In the context of the philosophy of being as developed by Aquinas, the Latin esse is considered to be the cognate word for both the English term ‘existence’ and the English phrase ‘act of being.’ Reading the works of Aquinas one certainly 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 point.
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. Taking esse in the first sense, we cannot understand God’s esse nor His essence; but only in the second sense we can understand the esse of God. For we know that this proposition which we form about God when we say ‘God is,’ is true; and this we know from His effects.
In the first sense God’s esse is His actus essendi; in the second sense, esse applied to God means ‘God exists.’ Here is the Latin text as it appears in the 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.
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.