Galen : Ancient and Medieval Theories of Vision

Rhazes’ Al-Shukuk ’Ala Jalinus (Doubts on Galen), although an important and controversial contribution in his era and thereafter, has seldom been examined in the literature. In this article, we give a brief account of Rhazes’ life and works, and provide an English translation of a passage from his treatise “Doubts on Galen,” concerning the theory of vision. An overview of the evolution of vision theories in ancient and medieval times is given with respect to both intromission and extramission theories. Refuting the Stoic and Platonic conjectures, which were then followed by Galen, Rhazes adopted Pythagoras’ and Aristotle’s views of the intromission theory and for this, his own reasoning was sufficient to cast doubt on Galen. The bearing of this notion is even more palpable if one realizes what an avid follower of Galen Rhazes was in his diagnostics and treatments, praising him in almost any of his writings. The extramission theory ultimately gave way to an intromission thesis with emerging empiricism, but the history of evolution of theories of vision has witnessed a dramatic fluctuation of old scientific thoughts across centuries and among philosophers. This article also gives a brief historical snapshot on how vision theories lead to the development of geometrical and physical studies that further contributed to advances in ophthalmology.