In his gigantic post on existential inertia (section 7.13), Joe Schmid gives an argument against the act-potency distinction which can be roughly summarized as follows: the act-potency distinction commits us to pluralism about being, but pluralism about being is false, therefore we should reject the act-potency distinction. I had not heard of pluralism about being before, but Schmid quotes Merricks (a recent critic of the view) giving the following general idea:
Monism about being (monism for short) says that everything enjoys the same way of being. So monism implies, for example, that if there are pure sets and if there are mountains, then pure sets exist in just the way that mountains do. Monism can be contrasted with pluralism about being (pluralism for short). Pluralism says that some entities enjoy one way of being but others enjoy another way, or other ways, of being.
Schmid’s ultimate target is the Thomistic view of existence as articulated by Aquinas in On Being and Essence, but he gets at it through the more general act-potency distinction. On the Thomistic view, God and creatures exist in different ways, since in creatures there is a real distinction between essence and being (essence is in potency to the act of being), whereas in God there is no such distinction—he is being itself, pure act. While this is true so far as it goes, it is not obvious that the “ways of being” in the Thomistic sense corresponds to the “ways of being” in Merrick’s sense, and my aim here is to show that it in fact does not.
Now, I’ll admit that the overall structure of Schmid’s argument is somewhat confusing to me. He actually takes aim at something he calls act-potency pluralism—the view that there are two ways of being (in Merrick’s sense), namely being-in-potency and being-in-act. His discussion ends once he establishes the conclusion that we should not accept act-potency pluralism, but he doesn’t show that the Thomistic view of existence commits us to such a view in the first place. As best I can tell, he assumes that the Thomistic view of existence requires the act-potency distinction and that the act-potency distinction entails (or is equivalent to) act-potency pluralism. Thus, an argument against act-potency pluralism amounts to an argument against the Thomistic view of existence. On the contrary, I do not think the act-potency distinction commits us to act-potency pluralism, and that therefore the conclusion of Schmid’s argument is irrelevant to the Thomistic view of existence.
In order to show this, we must clarify what pluralism means when it speaks of “different ways of being”. If we look at premises (18) and (19) in Schmid’s argument, we can see what he takes to be the salient feature of pluralism that makes it problematic:
18. Act-potency pluralists should not accept generic existence.
19. If act-potency pluralists do not accept generic existence, then they cannot—by their own lights—state that everything is thus and so.
To “generically exist” means “to exist, or to have being, or to be something”. This is the only sort of existence the monist thinks there is, while the pluralist could in principle accept that there is this sort of existence in addition to others. Nevertheless, as Schmid points out when defending (18), the motivations for pluralism militate against the acceptance of generic existence. This is all the more true of the Thomistic understanding of act and potency, since we take this distinction to be exhaustive.
This brings us to (19). If there is no generic being alongside being-in-potency and being-in-act, then there is no X such that we can say simply that everything is X, since everything a thing is is in virtue of its being, and there is no way of being that applies to everything. At best we can say something is(-in-potency) X or is(-in-act) X. As Schmid’s argument goes on to say, this is a problem for act-potency pluralism because it is the position about what everything simply is, and is therefore self-undercutting (see his post for more detail).
This implication of pluralism helps us to better understand what it means for there to be different “ways of being” in Merrick’s sense. If being is divided in this way, then there is no sense in which a thing simply is anything because there is no sense in which a thing can simply be anything—it must either be-in-potency or be-in-act.
The thing is, this isn’t how Thomists understand the act-potency distinction. For us, act just is the generic being, in virtue of which everything exists. Potency, or more specifically potentiality (a difference we’ll unpack later), is not strictly speaking being or a way of being, but something else. But, we may wonder, what could it be if not some other way of being? There are two answers we can give, which we’ll introduce each separately before showing that they are in fact equivalent.
Potentiality as relative nonbeing
The first account arises in response to the problem of the one and the many, motivated by Parmenides’ against multiplicity. In order for one thing to be distinct from another, it needs to have some differentiating feature that the other doesn’t. On the one hand, it can’t be being, since this is something that both things already have. On the other, it can’t be nonbeing, since nonbeing is nothing and if nothing differentiates the things then they are not differentiated. For Parmenides, the conclusion we should draw from this is something we could call “Parmenidean collapse”—that there is no multiplicity of being, all is one and any diversity in being is merely an illusion.
But surely our experience of multiplicity in being is more evident to us than the premises of this argument! In particular, while it is initially plausible to think that being and nonbeing are exhaustive, perhaps there is some third option that enables us to avoid the Parmenidean collapse. This third thing, whatever it is, must have some influence over being (similar to being) but without contributing any being of itself (similar to nonbeing). But if it makes no positive contribution to being (as being does) then its influence must be negative, demarcating what being is not. And if it is to avoid completely excluding being (as nonbeing does), then this demarcation cannot be absolute. Thus, this third thing must be what we could call relative nonbeing, a midpoint between absolute nonbeing and absolute being.
Relative nonbeing, then, is that whereby being is multiplied, it is the otherness by which something is other than other things. Alice is not Bob because there is a principle within Alice by virtue of which she is not Bob, Charlie, Eve, or anything else—that is, relative to all these other things, Alice is not. We call the principle of relative nonbeing potentiality, and we call the principle of being actuality (or act). Both actuality and potentiality must be real principles of things lest they not be able to play the role they do in the multiplicity of being. Furthermore, they need to be really distinct from one another because their functions are in some sense opposed to one another, with actuality providing being and potentiality limiting or constraining it.
We may worry that there is an implicit (and unjustified) assumption lurking somewhere in the background here, of the principle of identity of indiscernibles. Why do we need some feature of a being to explain its distinction from other beings? Why not simply assume that such a distinction is primitive? In an important sense, we do take it to be primitive—relative nonbeing is postulated as an ontological primitive that isn’t analyzable into any more fundamental principles. We are not suggesting that the distinction between beings is accounted for by some property they have, since the properties themselves are part of being’s diversity which we seek to explain. What we don’t do is consider distinction to be theoretically primitive, in the sense that our theory simply assumes it without looking for its basis in reality. If we are faced with two theories, one of which is able to explain a phenomenon and the other of which simply refuses to explain it, the former is preferable when our entire aim is to give an account of reality.
Potentiality as the passive capacity for act
The second account of potentiality arises from the analyses of causation and change. Suppose Alice teaches something to Bob, which is to say that she causes him to change from being uneducated to being educated. Again, we say that act (or actuality) is the principle of being, so Bob’s change would be from being actually uneducated to being actually educated.
Now, throughout this process Bob must have the capacity for being educated—if we considered him apart from his educatedness, we would find that nothing determines it one way or the other. This is in contrast to what we find in the case of a tree or a squirrel, where the nature of each excludes the possibility of them being educated. Bob must continue having the capacity for being educated once Alice has educated him, because his being educated just is the fulfillment of this capacity—take away the capacity and he stops being able to be educated! Just as a water jar retains its capacity for holding water whether it is empty or full, so too does Bob retain his capacity whether he is uneducated or educated.
Similarly, Alice must have a “productive” capacity for realizing Bob’s “receptive” capacity. If we considered her apart from Bob’s being or becoming educated, we would find that nothing determines one way or the other about Bob. After all, Alice has this capacity before and after Bob’s being educated, and it exists even if something gets in the way of Bob being able to learn (lack of attention, distraction, misunderstanding, etc.).
Generalizing this example, a potency is a capacity for some act, and we can distinguish between two types. The capacity for producing some act in another is called an active potency, which today is more commonly called a power; and the capacity for receiving some act into one’s being is called a passive potency, or a potentiality. So, in our example, Bob has the potential for being educated while Alice has the power to educate him—to actualize this potential of his. An important difference between potentiality and power is that the former involves an indeterminacy within the thing that has it whereas the latter does not. This is a consequence of how we arrived at each concept: a capacity is conceived in abstraction from its act, so that if the act is within something (as in the case of potentiality) then the capacity corresponds to a non-actuality (and therefore indeterminacy) within it, but if the act is outside of something (as in the case of power) then the capacity corresponds to how it actually is (and is therefore determinate).
While this is all worth exploring in more detail, here we are concerned solely with what this means for potentiality. From the perspective of causation and change, potentiality is a capacity of a thing that makes it “open” to some act without determining one way or the other whether it is in fact actual in that way.
What these accounts mean for act-potency pluralism
So, then, we have two accounts of potentiality arising from two different observations about reality, and it is relatively easy to see that they are equivalent. Indeterminacy regarding some actuality (passive potency) is equivalent to making no positive contribution of being (relative nonbeing), and being contrary to Y, Z, and so on for all options other than X (relative nonbeing) is equivalent to being for X (passive potency). Thus, these two accounts just give us two complementary perspectives on the same fundamental principle, potentiality.
We can now return to the question of whether this distinction between actuality and potentiality entails (or is equivalent to) a pluralism about being in Merrick’s sense. Do our reasons for positing potentiality as a real principle of things motivate a denial of generic existence? Or does potentiality amount to some second kind of existence alongside actuality? Hopefully our discussion up until this point will have made it clear that the answer to both of these questions is no. Both act and potentiality are real and distinct principles of things, but there is only one “generic existence”, and that’s act—when we say things are thus and so we mean that they are actual. Potentiality is not another way of being (in Merrick’s sense), it is relative nonbeing and of itself is indifferent to what is actually the case.
This remains true even when what we affirm a potentiality to be thus and so, because potentiality only ever exists in virtue of actuality, either through composition with some act (its actualization) or as the second potentiality of some act. Composition is what obtains when potentiality comes to limit or circumscribe act, as we needed to posit in order to avoid the Parmenidean collapse. In this case, act supplies being while potentiality limits it to this being rather than that being. It is in virtue of the actuality, then, that the potentiality can be operative as a principle of limitation since without it there would be no positive being to limit. Second potentialities are the more intuitive way in which potentials are said to exist. These occur when the actualization of a potentiality leaves some residual indeterminacy, thereby constituting a further potentiality. Once Alice actualizes her potential to be able to read (by learning how), this results in the further potentiality to actually learn from this or that book. Or, using other Aristotelian categories, Bob exists by virtue of his substantial form actualizing his underlying prime matter, which results in further potentials for having different heights, skills, knowledge, and so on. It’s second potentialities which accounts for things being in potency to various things, ie. to have the unactualized potential for being something. A second potential can exist without being actualized because it is grounded in a lower-level actuality, on account of which we say that the potential is thus and so.
So, then, I don’t think Schmid’s argument holds much weight against the Thomistic view of existence or the act-potency distinction more generally. Even if we accept premises (18) and (19), we should reject the implicit assumption that the act-potency distinction commits us to act-potency pluralism. In Merrick’s sense, the Thomist thinks there is only one way of being, namely by act. Potentiality is not another way of being, but relative nonbeing and the capacity for act.
 He does note that his argument is “applied to act-potency pluralism, but it applies mutatis mutandis to the De Ente argument—simply replace ‘a-existence’ and ‘p-existence’ with ‘d-existence’ and ‘c-existence’ (representing ‘divine existence’ and ‘creaturely existence’, respectively).” However, he doesn’t explain why such a translation would leave the justifications for the relevant premises unaffected. After all, c-existence is not the same as p-existence.
 In footnote 25, he suggests that act-potency pluralism encapsulates the first of the 24 Thomistic Theses. However, this thesis does not say that act and potency are two ways of being, but rather that everything which exists is either purely actual things or an act-potency composite. As we shall see, this does not require that act and potency be two ways of being in Merrick’s sense.
 Merricks, 599, quoted by Schmid.
 For a very detailed discussion of Aquinas’s account in response to Parmenides, as well as the discussion of the notion of relative nonbeing, see John Wippel’s The Metaphysical Thought of Thomas Aquinas.
 Modern readers will recognize this as a Moorean shift, but it is also a consequence of the methodology outlined by Aristotle in Physics I. We do not have direct access to the principles of things, but only the composites of which they are principles. Thus, we are justified in believing some purported system of principles only insofar as it accounts for those things which are more manifest to us. Parmenides’ argument is rejected, therefore, because it proposes a system of principles that fails to account for the manifest multiplicity of being.
 We should note that Aquinas tends to use the word “power” as equivalent to “potency”, so that for him there are active powers and passive powers. In modern times, we tend to think of powers as exclusively active.
 We connect non-actuality with indeterminacy because if something is neither actually X nor actually ~X, then it is indeterminate between X and ~X.
 Of course, there is also an active potency at play here, but we are interested in the passive potency which is realized when Alice gains knowledge from her reading.