Naṣīr al-Dīn al-Ṭūsī’s revision of Theodosius’s Spherics

We examine the Arabic edition of Theodosius’sSphericscomposed by Naṣīr al-Dīn al-Ṭūsī.Through a comparison of this text with earlier Arabic and Greek versions and a study of his editorial remarks,we develop a better understanding of al-Ṭūsī editorial project.We show that al-Ṭūsī’s goal was to revitalize the text of Theodosius’sSphericsby considering it firstly as a product of the mathematical sciences and secondarily as a historically contingent work.His editorial practices involved adding a number of additional hypotheses and auxiliary lemmas to demonstrate theorems used in theSpherics, reworking some propositions to clarify the underlying mathematical argu