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 III: In I Sent., 8, 5, 2, c
In compositis autem ex materia et forma ‘quo est’ potest dici tripliciter. Potest enim dici ‘quo est’ ipsa forma partis, quae dat esse materiae. Potest etiam dici ‘quo est’ ipse actus essendi, scilicet esse, sicut quo curritur, est actus currendi. Potest etiam dici ‘quo est’ ipsa natura quae relinquitur ex conjunctione formae cum materia, ut humanitas; praecipue secundum ponentes quod forma, quae est totum, quae dicitur quidditas, non est forma partis, de quibus est Avicenna.
In this text Aquinas further clarifies his understanding of the metaphysical principles of ‘essence’ and actus essendi. The example with which Aquinas forcefully accentuates the validity of his insight is the human soul taken in its spiritual substantiality.
In essence, the text talks about three realities that can be described with the Latin expression quo est, (1) the ‘form,’ as in the Aristotelian distinction matter and form; (2) the actus essendi, as in the distinction ‘essence’ and actus essendi; and (3) the substance, as in the Aristotelian distinction substance and accidents. In this context, Aquinas explains that he takes the term quidditas to mean ‘essence’.
As for the Aristotelian distinction matter and form, the human soul, in its spiritual substantiality, is never composed of matter and form. The ones who are in fact composed of matter and form are individual men existing here on earth, as long as they are here on earth, as well as individual men who are already enjoying the state of glory with body and soul in heaven. For these, the composition of matter and form is the composition soul and body.
As for the distinction ‘essence’ and actus essendi, the human soul, in its spiritual substantiality, is always affected by the actus essendi. But the ones who participate in the actus essendi are individual existing men and only through them does the soul get its actuality.
As for the Aristotelian distinction substance and accidents, the human soul, in its spiritual substantiality, can in one sense be said to be a quidditas and in another sense, not to be. Here Aquinas speaks most clearly about how he conceives the metaphysical principle of ‘essence.’
For one thing is the existing real thing which has quidditas, another the quidditas which makes that thing to be what it is, and yet another the actus essendi of the thing which places both the thing itself and its quidditas in actual existence.
In material beings in general, Aquinas conceives quidditas or ‘essence’ as the bare support of matter and form at the exclusion of two "things," (1) all accidental perfections and (2) the actual existence of the thing in the external world. In describing this conception of ‘essence’ Aquinas stresses a double emphasis. On the one hand, the quidditas or ‘essence’ of a material thing must include the matter. On the other, the quidditas or ‘essence’ of a material thing must exclude all the accidental perfections and the actus essendi.
Now since man is not only a material creature composed of matter and form, but also an everlasting spiritual person, this conception of quidditas is applicable to men under certain circumstances but not, under others.
For the individual men existing with body and soul (either here on earth or in the state of glory), Aquinas takes ‘essence’ or quidditas to be not the soul in its spiritual substantiality but ‘humanity,’ the ‘humanity’ of an individual person. By ‘humanity’ Aquinas means what is essential for a man to be a man and nothing else, other than that. In this context, the term ‘humanity’ comprises everything in the individual person except (1) the accidental perfections and (2) the ‘act of being.’
For the individual men existing in heaven as they await the resurrection of the body, Aquinas takes ‘essence’ or quidditas to be the soul itself in its spiritual substantiality. In this regard the separated soul and the angels have something in common, namely this, that their ‘essence’ does not include matter. In “defining” the ‘essence’ of the separated soul and angels, there is only one emphasis, namely, that the ‘essence’ of a spiritual being excludes (1) all the accidental perfections that that spiritual being might have and (2) the actus essendi. The emphasis on excluding matter is not needed.
Thus we have the following.
(1) The form, as quo est for the matter, is that without which the matter cannot be matter.
(2) The actus essendi, as quo est for the essence, is that without which the essence cannot be real in an individual thing.
(3) The ‘essence,’ as quo est for the thing, is that without which the thing cannot be what it is.
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.")