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:

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.^{Footnote 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.^{Footnote 5} Indeed, powers are that through which something acts: powers are instruments or quasi-instruments in virtue of substantial forms.^{Footnote 6} But what does it mean for something to act through a power? ‘To act’ has different meanings for Aquinas.^{Footnote 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, heat^{Footnote 8} is said to be the formal cause of the action of heating,^{Footnote 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.”^{Footnote 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.^{Footnote 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:^{Footnote 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.^{Footnote 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.^{Footnote 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:

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:

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.^{Footnote 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.”^{Footnote 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.^{Footnote 19} Being is a perfection and so an act: when it is received, it gets limited to this essence or that essence,^{Footnote 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.^{Footnote 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).^{Footnote 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.^{Footnote 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.^{Footnote 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.”^{Footnote 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:

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.