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 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.’
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.
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.")