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 X: De Potentia, 7, 2, ad 1
Ens et esse dicitur dupliciter, ut patet V Metaph. Quandoque enim significat essentiam rei, sive actum essendi; quandoque vero significat veritatem propositionis, etiam in his quae esse non habent: sicut dicimus quod caecitas est, quia verum est hominem esse caecum. Cum ergo dicat Damascenus, quod esse Dei est nobis manifestum, accipitur esse Dei secundo modo, et non primo. Primo enim modo est idem esse Dei quod est substantia: et sicut eius substantia est ignota, ita et esse. Secundo autem modo scimus quoniam Deus est, quoniam hanc propositionem in intellectu nostro concipimus ex effectibus ipsius.
"Ens and esse may be taken in two ways (Metaph. X, 13, 14). Sometimes they signify the essence of a thing and the ‘act of being,’ and sometimes they denote the truth of a proposition even in things that have no being: thus we say that ‘blindness is’ because it is true that a man is blind. Accordingly when Damascene says that God’s existence is evident to us, the esse of God is taken in the second sense and not the first. For in the first sense God's esse is the same as his essence, and as his essence is unknown so also is his esse. In the second sense we know that ‘God exists,’ because we conceive this proposition in our mind from his effects."
Although the text begins with an explicit reference to ens as the present active participle of esse, no mention is made of the fact that ens is more than just a verbal adjective. The stress is placed rather on the fact that esse has two well-defined meanings.
The text from De Potentia unequivocally differentiates esse in its restricted meaning of actus essendi from esse in its wider meaning of “the truth of a proposition.”
With the example of “blindness,” the text sends us back to the conception of “existence,” which I previously described as the consequence of an actual “state of affairs.”
A person lacking sight is a real person, an actual “state of affairs.” And “blindness” connotes the absence of a quality.
Thus, when we say that 'blindness exists,' 'caecitas est,' we are simply translating our knowledge of the fact of existence into a true statement. The statement is true because we affirm the existence of “that which is.”
This aspect of the verb est does not refer to the metaphysical principle of actus essendi; it refers to an actual “state of affairs,” to the fact of existing.
In its wider meaning, esse refers to “the truth of a proposition” which may simply state something about the absence of being.