 The treatise On the Heavens followed next in the Aristotelian order but no commentary of Avempace has reached us, and the following extant commentary is on Coming-to-be and Passing-away (IB-GC). Avempace coincided with Philoponus in a preliminary remark observing that Aristotle proved in his book On the Heavens the existence of four elementary bodies: fire, air, water, earth. Since one of the subjects of Coming-to-be and Passing-away is the mutual generation of these bodies, it is the logical continuation. Thus, Avempace’s lost commentary On the Heavens likely comes after the one on Physics and before the one on Coming-to-be and Passing-away.