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 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.” .
“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.” .
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 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) . 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)” .
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” .
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” .
- “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” .
- “Being properly signifies that something actually is, and actuality properly correlates to potentiality” .
So we can say, with Garrigou-Lagrange, that “all finite beings are composed of potency and act” . 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” . 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.
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 .
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 .
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.
- The 24 Thesis in Latin and English, along with commentary and references, can be found here: http://www.u.arizona.edu/~aversa/scholastic/24Thomisticpart2.htm
- The present translation is my own, but also draws from translations found in the above link, as well as here: http://www.catholicapologetics.info/catholicteaching/philosophy/thomast.htm
- 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: http://www.catholicapologetics.info/catholicteaching/philosophy/definitions.htm
- Klubertanz, Introduction to the Philosophy of Being, second edition, Wipf and Stock Publishers, 2005, page 55.
- Thomas Aquinas, Summa Contra Gentiles, Book II, chapter 54.10, https://dhspriory.org/thomas/ContraGentiles2.htm#54
- Aquinas, Summa Theologiae, I, Q. 77, Art. 1, https://isidore.co/aquinas/summa/FP/FP077.html#FPQ77A1THEP1
- Aquinas, Commentary on Aristotle’s Metaphysics, book VII, lesson 1.1245, https://isidore.co/aquinas/Metaphysics7.htm#1
- Aquinas, ST, I, Q. 5, Art. 1, https://dhspriory.org/thomas/summa/FP/FP005.html#FPQ5A1THEP1
- Garrigou-Lagrange, Reality, 32.
- Klubertanz, Introduction, 88.
- 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.
- For more on Parmenides’ argument, see Garrigou-Lagrange, Reality, starting on page 32.