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

We examine the Arabic edition of Theodosius’s Spherics composed 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’s Spherics by 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 the Spherics, reworking some propositions to clarify the underlying mathematical argument and reorganizing the proof structure in a few propositions. For al-Ṭūsı, the detailed preservation of the words and drawings was less important than a mathematically coherent presentation of the arguments and diagrams.