I. In I Sent., 8, 1, 1, c
II. In I Sent., 8, 4, 2, ad 2
III. In I Sent., 8, 5, 2, c
IV. In III Sent., 11, 1, 2, ad 2
V. De Veritate, 1, 1, c
VI. De Veritate, 1,1, ad 1
VII. De Veritate, 1, 1, ad sc 3
VIII. De Veritate, 10, 8, ad 13
IX. Summa Theologiae, I, 3, 4, ad 2
X. De Potentia, 7, 2, ad 1
XI. Quaestiones Quodlibetales, 9, 4, 1, c
XII. In Metaphysicorum, 4, 2, No. 6
XIII. In De Hebdomadibus, 2
Commentary on Text IX: Summa Theologiae, I, 3, 4, ad 2
Ad secundum dicendum quod esse dupliciter dicitur, uno modo, significat actum essendi; alio modo, significat compositionem propositionis, quam anima adinvenit coniungens praedicatum subiecto. Primo igitur modo accipiendo esse, non possumus scire esse Dei, sicut nec eius essentiam, sed solum secundo modo. Scimus enim quod haec propositio quam formamus de Deo, cum dicimus ‘Deus est,’ vera est. Et hoc scimus ex eius effectibus, ut supra dictum est.
"The Latin verb ‘esse’ can mean either of two things. It may mean the ‘act of being,’ or it may mean the composition of a proposition effected by the mind in joining a predicate to a subject. Taking ‘esse’ in the first sense, we cannot understand God’s ‘esse’ nor His ‘essence;’ but only in the second sense. We know that this proposition which we form about God when we say ‘God is,’ is true; and this we know from His effects, as said above in Question 2, Article 2."
Reading the works of Aquinas one finds that he used the Latin verb esse to signify in more than one way. In his Summa Theologiae (I, 3, 4, ad 2,) he is clear on this. Thus he writes,
It must be said that esse applies to a thing in two ways. In one way, it means the act of being, actus essendi. In another way, it means the composition of a proposition effected by the mind in joining a predicate to a subject.
In the first sense God’s esse is His actus essendi; in the second sense, esse applied to God means ‘God exists.’
By means of demonstration and reasoning one can prove the ‘existence’ of a thing without having to have recourse to the sense experience of an existing exemplifying individual. The grasping of the ‘act of being’ of a particular thing is indeed the strongest evidence that the thing exists, but the knowledge of the ‘existence’ of a particular thing and the grasping of its ‘act of being’ are entirely different issues. The grasping of the ‘act of being’ requires direct and immediate contact with individual, real sensible things. On the other hand, to answer the question of whether or not a thing exists, one does not have to interact directly with existing sensible things.
See also (a) This Journal, Entry 0048; and (b) Stephen L. Brock, "Thomas Aquinas and 'What Actually Exists,'" Wisdom Apprentice, P.A. Kwasniewski, Ed., The Catholic University of America Press, Washington, D.C., 2007, pp 13-39.
Note on Translation: The expression "actus essendi" is translated into English as "act of being," into Italian as "atto di essere," into French as "acte d'être," into Spanish as "acto de ser," and into German as "Akt des Seins" ("Seinsakt.")