Defending the 24 Thomistic Theses: The First Thesis

The “Twenty-Four Thomistic Theses” is a list of theses accepted by the Sacred Congregation of Studies as the major, central principles of the philosophy of Saint Thomas Aquinas. These principles identify the philosophy of Thomism and distinguish it from other philosophical schools.

The Thesis

The first of the theses states:

Potentia et actus ita dividunt ens, ut quidquid est, vel sit actus purus, vel ex potentia et actu tamquam primis atque intrinsecis principiis necessario coalescat.” [1].

“Potency and act thus divide being, that whatever is, either is pure act, or is composed by necessity from potency and act as from first and intrinsic principles.” [2].

The distinction between act and potency (or actuality and potentiality) is the foundation of the metaphysical systems of Aristotle and St. Thomas; everything else depends upon this point. But what exactly does this mean, and why is it so important?

The Explanation

The first point the thesis makes is that act and potency divide being. Being (ens) is “that whose act is to be” (id cuius actus est esse) [3]. In other words, something is “a being” if it exists, if it has existence. Another way to put it: “Being is something having esse (an act of existing)” [4].

Given this, what does it mean for act and potency to divide being? One way to think of it is that, if we were to take beings apart, dividing them into their more fundamental parts/constituents, and dividing those in turn into their more fundamental parts/constituents, eventually we would reach act and potency as the most fundamental constituents of a being. This is what the thesis means when it states that act and potency are the “first and intrinsic principles” of being. At bottom, beings are composed/made up of act and potency; and they cannot be divided further.

On this point, Saint Thomas writes:

“It is therefore clear that composition of act and potentiality has greater extension than that of form and matter. Thus, matter and form divide natural substance, while potentiality and act divide common being. Accordingly, whatever follows upon potentiality and act, as such, is common to both material and immaterial created substances, as to receive and to be received, to perfect and to be perfected” [5].

Matter and form are principles or constituents of material beings specifically, but not of being as such. For if there is such a thing as immaterial being — if this is even possible — then matter cannot be a fundamental principle of being. But any being, whether material or immaterial, can be composed of act and potency.

The claim that act and potency divide being as its fundamental constituents is explicitly taught by Saint Thomas. The above quote makes this clear, as do the following:

  • “Power [potentia] and act divide being and every kind of being” [6].
  • “Now essential being, which exists outside the mind, is divided in two ways, as has been stated . . . for it is divided, first, into the ten categories, and second, into the potential and the actual” [7].
  • “Being properly signifies that something actually is, and actuality properly correlates to potentiality” [8].

So we can say, with Garrigou-Lagrange, that “all finite beings are composed of potency and act” [9]. But what exactly do act and potency mean? Klubertanz defines “being in act” as “the condition of really possessing some perfection or modification,” while “being in potency” is defined as “the condition of not really having, but being able to acquire, some perfection” [10]. We can think of actuality as a perfection, a feature that is really and currently present in/possessed by something. For example, a brown horse is actually brown, it has the actuality of browness or of being brown. Water that is currently frozen is actually ice. A tree is actually a tree. We can think of potency, on the other hand, as the ability or capacity to actually have some feature. Ice is actually frozen but potentially liquid, because it has a real, innate capacity to become liquid. An acorn is actually an acorn but potentially an oak tree, because it has the real, innate ability to grow into an oak tree.

Another way to talk about act and potency is to say that act is determined being (i.e., being determined to a specific characteristic), while potency is undetermined but determinable being. So, ice is water that has been determined to its solid state; but while it is ice it is still determinable to a liquid state, i.e. it can be turned into a liquid state. When water is ice, it is determined with respect to one of its qualities (being solid), but undetermined with respect to other potential qualities (being liquid or gaseous). We can also use the concept of transformation to make sense of act and potency. Potency is being that is transformable into something actual.

To say that there is a real distinction between act and potency is just to say that act and potency are two objectively different, non-identical principles. It is to say that in objects which are made up of act and potency as constituents, its constituent of act is distinct from and non-identical to its constituent of potency.

The next part of the thesis tells us the manner in which the principles of act and potency divide being. A being can either be purely actual, or a being can be a composition of act and potency. In a being that is pure act, act is the sole constituent making up the being; in a being composed of both and act and potency, both act and potency are real and distinct constituents of the thing. Why can there be something purely actual but not something purely potential? This conclusion follows directly from the nature of what act and potency are. Act, as we’ve said, is the condition of really possessing some feature or characteristic. For something to be “pure potency” would thus mean for it to lack all real features, including the real feature of “existing.” But something that lacks the feature of existing, obviously doesn’t exist. Existence is the most fundamental kind of actuality; it is the prerequisite actuality of all other actualities. A thing cannot have the actuality of “being green” if it doesn’t even exist in the first place. So, we might be able to think of the idea of “pure potency” conceptually, but such a thing couldn’t actually exist as a being. Here we see that actuality is ontologically more fundamental than potentiality. Not only is the act of existing a prerequisite for all other acts; it is also a prerequisite for all other potencies. Something cannot have any potential whatsoever if it doesn’t first exist. Potency depends upon act.

Finally, potency and act are intrinsic principles of being. This is especially important to realize with respect to potency. Real potency is not mere privation or mere possibility in a broad, logical sense. For instance, it is logically possible for human beings to grow wings and fly; but human beings don’t have a real potential for growing wings and flying. A real potency is something innately possessed within the very nature of a being. An acorn has the potential to grow into an oak tree; but it does not have the potential to grow into a giraffe. This is why we can say that potency is a real constituent of being: because for something to have a potency is for it to have a real, objective capacity within its nature. Something about the nature of an acorn makes it the case that it really has the capacity of becoming an oak tree and not a giraffe.

So, to recap our explanation of the first thesis: being is that which exists. To be a being is to be something that has existence. The most fundamental constituents of all beings are the principles of act and potency. Act is the condition of really possessing some feature or characteristic; while potency is the condition of having a real capacity for some feature. Act and potency are distinct principles, and their very nature is such that, for every being, it must be the case that the being in question is either purely actual, or some composition of act and potency. Pure potency cannot exist, because existing itself is a kind of actuality.

The Defense

So, we’ve seen now what the first Thomistic thesis states and what it means. But why think that it’s true? And even if it is true, why is it significant?

The notion of act and potency first come from experience and analysis of the reality of change. We live in a world full of changing things; and we ourselves are constantly undergoing and influencing change. And as we shall see, the real distinction between act and potency is absolutely necessary for accounting for and explaining the phenomenon of change.

To understand why this is the case, let’s consider what exactly change involves and what is required to make sense of it. Let’s use as an example of change the process of ice melting into water. Ice melting into water is clearly an instance of change; but what does that mean? It means, at least, that the four following conditions are met (drawing from Klubertanz’s work): first, that there is some initial stage, with an initial set of features/characteristics (e.g., the ice existing as ice with the standard features of ice, such as being solid, cold, etc.); second, that at some other point there is a terminal stage, with some real difference in the features/characteristics present (the water now existing as liquid, which is different from solid ice); third, that there is some process between the two stages by/during which the features present in the initial stage cease to be present and the features present in the terminal stage become present (the melting of the ice into liquid water); and, finally, that there is some real continuity or connection between the two stages that underlies the process. So change requires/involves the four following conditions: an initial stage, a terminal stage, a process, and an underlying continuity [11].

If there is no real difference between the two stages — if the initial stage and terminal stage are entirely identical in every respect — then obviously no change has occurred. Similarly, if there is not some element of sameness, if there is not identity in at least some respect, then change likewise cannot happen. To understand why this is so, consider the following example: suppose I have a table painted brown, but that I want to change it to red. But instead of just painting it red, I throw the table away and buy a whole new table that is red. Now, obviously some sort of change has occurred; but it would be completely false to say that “the table changed from brown to red,” because there are two different tables entirely. To say that the table undergoes a real change requires that there is one and the same table that is the subject of change, that is first brown and then red and persists through the process of change. If there is nothing to connect the initial stage from the terminal stage, then we cannot properly say that there is any change; there are simply two distinct and totally unrelated, separate stages.

So we need both an element/principle of difference and an element/principle of identity or continuity. In other words, for an object to undergo change, there must be some underlying subject that at one point has one feature, and then at another point has a different feature. The underlying subject itself remains the same qua subject, but changes with respect to the different features. Note that the underlying subject cannot be identical with either of the separate features, for then it could not actually change with respect to its feature while remaining the same subject (e.g., the brown table cannot be identical to its brownness, because then it could not possibly become red while remaining itself. On the contrary, in saying that a table changes from being brown to being red, we need to say that one and the same table remains, but that at first it is brown, and later it, while remaining the same table, has become red).

So now we are faced with the crucial question of change: how can one and the same subject (S) go from really possessing one feature (F1) at one point (T1), to losing that feature and coming to possess an entirely different feature (F2) at some other point (T2). The ancient Greek philosopher Parmenides famously denied the possibility of this actually happening. His thinking went something like this: if S is a being and exists, and S possesses F1 at T1 as a real feature that also exists, but lacks at T1 some other feature F2, then at T1 we must say that F2 does not exist. If we want to say that at T2 F2 does exist, then we must say that F2 begins to exist or comes to be. But how does F2 come to be? Where does it come from? Parmenides’ answer was that it can either come from being that already is, or else it must come from nothing. It cannot come from being that already is, because the being that already is is F1, and F2 must be different from F1. F1 is F1 and is non-F2; so F1 cannot also be F2. But neither can it come from nothing, because nothing comes from nothing. So, Parmenides concluded, change must be impossible [12].

It was Aristotle who, in answering Parmenides’ challenge, developed the act-potency distinction which Aquinas further extrapolated. Being is not only act; it also includes real potency. S at T1 is actually F1 and potentially F2. At T2, S is now actually F2 because its potential for F2 has been made actual. So F2 does not come from either actual being or nothing; it comes from potency, or potential being. S at T1 has a real, objective capacity for F2.

So, if we deny the reality of potency and act as distinct principles of being, we are faced with the following possibilities in consequence: either change cannot occur at all, which is completely contrary to our experience; or, if change does occur, new features come into existence out of nothing, which is impossible given that something cannot come from nothing.

So the first of the Thomistic theses is vitally important insofar as it is necessary in order to give a metaphysical account of the reality of change. It also, as we shall see in later posts, does a lot of other interesting and significant metaphysical work. As a metaphysical theory it is extremely fruitful and has an extraordinary degree of explanatory power. It provides an important step in one of Saint Thomas’ central arguments for the existence of God; it helps Saint Thomas develop compelling answers for the philosophical issues of universals, concrete-particulars, persistence and identity through time, causation, and a host of others. In short, the act-potency distinction provides the fundamental framework from which Saint Thomas approaches pretty much every subject. It is central in his philosophy of human nature, his understanding of knowledge, ethics, and even theology and Scriptural interpretation.

I find the first thesis absolutely compelling and even necessary. It is the foundation of the entire Thomistic philosophical system; and, as we shall see, the other theses follow directly from it.


  1. The 24 Thesis in Latin and English, along with commentary and references, can be found here:
  2. The present translation is my own, but also draws from translations found in the above link, as well as here:
  3. From what I can tell, this is a standard Scholastic definition. I first found it in Garrigou-Lagrange, Reality: A Synthesis of Thomistic Thought, translated by Patrick Cummins O. S. B., Ex Fontibus Co., 2015, page 27. It can also be found here:
  4. Klubertanz, Introduction to the Philosophy of Being, second edition, Wipf and Stock Publishers, 2005, page 55.
  5. Thomas Aquinas, Summa Contra Gentiles, Book II, chapter 54.10,
  6. Aquinas, Summa Theologiae, I, Q. 77, Art. 1,
  7. Aquinas, Commentary on Aristotle’s Metaphysics, book VII, lesson 1.1245,
  8. Aquinas, ST, I, Q. 5, Art. 1,
  9. Garrigou-Lagrange, Reality, 32.
  10. Klubertanz, Introduction, 88.
  11. Much of this analysis of change and defense of the act-potency distinction, including specific terms and concepts used, here and throughout the whole post, is drawn from Klubertanz, Introduction, especially chapter IV.
  12. For more on Parmenides’ argument, see Garrigou-Lagrange, Reality, starting on page 32.

20 thoughts on “Defending the 24 Thomistic Theses: The First Thesis

  1. I’ve never been able to wrap my head around the Thomistic metaphysic, specifically struggling with essentialism and issues of demarcation. So I hope you don’t mind if I ask a few questions as you go through the series.

    In this post you spoke of act and potency in terms of “features”. What is the ontological status of these features? Are they inseparable yet distinguishable aspects of a being’s essence? Is the full set of all potentials and actuals equivalent to the full set of features for that entity? How do we distinguish between accidental and essential features?


    1. Thanks for the comment, I don’t mind the questions at all, keep them coming! I’ll try my best to answer what I can.

      In response, I should say first that concepts such as essence, accident, etc. are extremely important to the Thomistic metaphysical system, and I will definitely be discussing them throughout the series; but with respect to this first thesis on its own, one need not accept or even make reference to these concepts at all. I think the theories of essence, substance, accident, form, etc. follow from this first thesis, but technically speaking, one could accept the first thesis while rejecting the rest. As such, I intentionally tried to use “neutral” and ambiguous terms like feature and characteristic, which one can accept without any commitment to any further metaphysical concepts.

      So what is the ontological status of “features”? It depends. Act and potency are extremely general concepts that can be applied to a lot of different kinds/levels of being. As St. Thomas sees it, essence is potential that is actualized by an act of existing, matter is potential that is actualized by form, substance is potential that is actualized by its different accidents. But one could theoretically reject all of these categories and still accept the act/potency distinction. In the sense I’m using it, a “feature” of a thing is just anything real about it. Being brown is thus a feature of a brown horse, having heat is a feature of fire, etc. So, understood as such, it can be said that the full set of features of a thing is indeed equivalent to its full set of actualities and potentialities (although, some of the features are only had potentially. Ice does not have the actual feature of being liquid, but it does have the potential for having the feature of being liquid).

      Now, the Thomist wants to say that a thing has some features as necessarily following from its essence, and others which do not. How do we distinguish between these kinds of features? That can be a difficult epistemological question. It’s one I haven’t studied in too much depth yet, but I do have one suggestion. One simple way to distinguish could just be to ask the question: for any feature X, if the thing which has X were to lose X, would it still remain the same kind of thing that it is while it has X? In other words, if a horse loses the feature of “being brown” would it remain a horse? And I think the answer is obviously yes. On the other hand, if a horse were to lose the feature of “being alive,” it seems it would no longer be a horse.

      I hope this helps. Feel free to ask any follow ups


      1. Thanks. I’ll try to keep questions focused on act\potency for this post and leave questions of essence and accidents for future posts.

        I see an ambiguity regarding levels of description and how this translates to act and potency. For example, the color of the brown horse could be described as simply ‘brown’, or as a spectrum (wavelengths and intensities) for the entirety of the horse, or as a specific wavelength and intensity for each atom that makes up the horse at a specific moment in time. Are all of these levels equally real, or is there just one actual that is merely approximated to different degrees by each of the descriptions? If the latter, are we doomed to approximations without ever truly comprehending the true nature of the act or potency that we are describing?

        Another issue is with regard to the scope of the entity to which an act or potency applies. Using your ice example, note that it does not on its own have the potential of being liquid. It is ice because it is currently at a particular temperature. In complete isolation, it will remain ice. It is dependent on an outside contribution of energy to become liquid. So does the potential apply to the ice, or is it actually the ice + environment system that has the potential? Does a potential only exist for an entity that does not depend on anything outside of itself to actualize the potential, or do potentials include interactions? If the latter, is there a constraint on the set of interactions?


      2. 1. To say that the horse is actually brown, or that it has the actuality of browness, is just to say that whatever one means by stating “the horse is brown” is actually true. The sentence “the horse is brown” is certainly true/meaningful, but can be translated to much more specific/detailed/technical descriptions. For instance, to say that the horse is brown is really to say that the horse has brown fur. And to say that the horse has brown fur is to say something like the horse has fur with a molecular/atomic structure that interacts with wavelengths of light in this particular way such that our eyes percieve the light emitted from the atoms as this particular color. And we could get even more technical as we dive deeper into the structure of things. Again, all of these different levels are perfectly true and meaningful ways of stating how things are. So, on the level of act/potency, we can either say the horse has the actuality of being brown, or we can say that the horse has the actuality of having fur with a certain atomic structure that interacts with light in a certain way etc. etc. Or, to say that water is actually ice can be translated to “water molecules are actually arranged in this particular molecular structure,” and to say that water is potentially liquid when it is actually ice can be translated to “water molecules are actually arranged in this particular way, but the same molecules can potentially become arranged in this different way.” Does this help/make sense at all? Now, a Thomist might want to argue that there’s *more* to a horse being brown and to water being ice than just what can be described in molecular/atomic terms, that describing things just in terms of the latter framework is reductionistic and doesn’t capture the whole, qualitative reality; but that is a much more complicated and controversial issue that doesn’t necessarily affect the act/potency distinction itself.

        2. On your second question, your getting at a very important Thomistic principle that St. Thomas utilizes in his arguments for the existence of God: that a potential can only be actualized by something else already actual. This other thing is what Aristotle/Thomas call the agent or efficient cause. Whenever we say that “Object X has the potential for P,” we are really saying that “X has the potential for P to become actualized by some actual agent A.” Now, it is really X that has this potential. But this is what Thomists call a passive potency, a potential to be affected in some way. Ice has the potential to be affected by outside energy such that its molecular structure changes and it becomes liquid.
        The last question is really interesting. Potentials that things have include potentials to interact with/be affected by other things. Is there a constraint on such potentials that a things has? Yes and no. To answer this, we have to appeal to the Thomistic principles of form-matter composition. Pure matter on its own is infinitely determinable, i.e. has infinite potentials to interact with/be affected by things. As we’ll discuss later (probably with the second thesis), there’s a sense in which matter limits form. But there’s also, I think, a sense in which form limits matter. For instance: I, as a human being, do not have a real potential to become liquid. Now, some of my material components certainly do have that potential. But as long as they are material components of *me*, material components of the specific form of human being, they do not. In other words, my material components cannot melt into liquid *while remaining me*. If my body melts into a puddle of goo, I’m no longer me, no longer a living human being. So while matter on its own has pretty much limitless potential for interactions/determinations, matter in composition with a specific form only has certain potentials that can be actualized as long as the form remains. The form determines the matter, and determines what real potentials it has qua what it is.


      3. Thanks for unpacking that. A couple more clarifying questions:

        1. I think you’re saying that act\potency subsumes all of the levels of description. There is just one actuality for the browness of the horse, but it can be described in multiple ways? Is this correct? To further help me understand this, lets consider the case where one in every 100 hairs on the horse is white and all other hair is brown, so that we still just see a brown horse as a whole. Ontologically speaking, how many actualities are there? Is there one actuality for the horse’s color as a whole, and separate actualities for the collection of brown hairs and the collection of white hairs? Or is there just one actuality for the horse’s color as a whole that can then be described in different ways? I think that from your previous answer, it’s the second option. If so, I then want to ask how we distinguish ontological entities (actualities and potentials) from descriptions.

        2. I think I understand the concept of “passive potency” and I would agree that there seems to be no limit to the set of passive potentials. Maybe this is a question for a different thesis, but is form effectively the same thing as actuality. If so, then I think you’re saying that the set of passive potentials are uniquely tied to each distinct actuality, and anything which results in a new actuality will thus also introduce a new set of passive potentials. Is that correct?


      4. Hi Travis. Really sorry I haven’t gotten back to you yet. Your questions are really good and there’s a lot I’d like to say in response. Unfortunatley school has been really busy the past few weeks. I’ll try to finish my response as soon as I can

        Liked by 1 person

      5. Sorry again for taking so long to respond. Finals just finished up so I finally have a chance to work a bit on the blog again. Now to your questions:

        1. I’m not absolutely certain the answer to this question, but my thoughts at the moment are that the statement “the horse has the actuality of brownness” is really an imprecise description of the fact that the horse has individual hairs which overall appear brown. Its the individual hairs which actually have color, but the horse as a whole actually possesses the hairs. This reply needs more work, but what do you think of it now?

        2. I think this is largely correct. Passive potentials only exist as possessed by some thing, and what that thing is determines what potentials it has.

        I’m working on my post on the second thesis now, so hopefully it will be up soon. Feel free to keep any questions or comments coming


      6. Thanks Harrison. Hope your finals went well.

        Regarding #1, I think I understand you as saying that the actuality only exists at the lowest levels where the actuality cannot be further reduced, and any higher level attribution is just a description rather than an actuality. Correct? As an example, a sponge doesn’t have the actuality of wetness. Instead, the water in the sponge is the carrier of that actuality and we only describe the sponge as wet by virtue of the water it contains. But if we go low enough, an individual H20 molecule doesn’t have the actuality of wetness, so the reduction stops at some threshold below which wetness cannot be reduced. This raises something like Sorites Paradox for these actualities, but perhaps that is a topic for another time.


      7. Well, I want to be careful here about being reductionistic. To go back to our horse example, when we say “the horse is brown” we mean something like “the horse has hairs that appear mostly brown.” But the hairs are really parts of the horse, they are possessed by the horse. A Thomist wants to say that the horse is one individual overall substance which has different distinct parts, such as the hairs. It is the overall substance of the horse which possesses the actual feature of having brown hairs. The sponge example is more difficult, because we first have to determine 1) whether the sponge itself is an actual substance, or just an artifact, 2) whether the water, when it interacts with the sponge, remains its own unique substance or rather somehow becomes part of the substance of the sponge. We need to be able to answer these questions before we can decide whether the sponge itself has the actuality of wetness. Presumably, either way, I think we could say that the sponge in itself has the power to absorb water, such that whether or not the water becomes a part of the substance of the sponge or just enters into some kind of external relationship to it, the statement “the sponge is wet” still reflects reality.

        The question of whether an individual H20 molecule has the actuality of wetness is another interesting one. I’m presently inclined to say that it does, it just has it on a level so small it’s imperceptible to our senses.

        Also, I’m not familiar with the Sorites Paradox. I’ll have to look it up.


      8. I will be interested to hear your thoughts on Sorites Paradox. I see it – the issue of demarcation – as a very important concept to work through for any essentialist metaphysic.


  2. Good post, Harrison. Isn’t prime matter described by Feser as pure potentiality? All actual matter possesses form, but prime matter does not. If it it not pure potentiality, what then is prime matter?


    1. Prime matter as a concept does denote pure potentiality, but prime matter can exist only as an abstracted mental concept, never as a real being. Every real being as such must have at least the actuality of an “esse” or an act of existing. So prime matter never exists in things *as* pure potentiality, only ever as actualized matter. Something that is pure act, on the contrary, can really exist on its own


  3. Hello !
    I’m really fond of your blog and your replies which I found very informative and helpful in my own thomistic work.
    I’m a bit confused, but perhaps you’ll be able to help me, against a “competing” approach to explain change, namely, atomism. I mean, sure, we have the potential-actual distinction, but if we look at it through the lens of atoms, everything can be studied as small particles moving (with a discretization of space if needed), and we have ample evidence of “things moving”. Atoms are unchanging in themselves, and voilà… Why/How should we say that change is ontologically prime to atom position ? In other words, what reason do we have to think that “all change can be accounted as local minimal position difference” is false ?

    Thanks for your answer.


    1. Several thoughts:

      First, I think atomism still ultimately requires an act-potency distinction (as you note) in general and form-matter composition in particular. This is because *any* kind of change must be explained via potency and act. On atomism, the atoms must still be form-matter composites, but everything else will be reduced to these primary substances.

      Second, I’m reading Descartes’s “Principles of Philosophy” right now where he explicitly argues that local-motion is the only real kind of motion. I think this position doesn’t really remove other kinds of motion (e.g., qualitative change) from reality, it just relocates them to the subjective consciousness, which then creates a mind-body problem. We have direct experience of qualitative change: the only question is whether this change actually occurs in the external world, or only in our subjective experience of the world. Either way, other types of motion are real and hence in need of explanation.

      Does this help?


      1. It does help a lot, thanks!

        I think my question is thus more towards “why should we accept act/potency as the only explanation for change, and not treat it as a bad/wrong option compared to, let’s say, atomism or anything else?”.

        The second point you develop seems to be going the same way I thought about the question: if we deny that qualitative change happens in the real world, then we must relocate it somewhere else.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s