Causal Powers as Accidents: Thomas Aquinas’s View

Abstract

I argue that Thomas Aquinas maintains the view that (created) powers are accidental to their bearers not because powers pertain to bearers with limited essences, but because their bearers have limited actual being. Power tracks not only the essence of something but also its actual existence. Things have powers that are causally relevant when these things exist, that is, the nature of a power is determined by a thing’s essence, but the actual being of the thing of that essence accounts for the limitations of this power and for the extent to which a power can have causal effects.

J’interroge dans ce travail le sens dans lequel Thomas d’Aquin estime que les choses dotées de pouvoirs (créés) possèdent ceux-ci de manière accidentelle. Cette prise de position s’explique parce que le pouvoir retrace l’existence réelle d’une chose en plus de son essence. Les choses ont des pouvoirs pertinents à l’ordre de la causalité lorsque ces choses-là existent : la nature d’un pouvoir est déterminée par l’essence d’une chose, tandis que l’existence même de la chose ayant cette essence explique les limites de ce pouvoir et la mesure dans laquelle une puissance peut être cause d’effets.

Causal powers appear in almost every domain of medieval philosophy. They are present in physics, or natural philosophy, where medieval philosophers talk about substances and qualities as having powers and producing causal effects. In psychology, medieval philosophers speak of the causal powers of the soul. Our soul, according to medieval psychology, has faculties through which it acts. These faculties are conceptualized in terms of causal powers in virtue of which souls can have acts of understanding, volition, perception, etc. Causal powers also appear in ethics. Virtues are dispositional qualities of certain faculties of the soul — namely, the will, the intellect, and the sensitive appetite — and it is these qualities that enable us to act virtuously.

A common understanding of causal powers is that they are features that something has in virtue of what it is. For example, in virtue of being rational creatures, we have the power of intellect (our faculty for forming concepts, judgements, and arguments) and the power of will (our faculty for making choices and for forming intentions). God also has powers in virtue of what He is: He has the power to create because His essence is such that He is omnipotent. Yet, despite the consensus that things have powers in virtue of what they are, some medieval philosophers claim that the causal powers of created beings are not essential features of their bearers,¹ while God’s power is the same as His essence. If these powers are not essential features, they must be accidental. But if things have powers in virtue of what they are, why are the powers of God essential to Him, while the powers of created beings are only accidental to them?

In this paper, I inquire into what Thomas Aquinas means by (created) powers being accidental to their bearers.² I argue that he maintains this view not because powers pertain to bearers with limited essences, but because their bearers have limited actual being. Put differently, I argue that, for Aquinas, power tracks not only the essence of something but also its actual existence. In his view, things have powers that are causally relevant when and because these things exists: the nature of a power is determined by a thing’s essence, but the actual being of the thing of that essence accounts for the limitations of this power, and ultimately for the extent to which a power can have causal effects.

The paper has two parts. In the first part, I investigate one reason that Aquinas might have for thinking that powers are accidental to their bearers, that is, why he must follow Aristotle’s claim that powers are accidents in the category of quality. In the second part, I consider one of Aquinas’s arguments for why the powers of the soul are accidents.

Powers as Qualities

One might think that there is an easy answer to why Aquinas maintains that powers are accidental to their bearers: he is following Aristotle’s view. In Categories 8 (9a14–28), Aristotle mentions powers, together with habits, among the species of quality. Probably the reason Aristotle assigns powers to this category is that things that have powers to cause other things to burn, purge, break, etc., are things that are qualified in a certain way. For example, a knife has the power to cut because it is sharp, a fire has the power to heat because it is hot, a stone has the power to break glass because it is heavy, etc. Since ascriptions of causal powers refer to things that are in a certain way, they refer to some of these things’ accidents, namely, their qualities.

Although Aquinas agrees with Aristotle that at least some powers are in the category of quality, he does not deduce from this that powers are accidental. In fact, the order of explanation is different:

[Text 1] If accident is taken in contradistinction to substance, then there can be no half-way between them since one is the negation of the other. One presupposes a subject of inherence and the other does not. And thus, since a power of the soul is not its essence it has to be an accident, and in fact it belongs to the second subdivision of quality. But if we take the word ‘accident’ the way we do when we speak of the five types of predicable, then there is something half-way between substance and accident. Substance in this case includes everything essential to a thing, but not everything non-essential comes, in this way of speaking, to be described as accidental, but solely what does not follow necessarily from the thing’s nature. For what is said to be proper to a thing is not of its essence yet follows necessarily from its essence, putting it, as predicable, midway between essence and accident and in this sense the powers of the soul can be said to be midway between substance and accident, natural properties of the soul, as it were.³

In the background of this text lies an important metaphysical distinction. Some features constitute what something is — they are that thing’s essential features. For Aquinas, the powers of the soul, such as the intellect and the will, are not essential features of the soul.⁴ Any feature of a thing that is not one of its essential features is an accidental feature. Because the powers of the soul are not essential parts of the soul, they are accidental features of it. This seems wrong, for the will and the intellect seem not to be accidental features of a human soul: wouldn’t medieval philosophers think that a human being is rational precisely because she has an intellect and a will? In fact, the term ‘accidental’ has two meanings. First, it can refer to features that a thing can lose. Consider, for example, a pot that becomes hot when put on the stove; when it is removed from the stove, it slowly becomes colder. In scholastic parlance, the quality of heat is accidental to the pot, because the pot can lose this quality without suffering any other change, thus remaining what it is, namely, a pot. The powers of the soul are not accidental in this first sense, for a rational being such as a human, if it were deprived of intellect and will, would cease to qualify as a rational being. In its second sense, ‘accidental’ refers to features that necessarily pertain to something: they always follow from the essential features of a thing. For example, heat is a necessary accident of fire because being hot always follows from what fire is. These accidental features are also called proper accidents: because they follow from what a thing is, they are the proper features of that thing. In contrast to simple accidents (accidents that fall under the first meaning of ‘accidental’), proper accidents cannot be lost unless the essence that has them is also destroyed.

In Text 1, Aquinas takes the powers of the soul to be proper accidents of the soul. They are features that any being that has a rational soul has necessarily, but they are not essential features of the soul. Because proper accidents remain ontologically accidents, the powers of the soul will be accidents. Aquinas says that the powers of the soul fall in the category of quality: any power of a created being is a quality. However, in Text 1, Aquinas does not give any argument for why powers are qualities — to my knowledge, he never explains why (created) powers are qualities.

One might think that powers must be qualities of their bearers because of how Aquinas understands the causal contribution of a power to an action. According to Aquinas’s account of how powers causally contribute to an action, they must be qualities, and so accidental. So, let us focus on his view of the causal contribution of powers. For Aquinas, something can exist in virtue of the essential features (substantial form) it has, yet this thing acts through its powers.⁵ Indeed, powers are that through which something acts: powers are instruments or quasi-instruments in virtue of substantial forms.⁶ But what does it mean for something to act through a power? ‘To act’ has different meanings for Aquinas.⁷ It can mean that something acts as an efficient cause, that is, in the manner of something bringing about an effect; for example, a builder is the efficient cause of a house because she brings about the house. To act also means to act as a final cause, that is, in the manner of something that provides an aim for something else to act as an efficient cause; for example, health is that for the sake of which some people go to the gym. Finally, ‘to act’ also means to act as a formal cause, that is, in the manner of something that makes something else be a certain kind of thing; for example, the shape of a cup makes something a cup. In several texts, Aquinas says that a power acts as a formal cause when it formally causes the action of the principal agent, which is the efficient cause of the action. For example, heat⁸ is said to be the formal cause of the action of heating,⁹ whereas it is fire that produces or brings about heat by the power of heat. The view that causal powers act formally is expressed in Aquinas’s claim that “what the agent does effectively is what the form does formally.”¹⁰

What does it mean for a power to cause an action formally? Consider a case in which a fire causes heat in a haystack; the fire itself has heat, an active quality and a power that enables fire to have a specific action, that of heating. The form of heat not only allows fire to cause heat in something else but also accounts for how fire causes heat in that thing, and thus for the specific way in which fire acts; this is so because usually agents produce something similar to themselves.¹¹ Given this, a power, being also a form, is the formal cause of an action because it accounts for the kind of action the bearer of the power performs. But since what the agent does efficiently the form does formally, the causal contribution of powers is in the framework of formal causality, while that of the whole composite is in the framework of efficient causality. Thus, there seems to be only one proper agent, the whole thing, while the powers themselves do not act efficiently, but only formally; that is, they account for how the action of the agent is done. This explains why powers have their own action, as Aquinas sometimes says:¹² the whole agent acts, but because its powers account for the manner of its action, each specific action can be traced back to a power of the agent. For example, fire’s action of heating can be traced back to heat, but not its action of illuminating.

In Aquinas’s view, powers have the role of specifying the manner of an action or operation; thus, it makes sense to think of powers as qualities. The causing of an effect is attributed to the whole subsistent thing as the agent, but the specific manner of this causation is attributed to the agent’s active and passive qualities. For example, when fire heats a pot, we say that in this case of alteration, the causation of heat in the pot was done by fire by means of heating. But the action of heating is attributed to fire only insofar as it has heat, and not, for example, because it can illuminate; that is, it is attributed to fire insofar as it has a certain quality.

Yet, this view about the causal contribution of powers does not fully explain why Aquinas thought that powers must be accidental to their bearers in the first place. One might think that one can trace a specific way of acting to an agent’s power, even when the agent’s powers are not accidental to it, but essential features of it. For example, one might say that the whole soul acts, whereas the powers refer only to how the soul is related to specific operations.¹³ According to this view, which was defended by William of Auvergne before Aquinas, and by Peter John Olivi after him, the soul can act through its own essence — in other words, the powers are included in this essence. These authors have a strong reason to maintain this view: in God, His power is His essence and the soul is made in the image of God. So, Aquinas needs to explain why we must accept his view, which seems needlessly complicated.

Powers as Accidents

Why does Aquinas hold that the powers of created things are accidental to their bearers? Why is it impossible for the powers of the soul to be identical to the soul’s essence? One cannot answer these questions simply by stating that any power is accidental to its bearer. Medieval philosophers, including Aquinas, agree that powers can be the same as the essence of their bearer. This is evident in God, in whom the powers are the same as the divine essence. It also happens in the case of a substance’s passive power to receive its first accidents (its necessary proper accidents) — this power is the same as the substance.¹⁴

It is precisely the case of God’s power that divides medieval philosophers. Some of them, such as William of Auvergne and Peter John Olivi, take this case to be paradigmatic, at least for immaterial substances. Others are worried that, by positing that the powers of created beings are the same as the essences of those beings, an important difference between God and creatures is overlooked. The early modern scholastic Francisco Suárez raises precisely this point in his discussion about how the powers of the soul are accidental to the soul:

[Text 2] To be able to immediately cause all its activities through its substance seems to be repugnant to the limitation and imperfection of a creature.¹⁵

Suárez links the accidentality of causal powers with the imperfection and limitation of a creature, and as I will show, Aquinas does the same. But a creature is limited in various ways; so, which imperfect aspect of a creature is the one that grounds the view that powers are accidental to it?

An answer to this question is suggested by Aquinas in Summa Theologiae I, q. 79, art. 1:

[Text 3] A thing’s essence is the immediate source of its activity only when its activity is its being. For as any power (potentia) is related to its activity (operatio) as to its actuation (suum actum), so is essence related to being. Now in God alone is understanding the same as being. In creatures with intelligence, the understanding is a power of the one that understands.¹⁶

Aquinas’s point is that, because the operation or the act of a power is not the same as the being of a thing, the power for that operation cannot be the same as the essence of that thing. The argument suggests that the powers of the soul must be accidental to the essence of the soul because the operations of the soul are accidental to it.

The argument has the following form:

1. (A) A thing’s essence is its power only when the operation of its power is its being.

2. (B) Any power (potentia) relates to its activity (operatio) as its actuation (suum actum).

3. (C) Any essence relates to its being as its actuation.

4. (D) In creatures, the operation of a power is not the same as its being. (This premise is not stated in the text, but it is presupposed given that only in God is the operation of the power the same as His being.)

Therefore, in creatures, the power is not the same as the essence, so the power is accidental.

In this text, Aquinas does not bother to explain any of the premises, probably because he thinks that they are explained in other parts of his Summa Theologiae. So, let us see what explanation we can find for these premises.

Let us start with premise (C), which relies on Aquinas’s famous discussion of being and essence. According to premise (C), the being of an essence is its actuation. This means that no nature or form actually exists unless it has being. In Aquinas’s metaphysics, only God has being through Himself; other natures or forms or essences have being insofar as they get it from God. But note that there is another side to the actualization of forms or essences: when forms are actualized, the being something receives is determined or limited.¹⁷ Creatures of a certain kind get as much being as their kind (that is, their genus and species) allows them. In the background of Aquinas’s discussion lies an important principle, namely that “an unreceived act is unlimited.”¹⁸ Briefly, the intuition behind this principle is that anything that is limited requires an explanation for its limitation; something that is perfect does not.¹⁹ Being is a perfection and so an act: when it is received, it gets limited to this essence or that essence,²⁰ which by receiving being gets actualized. Actuation thus has two aspects: in actuation, essences get actualized in things, while being gets determined or limited in them. From premise (C) alone, it is unclear which of these two aspects Aquinas thinks is relevant for the argument in Text 3.

Premise (B) needs explanation, especially because of the use of the term ‘potentia.’ In a previous question (ST I, q. 77, art. 1), Aquinas uses an argument similar to the one in Text 3, but instead of ‘potentia’ and ‘operation,’ he uses ‘potentia’ and ‘act’ as divisions of being.²¹ ‘Act’ and ‘potency,’ in the sense of divisions of being, refer to what is actual as opposed to what is in potency. For example, a tree in potency is different from a tree that is actual. Later in the same text, Aquinas seems to equate act and operation, implying that ‘potentia’ in the sense of power means the same as ‘potentia’ in the sense of what is in potency. Given this text from q. 77, premise (B) in q. 79, art. 1 seems to say that power (potentia) relates to operation as potentia relates to its act. However, this is problematic for what is in potency cannot be a power: a power is a feature that is actual, but what is in potency is the opposite of what is actual. Put differently, only actual things can act, and what does not yet exist cannot act. Thus, premise (B) is probably not about power being something in potency.

So, what does Aquinas mean in premise (B)? Recall that premise (B) is related to premise (C) and they must be understood together: “For as any power (potentia) is related to its activity (operatio) as to its actuation (suum actum), so is essence related to being.” As we saw, the actuation of the essence by being is not only about having actual existence, but also about being determined and limited. Thus, if we take ‘actuation’ in premise (B) to refer to the limitation or determination of the power by the operation, then premise (B) makes the following point: power is related to an operation as that which determines or limits the operation in the same way in which essences determine or limit being to this or that nature. This interpretation will also fit Aquinas’s mention of act and potency in q. 77, for what is potential also determines or limits to a certain extent what is actual: a tree in act is the opposite of a tree in potency, and not, for example, of a flower in potency.

The same aspect of actuation, as determination or limitation, appears in Aquinas’s discussion of premise (D). He argues for premise (D) in Summa Theologiae I, q. 54, art. 2. His argument starts from the distinction between transeunt actions, that is, actions that pass into another thing, and immanent actions, or actions that remain in the agent. In transeunt actions, for example, in the case of a fire burning a haystack, the action (burning) is not the being of the agent (fire), for the action happens in the patient (the haystack).²² But neither is an immanent action such as understanding or willing the same as the being of a human, for example. This is so because an operation, according to its account (ratio), is infinite in scope: for example, any being can fall under the scope of an act of understanding or willing, that is, any being can be understood or desired. Yet, the being of creatures is always finite, limited to a species or a genus, so creatures will never be able to exhaust the scope of an operation. For example, human beings cannot have an infinite operation of understanding, one that comprises everything that can be understood. It is unclear what aspect of our being is responsible for this. Aquinas’s reference to species and objects of operations suggests that human beings cannot exhaust the scope of an operation because human understanding depends on sensorial input, which is always limited to existing in a certain space and time. Yet, when Aquinas concludes the argument, he says that an operation is not the being of a thing, when that being is limited to a species or genus.²³ This suggests that the problem is not with the actual existence of an individual of a certain species, but at the level of the species or kind to which the individual belongs. For example, human understanding, in contrast to angelic understanding, is dependent on having a body; generally, having a body puts constraints on how much sensorial input human beings can receive.²⁴

Aquinas’s argument seems counterintuitive. While one might agree that any being with a limited existence is incapable of attaining the whole perfection that is possible for an operation, it is unclear why it follows from this that an instance of an operation is not the same as the being of a certain limited thing. Aquinas’s point goes against what one would expect, namely, that an operation such as understanding is not the existence of a human being, since there is more to a human being’s existence than her act of understanding. Instead, Aquinas insists that, according to its account, the operation of understanding has an infinite scope, while the human being is limited to a certain kind. To understand his intuition, recall the principle that “an unreceived act is unlimited.”²⁵ An operation is an act, and so a perfection. Thus, the operation of understanding, when unreceived, that is, when uninstantiated, is perfect. But when the operation is instantiated in a human being, it becomes limited. Since it becomes limited only at the moment of its instantiation, and since, according to its account, the operation remains infinite in scope, the operation can never be the same as the being of its receiver: there is always more to the operation than what is instantiated.

Let us pause to take stock. Premises (B), (C), and (D) deal with how something that is a perfection (operation or being) gets limited by that in which it is received. If my interpretation up to this point is correct, then premise (C) is about how essences limit being to this or that nature; premise (B) is about how power limits operation to this or that power; and premise (D) is about how, because the actual being of creatures is limited to a genus or a species, their operations are limited. By transitivity, it appears that essence is what limits the operation, because essence is what limits being, and actual being is what limits operation. But if this is all there is to Aquinas’s argument, why didn’t he say it from beginning? Moreover, what is the role of powers in this argument?

A text from Aquinas’s Quodlibet III attempts to explain the connection between essence, power, and being, and nuances the view that essence is the ultimate cause for why an operation is limited in creatures:

[Text 4] It must be said that the active power of anything must be assessed according to its essence, so that something acts insofar as it is a being in act. For if in something a certain form or nature not limited and not contracted were to be found, its power would be extended to all acts and effects that are fitting for that nature; for example, if heat were understood as subsisting per se, or in some subject that receives it according to all its power, it would follow that it would have the power to produce all acts and effects of heat. However, if a subject did not receive heat according to all its power, but with a certain contraction and limitation, it would not have an active power with respect to all acts and effects of heat.²⁶

It might seem that, when it comes to gauging the powers of a thing, it is sufficient to consider its essence: human beings have intellectual capacities precisely because they are essentially rational beings. Yet, Aquinas immediately qualifies this claim by pointing out that in fact we must look to how something actually exists; that is, to gauge the powers of a thing we must consider not its essence in itself, but its essence insofar as it is actualized. He explains why we must do this. Consider heat: if this form were to exist by itself, not received in anything and not limited by anything, it would be able to bring about all the actions and effects that are suitable for such a form. Of course, if it were to exist in this way, it would still be a limited form, but it would be perfect insofar as its power and operation are concerned. Even if heat existed in a subject, if that subject could receive all the effects and actions, the power and operation of heat (or the heat-having subject) would extend to all the effects that are suitable for heat. But this is a counterfactual situation, for heat is received in something, i.e., in a subject. Once heat is received in a subject, this power is contracted and limited, extending only to certain objects, because the subject in which heat inheres gives to the quality of heat only a limited being. This is so because Aquinas follows the principle that, when A is received in B, it is received according to the capacities of B. For example, when water is poured into a vase, the vase will take only as much water as it is able to hold. Thus, when heat is received in a subject, the subject will receive heat according to the subject’s capacities, including to the subject’s own limited being.

Given Text 3 and Text 4, Aquinas’s point is that because an essence receives being, and because the essence limits this being, the powers and the operations of this actualized essence are in turn limited. His point is not that essence first limits being and then it limits both the powers and operations of an actualized thing, for it does not follow from how essence limits being that it also limits operation. In fact, once an essence has been actualized, that is, when it exists, it is this actualized being (comprising an essence and limited being) that is responsible for the limitation of the power and its operation, while presumably it is essence alone that is responsible for the kind of powers that an essence has. Thus, it is not the essence alone that powers and operations track, but the essence insofar as it has a limited being.

Let us return to the argument in Text 3 to see how we should understand it. Recall premise (A), which states that an essence is the same as its power when the operation is the same as its being. In the light of Text 4, there are two cases that must be considered. In the case in which a form exists by itself, its being is unlimited: Text 4 tells us that its operations and powers are the same as its essence and being. But this situation is counterfactual, since only God exists by Himself. In the case in which a form is instantiated, it has actual limited being, and the form’s powers and operations are limited too.

Why does Aquinas take the powers to be accidental? Consider the case of the soul. If the essence of the soul were to exist uncontracted and unlimited, it would have powers that could produce all the acts or operations that are proper to a soul. But any soul has only a limited being, and because of this the powers of the soul are limited. Indeed, considering the essence of the soul alone, these powers should be able to bring about all the suitable operations of such a form. Considering the essence of the soul as it exists, however, these powers are limited. Just recall how the power of understanding is limited by sensorial input. But this means that, when the soul exists, the powers of will and intellect are not grounded in the essence alone, but in both the essence of the soul and its being. A similar interpretation can be constructed for the case of accidents: once a composite of a substantial form and matter is informed by an accident, the power of heat is grounded in both the composite’s essence and being, and the accident’s essence. For example, once fire receives heat, it extends its being to it; the being that heat receives limits its power. This power is no longer the power of heat alone, but of the whole composite. Thus, to an important extent, the powers of an actually existing being are accidental to it, for they are dependent not only on its essence but also on its limited being. Recall that, for Aquinas, a feature is accidental when it is outside of the essence, but follows necessarily from it. According to Aquinas, on the interpretation I have proposed, the nature of the powers of an actual thing cannot be explained simply by a thing’s essence; its actual being also must be considered. In fact, actual being is relevant not only to explaining how powers are limited in this world: things have powers only when they actually exist in this world. Recall that the case of something other than God existing by itself is a counterfactual situation.

In short, Aquinas’s point is that ultimately the way something exists explains the limitations of its powers and operations. If we consider a thing by considering only its essence as uninstantiated, that thing has powers and operations that are the same as its essence. But created things have limited acts of existence. While it is the essence of a thing that explains the powers the thing has, it is this thing’s limited act of existence that explains that it now has powers that can actually do something and that these powers are limited. Thus, the actual being of creatures explains why their powers and operations are outside of their essences, and so accidental to them. It is only the infinite act of God’s existence that does not limit His powers (and operations), and therefore in God alone are the powers (and operations) the same as His essence.

Conclusion

The interpretation I have proposed does not explain how the powers of the soul are accidental qualities. Recall that, in Text 1, Aquinas puts powers in the category of quality, without advancing any argument for why this is the case. My interpretation, however, allows one to maintain that there is a distinction in power between creatures and God. In this interpretation, the powers of creatures are accidental to them because the way in which these creatures exist limits not only their operations but also their powers. Only a being such as God who has an infinite way of existing has a power that matches His essence.

I will conclude by briefly mentioning two aspects of my interpretation that make it plausible as an interpretation of Aquinas. First, my interpretation fits with important aspects of Aquinas’s view about actions: actions are attributed to whole things (actiones sunt suppositorum), not to their components. Recall Text 4 on heat: when heat is received in a subject (in the substantial form of fire), the power of heat is limited because of the actual being that fire has. This power is now grounded in the being and essence of heat as it is instantiated in the subject that receives it. But one outcome of this is that we cannot attribute this power to heat alone: the power is not the same as the essence of heat, because heat, in virtue of its essence, has a power that is infinite in scope; however, fire does not receive the essence of heat according to all its power, for the substantial form of fire as instantiated has only limited actual being. The power cannot be ascribed to the essence of fire, for fire did not have this power before it received heat. If this power tracks the actual being of heat, as I have argued, then the power must be attributed to that which has heat, namely, the actual composite of heat and fire.²⁷ Because the power is attributed to the whole composite, the action too will be attributed to the whole composite.

Second, my interpretation allows us to distinguish between two kinds of power. On the one hand, there is the power of a form or essence alone, a power that remains infinite in its scope and is linked with an infinite operation. Note that this power will be the same as the essence only if the essence has its own being. Since this is impossible (only God is His own being), it follows that this power is only improperly called a power because it will never bring about an operation. On the other hand, there is the power of an actual existing being. This power is a proximate principle of an operation, and so something that really can have a causal contribution in this world.²⁸

Thinking about the status of this power — albeit a power only in an improper sense of the term in contrast to a power properly so called — is helpful for two reasons. First, it shows that, although Aquinas associates power with essence, he also thinks that power is much more than a mere possibility for action. Only actual things can act, yet we are also correct to say that things can act in virtue of what they are, namely, their essences. Second, thinking about how the actual being of a thing explains its power allows Aquinas to emphasize that created beings’ power is contingent and dependent on God’s power. Finite power is contingent because it depends on God giving being to essences: without the actuation of essences, we cannot properly speak of powers as principles of operations.

There is a further issue with the interpretation I proposed: it is unclear to me to what extent Aquinas pursues the claim that the actual being of a thing limits its powers. For it is one thing to think about a power of something while abstracting from actual existence, and another to think of a power of something while considering it as an actual created thing of this or that nature. Yet, it is quite different to think about a power of something while considering this thing as a being of a certain nature that actually exists in space and time, and in a cultural, political, and economic system. Most of the appeal of my interpretation rests on what Aquinas understands by ‘limited actual being.’ When Aquinas says at the end of his discussion about how an operation, according to its account, cannot be the same as the being of something that falls in a genus or a species, he seems to mean that we must consider a thing insofar as it is a being of a certain genus or species. This would mean that, to understand the powers of a thing, we must consider its actual existence insofar as it is something pertaining to a certain species or a genus. In other contexts, Aquinas has a richer understanding of actual being or existence. He says that, although a thing falls under a genus or species, because of its form, its actual being is proper to it and distinct from the actual being of another thing that falls under the same genus.²⁹ Given this richer sense of ‘actual being,’ when we consider the limitations of a thing’s power, we might need to track that thing’s actual existence in space and time.

The outcomes of these two ways of tracking actual existence will be quite different. When we consider the extent of a thing’s capacities by tracking its actual being as a member of a genus and species, we will make general comments about contingency and limitation. But when we consider the capacities of a thing by tracking its actual being in space and time, our comments will be more nuanced. For example, on the first sense of ‘actual being,’ we will say that medieval people could have understood the theory of relativity, while on the second sense of ‘actual being,’ we will say that medieval people could not have understood it. These ways of assessing the limitations of the powers that things have make a difference when it comes to ascribing responsibility.³⁰

Like other medieval philosophers, Aquinas maintains the distinction between divine and created powers. He preserves this distinction by arguing that creatures cannot have operations that match the actual being of these creatures, although by considering their essences alone, these operations are possible for them. For Aquinas, powers depend on these things’ essence but are also affected by these essences’ limited act of existence.