Entry 0182: The Thirteen Texts in which Aquinas Uses the Expression "Actus Essendi" (XII)
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 XII: In Metaphysicorum, 4, 2, No. 6
Sciendum est enim quod hoc nomen homo, imponitur a quidditate, sive a natura hominis; et hoc nomen res imponitur a quidditate tantum; hoc vero nomen ens, imponitur ab actu essendi: et hoc nomen unum, ab ordine vel indivisione. Est enim unum ens indivisum. Idem autem est quod habet essentiam et quidditatem per illam essentiam, et quod est in se indivisum. Unde ista tria, res, ens, unum, significant omnino idem, sed secundum diversas rationes.
For it must be borne in mind that the term "man" is derived from the quiddity or the nature of man, and the term "thing" from the quiddity only; but the term "being" is derived from the 'act of being,' and the term "one" from order or lack of division; for what is one is an undivided being. Now what has an essence, and a quiddity by reason of that essence, and what is undivided in itself, are the same. Hence these three—thing, being, and one—signify absolutely the same thing but according to different concepts.
Just as he did in De Veritate, 1, 1, c, in the present text, Aquinas once again clarifies his understanding of the transcendental notion of res (thing), a term which derives its content from the quiddity or essence of the thing.
The underlying principle of this doctrine is this. In the real world one cannot have the metaphysical principle of ‘essence’ in isolation from the metaphysical principle of actus essendi. One cannot have one without the other. In the real world essences exist with the actus essendi, and vice versa, the actus essendi always appears instantiated in an essence. It is for this reason that the term "res" (thing) expresses a transcendental notion; res stands for a universal mode of being that follows upon the fact of having an essence. Essences are found in every existing thing without exception.
It is this inherent unity of the two most fundamental metaphysical principles what allows Aquinas to provide a coherent metaphysical account of the transcendental ‘one’ and the charaterisctic ‘unity’ of a subsisting thing.
From the side of the actus essendi every existing thing is said to be ens and from the side of the essence every existing thing is said to be res. But it is from the internal inseparability of these two metaphysical principles that every existing thing is said to be unum. Unum is as well a transcedental notion; unum derives its content from the indivisibility of 'essence' and actus essendi.