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 VII: De Veritate, 1, 1, ad sc 3
Cum dicitur: ‘diversum est esse et quod est,’ distinguitur actus essendi ab eo cui ille actus convenit. Nomen autem entis ab actu essendi sumitur, non ab eo cui convenit actus essendi.
"In the statement, ‘the term esse is not equivalent to the expression ‘that which is,’” we are saying that the ‘act of being’—which is one of the meanings of term esse—is being distinguished from ‘that, to which that act belongs.’ The term ens—translated as 'being'— refers indeed to the ‘act of being,’ and not to ‘that, whose act it is.’"
It is to be noted that in this text Aquinas uses the term esse to signify actus essendi. This meaning of esse goes beyond the mere fact of existing; this meaning of esse refers to something deeper. It refers to a metaphysical principle, namely, to the metaphysical principle of actus essendi.
After clarifying that from the side of the actus essendi every existing thing is said to be ens and that from the side of the essence every existing thing is said to be res, Aquinas now examines the relationship that exists between ens and quod est.
An existing thing is both ‘that which is’ (quod est) and a thing possessing the ‘act of being’ (ens.)
The point is this: the term ens does not take its meaning from ‘that which is,’ that is to say, from the quod est. The term ens takes its meaning from the metaphysical principle of actus essendi.
Just as laudans (laudantis) is the present active participle for the Latin verb laudare (laudo, laudare, laudavi, laudatus),’ ens (entis) is the present active participle of the Latin verb esse (sum, esse, fui, futurus.)
The term ens (entis) indeed is at times taken to signify ‘that which in any way whatsoever is,’ but in the present context Aquinas clearly puts aside this aspect of the meaning of ens to stress the direct relationship that exists between ens and actus essendi.
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.")