Aquinas teaches that every substance is on account of its operation (Summa contra Gentiles, I, ch. 46). The operation proper to a thing is the end thereof: for it is its second perfection (Summa contra Gentiles, III, ch. 25). The finite substance, with its act of being measured by its correlative essence, is ordered to an ulterior perfection attainable through operari. The metaphysical source of this inclination is found in the expansiveness of act, which per se tends to communicate itself:
It is in the nature of every act to communicate itself as far as possible. Wherefore every agent acts forasmuch as it is in act: while to act is nothing else than to communicate as far as possible that whereby the agent is in act (De potentia, q.
2, a. 1).
Every ens, therefore, is operative in proportion to the degree of its actuality. In the finite ens, the act of being first actuates the potentia essendi, defined by the essence; once this occurs it can then develop into operative energy.
The ordo ad operationem of finite ens contains a paradox: 1) on the one hand, this order reveals the generosity of the act of being which expands in the proper accidents and in the operations of the thing; 2) on the other, this order manifests the indigence of the being-in-act of the substance, which requires its own operative perfectioning.
Aquinas sees in the act of being the source of all the levels of actuality of the suppositum, and in the essence the potential determination that structurally constitutes esse as the esse of this thing. Ens, actuated by esse, is operans since it implies an ordo ad operationem. The theoretical steps that justify this necessity are the following:
 The resolutio of created ens flows into the couplet of essence, originary potentia essendi, and esse, the emergent act of being.
 Actuated by esse, the essence acquires a formal actuality which is able to be described as the primary content of ens.
 But the emergent act of being transcends, by its nature, its primary content.
 Consequently, the act of being tends to diffuse itself in ens beyond the essence in act, to the measure or degree consented by the essence.
This theoretically grounds the principle: “quamlibet formam sequitur aliqua inclinatio”. The principle would be unintelligible if esse exhausted its actuality in making the essence be; on the contrary, it becomes enlightening when the form or substantial essence is founded on an act that surpasses it.
On the one hand, all the perfections of an ens stem from esse. The being of operation, and the goodness that results from the being of the operation, proceed from the founding act of being. On the other hand, the being of operation is added (superadditum) to the constitutive being of the substantial essence. Therefore, it seems that the ontological actuality of operating is simultaneously under and over that of substantial being. It is, at the same time, a participant of substantial being and added to it.
The problem can be solved by distinguishing between esse ut actus and esse in actu. When we consider esse and essence as principles of this ens, namely, “before” ens, they correspond to one another as act and potency. When, however, we consider them “after” and “within” the real ens, it is clear that the essence is an essence-in-act in the sense that it is really and that that which in this way is, is according to that substantial quiddity. This essence-in-act is, therefore, in act by the act of being, but its being-in-act does not coincide with its act of being.
Esse ut actus – or the act of being – is that by which the substance is, and that which it has in se and not in another. Esse in actu – or the fact of being – is that which the substance is actually, and that functions as a subject of esse ut actus. For this reason, substantial esse can be understood in two senses: either in the dimension of intensive and constitutive act of being, or in the dimension of the being-in-act of the essence. In the first, esse is one and unique insofar as it is esse, and this is referred expressly to the firs substance or suppositum. In the second, esse (in actu) is unique only insofar as it is substantial-essential, but not insofar as it is esse, since it is placed in confrontation with accidental esse. Therefore, there is only one esse ut actus in the created suppositum, while there are diverse beings in act (esse in actu), that of the substance, those of the accidental forms, and those of the operations.
With respect to the act of being, the being (esse superadditum) of the accident and of the operation do not add any new esse ut actus to the suppositum, but rather are an ultimate expansion of the esse ut actus by means of the substance, consequently they are a new esse in actu, such that the perfection added is to that to which they are added as participant to participated. There is nothing more formal in a thing that intensive esse. On the other hand, the accidental form and the transitive or immanent activity refer to the esse in actu of the essence or substantial form as a supplement of ontological perfection, in such a way that the accident or operation increment the actuality of the substance: here, substance is to all that to which it is subject as a potency to act. Within this speculative framework, the two implications of accidental forms can be reconciled: that they come from the substance and that they inhere in it:
Actuality is observed in the subject of the accidental form prior to its being observed in the accidental form; wherefore the actuality of the accidental form is caused by the actuality of the subject. So the subject, forasmuch as it is in potentiality, is receptive of the accidental form: but forasmuch as it is in act, it produces it. […] it has already been said that the accident is caused by the subject according as it is actual, and is received into it according as it is in potentiality (I, q.
77, a. 6).
Because the subject of the proper accidents is simultaneously in act and in potency with respect to them, it should be so according to really different aspects. Thanks to the distinction made between esse ut actus and esse in actu, we understand that the substance, and more precisely the substantial form, has at its disposition a potential of actuality – because of its originary esse ut actus – that allows it to produce its accidents, while the same substance, if considered according to the reality that belongs to its esse in actu, is in receptive potency to those same accidents.
If operation shares with the other accidents the characteristic of adding an ens secundum quid to the ens simpliciter of the substance in act, it stills has the need of proceeding from its own operative potency, due to the due proportion between operative act and the principle from which it immediately comes from: omne agens agit sibi simile. Thus, the action of a created substance is always mediated by a proportioned active potency. The position of the created suppositum institutes from within a succession of relationships of potency and act: substance, in act with regard to its essence, is in potency to its operative potencies, which once actuated like accidental forms, are in potency to their operations.
The intensive act of being actuates the essence, then, by means of this, the accidental forms, as well as, by means of the operative potencies, the operations, such that this is participated by ens by means of these mediations in formally different modes, but teleologically directed to the ultimate perfection in act. Beginning with esse ut actus, delimited by the correlative essence, one descends along the chain of successive participates, in such a way that each participant is then participated by the following ontological moment. This manifests the emergence of esse over the different levels of ens that it founds. At the same time, the descent becomes, from the esse in actu of the substantial essence, an ascent to the qualitative and operative complement of the entire suppositum.
A term that can help understand the function of actus essendi in relation to the other principles of ens is that of virtus essendi, which refers to the actuating capacity enclosed in being itself. The virtus essendi is correlative to potentia essendi, to the capacity of being constitutive of the essence and delimited by the form. In the ens per participationem, the diffusion of the originary actus essendi is “filtered”, so to speak, by the substantial form, such that it founds two levels of being-in-act, that of the form or essence, with the proper accidents that flow from this, then that of operari, thanks to which the finite ens comes to its ultimate perfection. The virtus essendi will be converted successively, in the finite substance into virtus operandi, proportioned to the ontological density of the essence and destined to bring it to its ultimate end.
Hence, there are different levels of actuality in the created suppositum. The dynamic of participation that we want to analyze can be considered starting from both constitutive principles of ens. In the “descending” line (far-right column below), with respect to foundational esse itself, the successive participated “acts” are: the being-in-act of the substance, the being-in-act of the accidental forms, and the operating-in-act of the operative faculties; in the ascending line (second column), with respect to the essence as transcendental principle, the participants are: the essence of the substance as potency of being (in correlation to the being-in-act of the substance), the essence of the substance as limited being-in-act (in correlation to the being-in-act of the accidents), and the suppositum constituted in “first act” by the being-in-act of the essence and the superadded being-in-act of the accidental forms (in correlation to the being-in-act of operari). The intermediate levels of actuality depend on the emergent actus essendi, while the intermediate levels of potentiality refer to the originary potentia essendi.
Transcendental principles that constitute an ens per participationem
Potentia essendi: essentia
Actus essendi: esse ut actus
Levels successively constituted by participation in esse
esse in actu
insofar as it is the being-in-act of the substance
as formal and limited ens in actu
insofar as it is the being-in-act of the accidents
Suppositum in first act
by the being-in-act
of the substance and of the accidents
insofar as it is the second being-in-act
of the entire suppositum
The analogy of proportionality between participant and participated is the following: the substantial essence is to its being in act (esse substantiale) as the accidental form to its accidental being (esse accidentale), and as the operative power is to its operari. The esse ut actus and the potential essence tend, actively and passively, to the esse in actu of the substance, of its properties, of its operations. Thus, the real composition instills a teleology in ens, which directs it from within to its end. Action, on the one hand, perfects ens and action, on the other, is demanded by the esse specified by the essence as its ultimate expansion. Therefore, if, in a sense, the operari is the end of esse, insofar as the act of being tends ultimately, in finite ens to the act of operari, it is also true, in another sense, that esse is the end of operari, since the operation has as its scope the actuation of the constitutive virtus essendi of esse itself. Esse is thus the alpha and omega of ens: starting from esse as source of being, the dynamism of created ens, mediated by the essence, returns as to its end when it achieves the maximum actuality of which it is capable.
The act of being, insofar as it is act, transcends in sequence its primary essential determination, then its secondary accidental determinations, in order to expand itself ultimately into operation. In this process, the actuality of being explains why it surpasses the formal contents, while its limitation by the essence makes us understand why its ulterior expansion is of an operative type, in such a way that, in the creature, action is always other than the substance. Thus, the operative difference is anchored twice in the ontological difference: finite ens operates due to its esse and its intrinsic communicability; but the operating of finite ens differs from its substance due to the essence and the limitation that this imposes on the virtus essendi of esse.
Nous remercions vivement le R.P. Jason Mitchell L.C., à l’obligeance duquel nous devons la traduction anglaise du texte que l’on vient de lire.