Entry 0190: The Thirteen Texts in which Aquinas Uses the Expression "Actus Essendi"
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
Text XIII: In De Hebdomadibus, 2
Deinde cum [Boetius] dicit, "ipsum enim esse," manifestat praedictam diversitatem [i.e., differentiam eius quod est esse ad id quod est] tribus modis: quorum primus est, quia ipsum esse non significatur sicut ipsum subiectum essendi, sicut nec currere significatur sicut subiectum cursus: unde, sicut non possumus dicere quod ipsum currere currat, ita non possumus dicere quod ipsum esse sit: sed sicut id ipsum quod est, significatur sicut subiectum essendi, sic id quod currit significatur sicut subiectum currendi: et ideo sicut possumus dicere de eo quod currit, sive de currente, quod currat, inquantum subiicitur cursui et participat ipsum; ita possumus dicere quod ens, sive id quod est, sit, inquantum participat actum essendi: et hoc est quod [Boetius] dicit: "ipsum esse nondum est," quia non attribuitur sibi esse sicut subiecto essendi; sed id quod est, accepta essendi forma, scilicet suscipiendo ipsum actum essendi, est, atque consistit, idest in seipso subsistit.
[In the paragraph] where it says, "For to be itself...," he [Boethius] states this difference [i.e., the difference between what is meant by "to be" and what is meant by "that which is"] in three ways, of which the first is this: [When attributed to something] "to be itself" is not signified as if it were the subject of being, any more than to run signifies the subject of running.
Just as we cannot say that "to run itself" runs, so we cannot say that "to be itself" is, and just as "that which is" is signified as the subject of being, so "that which runs" is signified as the subject of running.
Therefore, just as we can say of him who runs, or the runner, that he runs, insofar as he is the subject of running and participates in it, so we can say that an existent--a being--or "that which is," is.
That is what he [Boethius] means when he says, "to be itself is not yet," because to be is not attributed to something as to a subject of being; but "that which is," having received a form of being, that is to say, by receiving the very act of being, "is," and consists, that is to say, subsists in itself.
In this text Aquinas further explains how he understands the notion of actus essendi. One cannot speak meaningfully about the 'act of being,' without referring to a particular subject in which 'act of being' is instantiated, because the 'act of being' is a metaphysical principle.
'Actus essendi' is the metaphysical principle that goes 'side by side' with the metaphysical principle 'essence' in a subsistent extramental thing. And metaphysical principles as such do not subsist by themselves in isolation.
In the real world 'essence' and 'actus essendi' are inseparable. The metaphysical principle of 'actus essendi' always appears instantiated in an 'essence.' And the 'essence' of the thing is what put limits to the thing's participation in 'actus essendi.
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.
Text XI: Quaestiones Quodlibetales, 9, 4, 1, c
Sed quia substantia Angeli non est suum esse -- hoc enim soli Deo competit, cui esse debetur ex seipso, et non ab alio --; invenimus in Angelo et substantiam sive quidditatem eius, quae subsistit, et esse eius, quo subsistit, quo scilicet actu essendi dicitur esse, sicut actu currendi dicimur currere. Et sic dicimus Angelum esse compositum ex quo est et quod est, vel secundum verbum Boetii ex esse et quod est. Et quia ipsa substantia Angeli in se considerata est in potentia ad esse, cum habeat esse ab alio, et ipsum esse sit actus; ideo est in eo compositio actus et potentiae; et sic posset in eo concedi materia et forma, si omnis actus debeat dici forma, et omnis potentia materia. Sed hoc non competit in proposito; quia esse non est actus qui sit pars essentiae, sicut forma; ipsa quidditas Angeli vel substantia est per se subsistens, quod materiae non competit.
This segment addresses the issue of the metaphysical identity of the substance of an angel. Two reference points are highlighted, namely, (a) the Aristotelian composition of primary matter and substantial form against (b) Aquinas’ distinction between the metaphysical principles of 'essence' and actus essendi.
With respect to the Aristotelian composition of primary matter and substantial form, the substance of an angel is not at all the result of two components coming together to generate a composite.
The substance of an angel is a substantial form that does not need primary matter to subsist. The substance of an angel is a substantial form that subsists by itself.
In the material world, on the other hand, a substantial form does not exist by itself. In the material world, a substantial form exists in the composite.
With respect to the distinction of 'essence' and actus essendi, the substance of an angel is called 'potency' in the most radical way: ipsa substantia Angeli in se considerata est in potentia ad esse, cum habeat esse ab alio. In other words, in Aquinas' metaphysics, angels can be thought of as not existing.
Of the two meanings of esse, in this context, Aquinas evidently is not referring to the truth of a proposition. The structure of the text unmistakably forces the term "esse" to mean actus essendi.
Here Aquinas departs from Aristotle. For Aquinas, a substantial form is a 'potency' with respect to the actus essendi.
Regardless of whether they are substantial or accidental, and of whether they belong to angels or to material things, all forms are 'potency' with respect to the actus essendi.
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.
Text IX: Summa Theologiae, I, 3, 4, ad 2
Ad secundum dicendum quod esse dupliciter dicitur, uno modo, significat actum essendi; alio modo, significat compositionem propositionis, quam anima adinvenit coniungens praedicatum subiecto. Primo igitur modo accipiendo esse, non possumus scire esse Dei, sicut nec eius essentiam, sed solum secundo modo. Scimus enim quod haec propositio quam formamus de Deo, cum dicimus ‘Deus est,’ vera est. Et hoc scimus ex eius effectibus, ut supra dictum est.
"The Latin verb ‘esse’ can mean either of two things. It may mean the ‘act of being,’ or it may mean the composition of a proposition effected by the mind in joining a predicate to a subject. Taking ‘esse’ in the first sense, we cannot understand God’s ‘esse’ nor His ‘essence;’ but only in the second sense. We know that this proposition which we form about God when we say ‘God is,’ is true; and this we know from His effects, as said above in Question 2, Article 2."
Reading the works of Aquinas one finds that he used the Latin verb esse to signify in more than one way. In his Summa Theologiae (I, 3, 4, ad 2,) he is clear on this. Thus he writes,
It must be said that esse applies to a thing in two ways. In one way, it means the act of being, actus essendi. In another way, it means the composition of a proposition effected by the mind in joining a predicate to a subject.
In the first sense God’s esse is His actus essendi; in the second sense, esse applied to God means ‘God exists.’
By means of demonstration and reasoning one can prove the ‘existence’ of a thing without having to have recourse to the sense experience of an existing exemplifying individual. The grasping of the ‘act of being’ of a particular thing is indeed the strongest evidence that the thing exists, but the knowledge of the ‘existence’ of a particular thing and the grasping of its ‘act of being’ are entirely different issues. The grasping of the ‘act of being’ requires direct and immediate contact with individual, real sensible things. On the other hand, to answer the question of whether or not a thing exists, one does not have to interact directly with existing sensible things.
See also (a) This Journal, Entry 0048; and (b) Stephen L. Brock, "Thomas Aquinas and 'What Actually Exists,'" Wisdom Apprentice, P.A. Kwasniewski, Ed., The Catholic University of America Press, Washington, D.C., 2007, pp 13-39.
Text VIII: De Veritate, 10, 8, ad 13
Ad decimumtertium dicendum, quod intellectiva potentia est forma ipsius animae quantum ad actum essendi, eo quod habet esse in anima, sicut proprietas in subiecto; sed quantum ad actum intelligendi nihil prohibet esse e converso.
"The intellective power is a form of the soul with reference to its ‘act of being,’ for it exists in the soul as a property in a subject. But there is nothing to prevent the opposite of this, from being true with reference to the act of understanding."
In this text Aquinas makes use of the principle of metaphysical priority. The application of metaphysical priority to the notion of ‘act’ results in the following gradation of acts:
Pure Act (God)
Actus Essendi (the metaphysical principle that goes with ‘essence’)
Substantial Form (which exists in both spiritual and material beings)
Accidental Form (like the intelligence of men and angels)
Activity of Accidental Forms (like reasoning in man)
Products of Certain Activities of Accidental Forms (like a conclusion reached after a process of reasoning)
In the text, Aquinas mentions four acts: 1) the actus essendi, 2) the soul--a substantial form, 3) the faculty of the intellect--an accidental form, and 4) the act of understanding--an activity of an accidental form.
Now, from the perspective of the actus essendi, it makes sense to say that the intellectual faculty of man inheres in the soul because the metaphysical principle of actus essendi refers to a self-subsisting individual that is actually existing here and now.
De Veritate, 10, 8, ad 13, expresses this as follows: "The intellective power is a form of the soul with reference to its ‘act of being,’ for it exists in the soul as a property in a subject." (Intellectiva potentia est forma ipsius animae quantum ad actum essendi, eo quod habet esse in anima, sicut proprietas in subiecto.)
But with respect to the activity of the intellectual faculty, the soul could be made the content of our thinking. The intellectual faculty of man can direct its activity towards getting information about our soul. In this sense, the soul informs, ‘gives form,’ to our act of understanding.
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.
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.
Text V: De Veritate, 1, 1, c
Probat etiam Philosophus in III Metaphys., quod ens non potest esse genus. Sed secundum hoc aliqua dicuntur addere super ens, in quantum exprimunt modum ipsius entis qui nomine entis non exprimitur.
Quod contingit uno modo ut modus expressus sit modus generalis consequens omne ens in se.
Et hoc affirmative:
Non autem invenitur aliquid affirmative dictum absolute quod possit accipi in omni ente, nisi essentia eius, secundum quam esse dicitur; et sic imponitur hoc nomen res, quod in hoc differt ab ente, secundum Avicennam in principio Metaphys., quod ens sumitur ab actu essendi, sed nomen rei exprimit quidditatem vel essentiam entis.
The underlying principle of this doctrine is that, in the real world, one cannot have one of these two metaphysical principles existing without the other being present. In the real world essences exist with the actus essendi, and vice versa, the actus essendi always appears instantiated in an essence. For this reason, the term res (thing) expresses a transcendental notion; it 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.
Thus, 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.
Text IV: In III Sent., 11, 1, 2, ad 2
The text addresses the following argument:1) De quocumque praedicatur inferius, et superius.
(2) Sed creatura est superius ad hominem.
(3) Ergo cum homo praedicetur de Christo, creatura de ipso praedicabitur.And here is how Aquinas responds:
Creatura non est superius ad hominem: quia creatio magis respicit esse quam naturam. Esse autem non est genus, nec inducitur in significatione alicujus generis, ut dicit Avicenna, cum ea quae sunt in uno genere, non conveniant in uno esse, sed in natura communi. Vel dicendum, quod creatura non est superius ad hominem, significans quid est homo: quia creatio non respicit naturam vel essentiam, nisi mediante actu essendi; qui est primus terminus creationis.
Humana autem natura in Christo non habet aliud esse perfectum quod est esse hypostasis, quam esse divinae personae; et ideo, simpliciter loquendo, creatura dici non potest: quia intelligeretur quod esse perfectum hypostasis Christi per creationem esset acquisitum.
Commentary on Text IV: In III Sent., 11, 1, 2, ad 2
In this text Aquinas addresses the issues that arise when one treats the terms “creature” and “man” as if they were concepts coming form the same kind of intellection.
The initial assumption is that the concept “man” is included within the category of “creatures.” But, if this were the case then, from the affirmation “Christ is man,” one would have to conclude “Christ is a creature.”
The response begins with a flat negation of the initial assumption. Simply expressed, the term “creature” cannot be conceived as a more general and wider category than the category of “man.”
The supporting argument is drawn from the two metaphysical principles “essence” and actus essendi.
Both the term “creature” and the term “creation,” says Aquinas, are rooted on the metaphysical principle actus essendi; not on the metaphysical principle of “essence.” Due to this connection then, the term “creature” does not have the properties of a concept derived from ordinary abstraction.
Christ is a man but Christ is not a creature. In Christ, the uncreated Divine Actus Essendi takes on a human nature and it is this Divine Actus Essendi what makes Christ a real existing human being. The Most Holy Humanity of Christ can be said to be a “creature” only as part of Christ in the uncreated Divine Actus Essendi. Compared to the human nature of ordinary men, the human nature of Christ is not instantiated on account of a substantial human actus essendi.
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.
Commentary on Text III: In I Sent., 8, 5, 2, c
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.
Text II: In I Sent., 8, 4, 2, ad 2
Hoc quod dico, ‘ens non est in subjecto,’ non dicit aliquod genus: quia in quolibet genere oportet significare quidditatem aliquam, ut dictum est, de cujus intellectu non est esse. Ens autem non dicit quidditatem, sed solum actum essendi, cum sit principium ipsum; et ideo non sequitur: ‘est non in subjecto:’ ergo ‘est in genere substantiae.’
Commentary on Text II: In I Sent., 8, 4, 2, ad 2
In this text Aquinas analyzes the expression ens non est in subjecto, to say that it refers to ‘beings’ which exist by themselves, that is to say, ‘beings’ which do not depend on another ‘being’ to subsist. Ens non est in subjecto means “a being that subsists in itself and not in another subject.”
As such, the meaning of ens does not include other ways of ‘being’ in which the reality signified is meant to exist as inhering in another subject. The latter meaning of ens applies to realities that fall within the Aristotelian categories of accidents; the former, to realities that fall within the category of substance.
The text is meant to show that from ens non est in subjecto one cannot conclude, therefore est in genere substantiae. In other words, from “an existing thing that does not need another subject on which to inhere” one cannot conclude that “it falls within the category of substance.”
To establish this Aquinas makes use of the two fundamental metaphysical principles of ‘essence’ and actus essendi.
Although properly speaking ‘substance’ cannot be defined, Aquinas proceeds as if one were able to define substance. When trying to explain what ‘substance’ is, Aquinas says that he is providing only a quasi definitio of substance. ‘Substance,’ as one of the supreme categories, cannot be defined by indicating its genus and specific difference.
In the present text, substance is first considered from the side of the metaphysical principle of 'essence' and then from the side of the metaphysical principle of actus essendi.
Thus, in trying to define substance from the side of the metaphysical principle ‘essence,’ Aquinas reasons that in this approach the emphasis falls on “that which has a quiddity” regardless of whether or not that quiddity actually exists in individual things.
From the side of the ‘essence,’ therefore, substance is conceived as “that which has a quiddity, a quiddity meant to be instantiated in something that does not inhere in another subject.” But the actual existence of individual things under that quiddity is irrelevant to conceive an answer to what substance is in this way.
Whereas, in trying to define substance from the side of the actus essendi, Aquinas reasons that in this other approach the emphasis falls on the actual existence of an individual thing.
Substance is still regarded as “that which has a quiddity” but now substance is more narrowly conceived.
From the side of the actus essendi, substance is ens, substance is something actually existing, it is an individual existing thing “which subsists in itself and not in another subject,” regardless of what exactly that thing is.
A key point is this. It is not on account of the actus essendi that individual things fall within a genus. The singular things contained in a genus are different in as much as one considers them from the side of their actus essendi. Contrariwise, the singular things contained in a genus are similar when considered from the side of their quiddity.
The metaphysical rigor of the philosophy of being makes known to us that the quiddity of a material thing is not its actus essendi. Thus quiddities can be conceived without making reference to actus essendi.
Now the expression ens non est in subjecto, as indicated, does not signify substance as “that which has a quiddity” because the expression ens non est in subjecto comes from the side of the metaphysical principle of the actus essendi, not from the side of ‘essence.’
And the fact is that the expression ens non est in subjecto include some things which are not substance at all, as is the case of the sacramental species remaining without a subject after the words of the consecration during holy Mass.
Therefore when going from ens non est in subjecto to ergo est in genere substantiae, Aquinas concludes with a non sequitur.
Cum in omni quod est sit considerare quidditatem suam, per quam subsistit in natura determinata, et esse suum, per quod dicitur de eo quod est in actu, hoc nomen res imponitur rei a quidditate sua, secundum Avicennam, hoc nomen qui est vel ens imponitur ab ipso actu essendi. Cum autem ita sit quod in qualibet re creata essentia sua differat a suo esse, res illa proprie denominatur a quidditate sua, et non ab actu essendi, sicut homo ab humanitate. In Deo autem ipsum esse suum est sua quidditas: et ideo nomen quod sumitur ab esse, proprie nominat ipsum, et est proprium nomen ejus: sicut proprium nomen hominis quod sumitur a quidditate sua.
Commentary on Text I: In I Sent., 8, 1, 1, c
In the text Aquinas addresses the issue of whether or not Qui est (He Who Is) is the proper name of God. The text highlights the familiar contrast between the metaphysical principle of actus essendi and the metaphysical principle of ‘essence.’ Here Aquinas begins with the things of nature where the 'essence' of the thing limits the actus essendi.
Accordingly, it is not on the basis of the actus essendi of the things of nature that we say what a thing is. It is the metaphysical principle of ‘essence’ what allows us to say what a thing of nature is.
God, however, is the only one in whom there is just one metaphysical principle, the actus essendi, and this metaphysical principle is his ‘essence.’
As explained earlier, the Latin est has more than one meaning, and when applied to the things of nature, if there is direct and immediate contact with an individual real sensible thing, est signifies the actus essendi, not the ‘essence’ of the thing.
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.")