  In section two of On First Philosophy, and several other short works that repeat the same arguments found in this section, al-Kindi follows arguments that derive from Philoponus. (Exactly which text or texts by Philoponus he used is unclear, but it would seem that he at least knew parts of Against Aristotle.) Interestingly, al-Kindi completely ignores a major aspect of Philoponus’ polemic: in the De Caelo, Aristotle had argued that the heavens must be eternal, because they have perfect, circular motion and are therefore not made out of any of the corruptible four elements of our lower world. Whereas Philoponus attacks this cosmology with a lengthy and detailed refutation, al-Kindi simply accepts that the heavens are made out of an ungenerable and indestructible fifth element – but blithely adds that they are nonetheless originally brought into being by God with a beginning in time. (For this see the treatise That the Nature of the Celestial Sphere is Different from the Natures of the Four Elements, edited at Abu Rida 1953, 40–6.)