A while ago I wrote a series of blog posts on potentiality, change, and limitation. I am always on the lookout for better ways of articulating the act-potency distinction, since it serves as a foundation for much of the Aristotelian-Thomistic framework that I find so useful in philosophy and theology. The original post, from two years ago, was occasioned by comments made by Joe Schmid about the relationship between the act-potency distinction and pluralism about being. In response to the post, a reader raised important questions about how this approach might help us to understand the Thomistic commitment that potency limits act. My original attempt to answer this is captured in the third post. However, since that discussion (and largely because of it) I have spent some more time thinking about the whole way of understanding the act-potency distinction and its relation to this limitation principle. My aim in this post is to give a brief overview of my updated approach.

We start with the act-potency distinction itself. In my original post, I jumped straight into a metaphysical account of act and potency. While this may be well-motivated historically-speaking, I think we can come up with a better analysis of the distinction, in light of the later applications of it. Aristotle originally conceived of act and potency in terms of causation of change,^[1] but since then it has been applied to being as such in the Thomist real distinction, as well as to logical distinctions, such as when we say that a genus is in potency to a specific difference. In hindsight, then, I think it’s best to start with a fairly general account of the act-potency distinction, which we can see at work in its various incarnations:

What this looks like will depend on the context in which it is applied, but the general relation between act and potency will always remain. Act is the focal or primary sense of being, in virtue of which we affirm of anything that it is, or is the case. A thing is red in virtue of its actual physical structure, and is human in virtue of the way its substantial form actualizes its underlying matter. Even when we say that something is in potency to X we are affirming something to be actually the case—not that the thing is X, but that some potency for X really obtains in it. For example, the potency to speak Spanish is actually present in a Spanish speaker in a way it is not present in me. We can also say that a genus (like animal) is in potency to some species (like rational animal) and that only the latter actually contains the specific difference (rationality), but we do not thereby mean to imply that these concepts as such have some independent existence. This is because when we speak of actuality in this case we are operating within a logical rather than an ontological context.

Of course, it can be quite difficult to determine exactly what the relevant act and context are in some cases. When we say that essence is of itself in potency to esse, or that prime matter is of itself in potency to various substantial natures, where is the act supposed to be? Or what about the simpler claim that P is a potency for some X? In such cases, we want to avoid any implication that a potency as such is somehow in act, lest the distinction between act and potency collapse entirely. It seems to me that whenever we consider a potency “in itself” or “as such,” what we are doing is considering it in abstraction from the concrete being it constitutes. Thus, strictly speaking, the affirmations we make of it occur in a conceptual or epistemological context. If act is that whereby anything is, and potency is really distinct from act, then potency will never exist by itself, detached from any act. So, if we are to ever analyze potency in isolation from act, this isolation will have to happen through such a process of abstraction.

Unlike act, potency does not make a thing to be X or not-X, but rather makes it open to both—if we abstract away the Xness (and not-Xness) of the thing we are not thereby left with nothing, precisely because it has this potency which makes it indeterminate with respect to Xness. Put a slightly different way, the potency is compatible with Xness and not-Xness, but the actuality is not. As Aristotle says:

The same thing that is said to be capable of [in potency to] being healthy is also capable of being sick, and at the same time, to be resting and to be moving, to build and to demolish, and to be built and to collapse. The capacity for contraries, then, is present at the same time. For the contraries to be present [actual] at the same time, however, is impossible… but being capable is of both alike, or of neither. (Metaphysics IX.9)

When a thing that is in potency to X is made to be actually X, then we say that that potency is actualized. This brings us to our first principle involving act and potency, which we will call the reception principle:

This principle appears in Aquinas’s On Being and Essence (IV.8) as part of his argument that immaterial beings must have potentials despite not having matter. I start with this principle because it calls us to introduce the important distinction between potency and potential.

Potencies can be intrinsic features of things, in which case they make a thing to be indeterminate between and compatible with contrary states. In this case we call them potentials, which are the focus of the reception principle. It is in virtue of having a potential that a thing may receive actualization from another without going out of existence, since the potential makes the thing which has it indifferent to what is actually the case. Incidentally, this is why every potential must be actualized by some other actuality, since the former is indifferent to (or open to, or compatible with, or indeterminate between) X and not-X, while the latter is not.

When one thing receives act from another, these are called the patient and agent respectively. Insofar as an agent acts on a patient we have action, and insofar as a patient is acted upon by an agent we have passion. Action and passion are really just the same thing considered from different perspectives, since both are realized insofar as the agent influences the patient (Physics III.3). Now, suppose we have an actuality in an agent that could actualize the potential in a patient but is not currently doing so. Perhaps this is because they are too far away, or because the actuality is not directed to this potential. Regardless of the reason, the point is that this agent is not acting on this patient and yet they are in some sense still open to doing so, and likewise this patient is open to being acted upon by this agent. This gives us two more potencies.

An active potency is grounded in an actuality, in which virtue of which a thing is open to affecting others in various ways, at various times, in various places, and so on. Since these have to do with affecting things, active potencies correspond to the modern notion of causal powers. For example, a fire’s heat (actuality) makes it capable of heating things (active potency) even if there are no things around to be heated. As this example illustrates, an active potency isn’t some separate intrinsic feature of the fire over and above the heat. Rather, it just is that heat considered extrinsically with respect to the possible influence it may have over others. Similarly, a passive potency is grounded in a potential, in virtue of which a thing is open to being acted upon in various places, manners, times, and so on. Like active potencies, passive potencies are strictly speaking extrinsic, but because they are grounded in intrinsic potencies (potentials) this nuance is rarely important. Given what we have thus far said, it should be clear that the actualization of an active potency is an action while the actualization of a passive potency is a passion. And since action and passion are the same thing considered from different perspectives, it follows that all three potencies (active, passive, and potential) are actualized by the same underlying reality, each in their own way.

With all this in hand, let us now say something brief in defense of the reception principle. In order for something to receive X from another, it must (1) be compatible with Xness (2) without being X itself. After all, (1) if it were incompatible then it would be destroyed when X were imposed upon it, and (2) if it were already X then there would be nothing to receive. But to be compatible and not X just is to be open to or in potency to X. Moreover, insofar as anything becomes X intrinsically so too must the potency be intrinsic, for it becomes X in accordance with how it is open to X. Thus, this potency for X must also be a potential for X.

Now, regarding (2) we may wonder about cases where something ostensibly receives X despite already being X, as when a hot thing receives heat or a tall thing receives height. Such cases don’t constitute counter-examples to our above argument, however, once we clarify what is going on. Suppose that something is temperate X, and it receives an increase in heat of Y. Then when it was X, it had the potential to be Y more hot, resulting in it having the temperature X+Y. Thus, while it may not have received heat as such (moving from no heat to some heat) it is still receiving more heat (moving from X heat to X+Y heat). The same goes for other quantitative features, such as height, or age, or speed.

So much for the reception principle. Let us now turn to our second principle.

Aquinas appeals to this principle throughout his works, but it seems that it originates from the Aristotelian account of sensation.^[2] Sensation consists in the reception of sensible forms through our various sense organs, but when our eyes receive the sensible form of redness they do not thereby become red. This is because, as the modality principle states, whatever is received is received according to the mode of the receiver. In this case, the sense organs are open to receiving the sensible forms not in just any way, but in such a way (or mode) that the sensible forms are encoded and processable by the brain.

On further reflection, the modality principle seems more generally applicable to potentiality as such. After all, whenever something is open to receiving X, it will receive X in accordance with the particular way (or mode) in which it is open to X. A chair is open to being red, but its shape will govern the shape the redness takes, its materials will govern how bright the redness can be, and so on. The atmosphere and the Earth each react differently to the thermal radiation from the Sun, because of the unique way or degree to which each of them is open to receiving that radiation. Examples could be multiplied, but hopefully these suffice to see the point.

It’s not clear to me whether the modality principle is supposed to be self-evident, an empirical generalization, or a metaphysical theorem deduced from more basic premises. If the last of these, then these premises would presumably be (1) the reception principle and (2) the fact that potentials are open to acts not simpliciter but in particular ways. That this is at least sometimes, or even often, the case is an empirically observable fact, and I’m not sure the modality principle needs anything more than this—we should just read it as saying that whenever the receiver has a mode, then reception occurs in accordance with that mode.

The modality principle is closely related to the third principle I want to discuss.

Together, the reception, modality, and infinity principles constitute what we can call the limitation principle: act is limited if and only if it is received into some potential. The connection between reception and potentiality is established by the reception principle. The reverse condition (“if”) is really just the modality principle, since if act is received into some potential then it is received according to the mode of that potential, and thereby qualified or limited. We already saw this in the case of redness and the chair, where the redness in the chair is limited in a way that redness as such is not. The forward condition (“only if”) is the infinity principle.

So, why think that unreceived act is unlimited? Suppose that X is limitable to Y by being received into something and limitable to not-Y (that is, to some Z incompatible with Y) by being received into something else. Then, X as such is not limited to Y or not-Y, for in either case it would be incompatible with the other. On the other hand, if X is limitable to Y but not to not-Y, then Yness may be part of Xness. Redness, for example, is limited to this or that shape, time, location, brightness, texture, etc. by the things which receive it. Redness as such cannot be limited in any of these particular ways, for otherwise it would be unreceivable in the others—if it were in itself limited to a particular place, say, then it could never be instantiated outside of that place, and likewise for other limitations. On the other hand, redness is always limited to a range of hues and never limitable to something outside that range, for in that case redness is replaced by some other color. Supposing, then, that redness were unreceived and free-standing in itself, then it would not be limited in any of the ways that receives limit it, and in this sense be unlimited or infinite.

Of course, red in itself would only be relatively unlimited, or have a qualified infinity. This is because redness itself has certain intrinsic limitations, such as the range of hues it imposes upon things which receive it. In order to get an act which is absolutely unlimited or infinite, it would need to be common to and receivable into all things, thereby excluding any limitation whatsoever in itself. In Thomistic metaphysics, this is esse, the act whereby a thing is. Unreceived esse, then, would be absolutely unlimited and infinite, thereby constituting the fullest and unqualified expression of the infinity principle.

Now, much of this argument can also be run for potency, by focusing on the receiver rather than the received: if X is open to Y and not-Y, then X as such is not limited to Y or not-Y, and so on. The result is an infinitely open thing, or something with unlimited potential. For example, prime matter is infinite relative to material being, and is limited to being actually this or that when composed with substantial form. A major difference between act and potentiality, however, is that there can in principle be no separately existing unreceiving thing of absolutely infinite potential, for such a thing would be in no way actual and therefore not even exist.

Another important difference is that infinite act is complete whereas infinite potentiality is incomplete. Aquinas notes that, “we call that perfect [complete] which lacks nothing of the mode of its perfection.” (ST I Q4 corp.) The mode of a thing’s perfection is determined by the kind of thing it is: a squirrel needs four legs, a musician needs a full grasp of their art, a French speaker needs a broad understanding of the language, a chair needs sufficient stability and sturdiness, and so on. A thing is not considered complete simply by potentially having the relevant features, but by actually having them: I am not a complete pianist, for example, simply because I have the potential to be one—I need to actually know how to play the instrument! Thus, redness as such is in some sense a more complete redness than received redness. When we say it is not limited to this or that shape, this is not because it is limited to some other shape, but because it is not limited by shape at all. Put another way, a red thing is an incomplete realization of redness itself, whereas unreceived redness would be the most complete realization. Unreceived esse is therefore infinitely complete, while prime matter is infinitely incomplete. Esse “contains” the full perfection of being, while prime matter excludes it.

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[1] W Norris Clarke, “The Limitation of Act by Potency: Aristotelianism or Neoplatonism,” The New Scholasticism 26 (1952): 167–94.

[2] John F. Wippel, “Thomas Aquinas and the Axiom ‘What is Received is Received According to the Mode of the Receiver.’” Metaphysical Themes in Thomas Aquinas II, Catholic University of America Press, 2007, 113–122.