In his third way, St. Aquinas, argues for the existence of God from necessity and contingency. Some philosophers have given physical-metaphysical interpretations of his argument while some others have given physical ones.
To claime that Aquinas`s argument and Avecenna`s proof are identical can be so problematic. In this article, we first present, in brief, Avecenna’s Seddiqin argument and express different readings of Thomistic necessity and contingency argument, then seek to reveal the differences, particularly their distinctions in their reliance on the principle of impossibility of infinite regress. It seems that we should, through this distinguishing, find out what conception of God the two philosophers have in their views, and what relation to the nature God has; Avecenna, can, by means of essential possibility, falsify the infinite regress in efficient causes, and so prove the existence of God as necessarily-existing being in itself. While, in physical-metaphysical reading of Thomistic necessity and contingency argument, the aim is to prove a necessarily-existing being that has created the nature from nihility. Hence, if such utterances only rely on essential possibility, the reasoning is not strong enough to prove the existence of an eternal creator who created the nature progressed from nihility. Also by refer to the Aquinas’ second way and comparing his argument for impossibility of infinite regress with that of Avecenna, the argument of medial and extreme, it becomes clear that even if the argument of Aquinas was derived from the Avecenna’s argument, the derivation is incomplete