In the article it is analyzed the question, whether philosophical assumptions included in the 13th paragraph of “A Dispute Over the Existence of the World” (“Spór o istnienie świata”) by Roman Ingarden allow to examine the so-called “ex possibili et necessario proof” for the existence of God? To put it differently, the aim of the article is to examine whether philosophical assumptions of the Ingarden’s ontology are consistent with the Aquinas’ third way of proving that God exists. More specifically, the article considers two main problems: (1) whether Ingarden’s conclusions are convergent with metaphysical conclusions drawn by Aquinas in his third way of proving the existence of God and (2) whether passing from the Ingarden’s ontological conception to metaphysical conclusions is fully justified. To answer these questions, the author analyses the Aquinas “third way” and then presents the philosophical assumptions of the 13th paragraph of Ingardens’s “A Dispute Over the Existence of the World”. After this short analyses, the author turns to comparing both Aquinas’ and Ingarden’s argumentations. Finally, the author tries to adjust Ingarden’s ontological assumptions with Aquinas’ metaphysical arguments for the existence of God. The conclusions drawn in the article are as follows: (1) philosophical assumptions included in the 13th paragraph of Inagrden’s “A Dispute Over the Existence of the World” can be adjusted with the assumptions of the Aquinas’ “ex possibili et necessario proof” for the existence of God. Argumentation in both cases displays strong similarities; (2) examination of the “ex possibili et necessario proof” for the existence of God based on the ontological categories of existential originality (entities that are not brought to existence but that are their own source of existence and for which it is not possible not to exist) and existential derivation (the dependence of entities on other ones in order to come into existence) does not solve the problem of logical incorrectness of the proof. Logical necessity of existence cannot be deducted and can be only assumed as an axiom what seems to be unjustified; (3) possibility of an idea of a necessary being results from the Ingarden’s ontology only if one accepts an assumption of determinism in the world, and this assumption could be hardly accepted in the Ingarden’s philosophy; (4) it is not excluded that there is a logic system in which it could be possible to describe the Ingarden’s ontology and therefore to justify the “ex possibili et necessario proof” (but only if one limits her considerations to the possibility of an idea of God as a necessary being without any conclusions about His actual existence); (4’) if there is not a logic system in which it could be possible to describe the Ingarden’s ontology, there are two following solutions: (4’a) to ignore logics; (4’b) to abandon the idea of logic necessity of existence of entities.