Frontmatter -- Acknowledgements -- Table of Contents -- Abbreviations -- 1. Introduction -- Part I. -- 2. The Foundation of Aristotle's Theory of Aether -- 3. The Existence of Aether: De cáelo I 2 -- 4. The Nature of Aether: De cáelo 13-4 -- Part II. -- 5. Philoponus' Rejection of Aether: Book I -- 6. On the Nature of the Celestial Region: Books II and III -- 7. The Celestial Body: Unchangeable but Corruptible -- 8. Conclusion -- 9. List of Fragments -- 10. Indices -- 11. Bibliography -- Backmatter