Entry 0189: The Thirteen Texts in which Aquinas Uses the Expression "Actus Essendi" (XIII)
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 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.