Entry 0056: - Actus Essendi: Commentary on Summa Theologiae I, 3, 4, ad 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. 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.’
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 Entry 01-0048 and 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.