Godefroid de Callataÿ presents a paper ‘The Ikhwān al-Ṣafā’ on the Theory of Rays’, Paris, CNRS, 18 January 2018, in the workshop ‘Theories of rays and their impact on Aristotelian physics in medieval Arab-Muslim, Christian and Jewish thought’ organized by Gad Freudenthal and Reimund Leicht.