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 VI: De Veritate, 1, 1, ad 1
Cum dicitur verum est 'id quod est,' li 'est' non accipitur ibi secundum quod significat actum essendi, sed secundum quod est nota intellectus componentis, prout scilicet affirmationem propositionis significat, ut sit sensus: verum est 'id quod est,' id est cum dicitur esse de aliquo quod est.
"In the statement 'The true is that which is,' the word 'is' is not here understood as referring to the act of being, but rather as the mark of the intellectual act of judging, signifying the affirmation of a proposition. The meaning would then be this: 'The true is that which is'--'the true' is had when the existence of 'what is,' is affirmed."
In this text Aquinas clarifies his understanding of the relationship that exists between “language” and the metaphysical principle of actus essendi. The underlying principle is this: The linguistic expression “est” does not necessarily refer to the metaphysical principle of actus essendi. In the text Aquinas clearly puts aside this aspect of the meaning of “est.” The text concentrates rather on the affirmation of the fact of existence.
Many philosophers define “existence” as “fact.” In realistic terms, when we say, for example, “It is a fact that the moon exists,” we simply acknowledge the presence of something having real, demonstrable existence. Whenever anything exists, its “existence” is a fact. In this context, “existence” is more rigorously defined as the consequence of an actual “state of affairs.” And consequently, a non-actual “state of affairs” is never credited with the connotation of “existence.”
Both artificial and natural things can be conceived as actual “states of affairs.” In an existing car, for example, because all the elements have been harmoniously put together, we find an actual “state of affairs” holding in reality. And, by the same token, a fox, an animal, that we suddenly catched crossing the garden, embodies within its being an actual “state of affairs,” namely, an entirety of internal constituents which holding together by the principle of life make it a living being. Because of the natural arrangement of these internal constituents, the fox exists as much as the car exists. The fox exists in accordance with God’s ordinance; the car exists according to the inventions of the human mind.
When we have evidence of a particular thing actually existing in the world and we proceed to affirm its existence, 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.
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.")