This article proposes a reconstitution of the philosophical tenor of al-Fārābī's lost treatise On Changing Beings (Fī al-Mawǧūdāt al-mutaġayyira). It is shown that this work is not only a response to book VI of John Philoponus' Contra Aristotelem, but that its real issues can only be grasped in the context of the author's metaphysical system. Although, for al-Fārābī, genuine demonstrations proceed from the cause to the caused, thus following the order of being, it will be explained how he also admits a strictly physical proof of the simple fact, independently of its cause, and that the physical demonstration of the eternity of the world pertains to this type of proof. This physical proof is specifically directed against the Kindian doctrine of creation.