. 1292), working from the Latin translation, borrowed all of these ideas for his own treatise, Perspectiva, and combined them with Avicenna’s Aristotelian-influenced faculty psychology. If not terribly original, Bacon’s treatise, along with works by his fellow Franciscan John Pecham (b. c. 1230–d. c. 1292), and the Silesian cleric Witelo (b. c. 1230–d. c. 1300), proved tremendously influential. Perspectivist ideas filtered into scholastic debates about natural causation and physics, cognition, epistemology, the Eucharist, and, even, the beatific vision. Various religious and pastoral treatises bear the imprint of perspectivist ideas, as do works by such renowned medieval authors as Jean de Meun (b. c. 1240–d. c. 1305), Dante (b. c. 1265–d. c. 1321), and Chaucer (b. c. 1343–d. c. 1400). Perspectivist ideas may even have played a part in the development of illusionistic painting in Italy, beginning with Giotto’s early-14th-century frescos at the Arena Chapel in Padua and culminating with Alberti’s 1435 treatise, On Painting. It was only in the early 17th century, with the publication of Johannes Kepler’s Emendations to Witelo, that the tradition of medieval optics as a comprehensive account of human cognition came to an end. Kepler (b. 1571–d. 1630) reimagined the eye as a camera obscura with a lens to focus light on the retina, effectively separating the study of optics from the study of cognition. The modern science of optics was born.