This article aims to reconstruct part of Book IV of Galen's lost treatise On Demonstration. This reconstruction is based on testimonies transmitted by Abū Bakr al-Rāzī (d. 925) in the Doubts About Galen, as well as on a source that until now has never been studied, namely the Solution to the Doubts About Galen of Abū al-ʿAlāʾ ibn Zuhr (d. 1131). This paper argues that, in Book IV, Galen maintains that no one can reach any certain conclusion regarding the eternity of the world, and that he consequently criticizes Aristotle's treatment of the subject in On the Heavens. Galen approves only of one of the arguments used by Aristotle in On the Heavens I, 3 (270b12–17), because it is based on empirical evidence. Al-Rāzī focuses on Galen's commentary on this passage. Our hypothesized reconstruction resolves some of the contradictions between the fragments of Book IV that are transmitted in al-Rāzī's report and Philoponus’ Against Proclus On the Eternity of the World. Finally, this article shows that al-Rāzī's interest in this passage from On Demonstration can be explained with reference to his own philosophical-theological goals. As my examination will make clear, al-Rāzī objects to Galen's arguments because he considers that Galen's teleology is based on a framework where the world is eternal (although Galen does not state this view explicitly), and that this framework does not allow Galen to understand that theodicy is prior to both cosmology and biology.