Defending the 24 Thomistic Theses: The Second Thesis

In a previous post, we began a series explaining and defending the “twenty-four Thomistic theses,” a list of theses which constitute the major, central principles of the philosophy of Saint Thomas Aquinas and the tradition which has followed him. The first thesis introduced the act-potency distinction. Here we continue with the second thesis.

The Thesis

The second thesis states:

“Actus, utpote perfectio, non limitatur nisi per potentiam, quae est capacitas perfectionis. Proinde in quo ordine actus est purus, in eodem nonnisi illimitatus et unicus exsistit; ubi vero est finitus ac multiplex, in veram incidit cum potentia compositionem.” [1].

“Act, in as much as it is perfection, is not limited except by potency, which is capacity of perfection. Hence, in which order act is pure, in the same only it exists unlimited and unique; where however it is limited and multiplex, in truth it falls in to composition with potency.” [2].

The first thesis established the real distinction between act and potency as fundamental principles of being; the second thesis establishes further what exactly their relationship to each other looks like.

The Explanation/Defense

An outline of the central claims of the thesis is as follows:

  1. Act is perfection; potency is capacity for perfection
  2. Since act is perfection, it is not limited in itself, but can only be limited by potency
  3. Hence, where act exists absolutely by itself, it will be entirely unlimited and unique
  4. So where act exists in a manner that is limited and multiplex, it cannot be existing on its own, but must be existing in composition with some potency.

We will take each of these points in turn. The first recapitulates our discussion of the nature of act and potency from the previous post, and is true by definition. “Perfection” and “act” are syonymous, at least in a sense. Recall Klubertanz’s definition of actual being: “the condition of really possessing some perfection or modification” [3]. If act is defined as the condition of really having some perfection, we can define a “perfection” in turn as some positive feature that is really possessed by something. If fire is hot, “heat” is the perfection which the fire possesses actually. We can say either “fire has the actuality of heat” or “fire has the perfection of heat.” More precisely, perfection refers to the positive feature in itself, while act refers to the manner in which the feature exists — actually, as opposed to potentially. But if a perfection exists actually, we can still refer to it as an actuality, i.e. an actually possessed positive feature.

Referring to actuality in terms of perfection connotes completion, i.e. that something is present and is not lacking. Saint Thomas writes:

“For according to this it is said that something is perfect, according to what degree it is in act, for the perfect is said of that which lacks nothing according to the mode of its own perfection.” [4]

Summa Theologiae I.4.1

Note here that something is perfect according to the degree that it is actual, and that something is perfect when it is not lacking, i.e. is complete. In the same article Aquinas states: “this word ‘perfect’ signifies whatever is not wanting in actuality” [5]. Thus what is merely potential or in potency is necessarily imperfect. What is merely potential lacks in the full measure of actuality and being and hence lacks the full measure of perfection. A perfection, considered as the particular feature in itself, can exist potentially, in the sense that there can exist a potential for a specific perfection. But a perfection that exists potentially does not exist as a perfection, but merely as the capacity for that perfection, which strictly speaking is imperfect. So St. Thomas writes: “A material principle is most imperfect. For since matter as such is merely potential, the first material principle must be simply potential, and thus most imperfect” [6]. And elsewhere: “Matter, by the fact that it is being in potency, has the nature of something imperfect; but the other causes [efficient, formal, final], since they are in act, have the nature of something perfect” [7]. In sum, a potential is always the capacity for some perfection, while act is the realization of the perfection.

We can now turn to the second point. Act, qua perfection, is not limited in itself; but is only limited by potency. This follows from the very nature of these two fundamental principles. Saint Thomas writes: “No act is found to be limited except by a potency that is receptive of the act” [8]. To illustrate this, let’s consider as an example the actuality “brownness.” Imagine that we take this actuality of brownness and separate it from all potencies, so that it exists entirely on its own as the pure actuality of brownness. In the real world, “brownness” only exists as a quality or feature of things, such as a brown horse. But if we remove it from everything else and suppose that it exists on its own, entirely separated from all potencies, it will now exist not as a feature of something, but rather as a thing in itself, a self-subsisting object in its own right: Brownness Itself, or pure Brownness. This Brownness will contain the full perfection of brownness, and will be completely unlimted as brownness. As Aquinas states, “Each thing is perfect according as it is in act, and imperfect according as it is in potency and lacking act. Hence, that which is in no way in potency, but is pure act, must be most perfect” [9]. (With a similar analogy, St. Thomas writes: “For it is manifest that, if some hot thing does not have the whole perfection of heat, this is for that reason, because heat is not participated in according to the perfect concept [of heat], but if heat were existing through itself subsisting, something of the virtue of heat would not be able to be lacking to it” [10] And elsewhere: “For example, if there were a separately existing whiteness, it could not lack any of the power of whiteness. For a given white thing lacks something of the power of whiteness through a defect in the receiver of the whiteness, which receives it according to its mode and perhaps not according to the whole power of whiteness” [11]).

Let’s consider why this is. “Brownness” as such is a perfection, an actuality. Brownness considered in itself, therefore, as totally abstracted from everything else, is a pure actuality, with nothing potential in it. In the real world, brownness cannot exist like this, separated from everything else. It only exists in other things. This means that, in the real world, brownness can only ever exist as mixed with potency. The fact that it exists in other things means that there must have been some potential for brownness that has been actualized in that thing. Furthermore, as it exists in the real world, brownness can only exist in this particular thing in this particular way at this particular time and place, etc. This shows that the actuality of brownness, as it exists in the real world, is necessarily limited. Our imagined “Brownness Itself,” however, which exists apart from all potencies, would not be limited in these ways. It couldn’t exist in this or that particular object, because those objects would be something other than brownness and hence not part of Browness Itself. (On this point St. Thomas writes: “Thus therefore in every composed thing there is something which is not itself. However, although this might be able to be said about something having form, that of course it has something which is not itself (for instance in a white thing there is something which does not pertain to the concept of white), however in the form itself there is nothing other [than the form itself]” [12]). It couldn’t exist at this or that particular time, because “time” is again something other than brownness and so couldn’t exist as an aspect of Brownness Itself. And so on. Brownness Itself, as we have said, would have no potentials, and so wouldn’t have the potential to be affected by other things, even other actualities.

So we see that, in the real world, act exists as limited by potency; and it can only be limited by potency. If act were limited by something other than potency, it would have to be limited by act, either itself or some other act. We’ve already seen that an actuality cannot be limited in itself, if it exists apart from all potencies, because it would be purely and absolutely itself. But could it be limited by some other actuality? For instance, if we have some brown object that exists at some particular time and place, isn’t the fact that it exists at this particular time and place an actual feature of the object? So isn’t it the case that the brown object’s being limited to this particular time and place is due to these actualities?

Ultimately, we know that it is impossible for act to limit act because act in itself would not have the passive potential to be affected and limited by some other act. The brownness of the brown object mentioned above is only capable of being limited to a specific time and place because it has first been limited to a specific object, which is due to that object having a potential for brownness that has been actualized. The object, however, only has the potential to be brown at this particular time and place, because it itself only has the potential to exist at this particular time and place. The actuality of brownness is received by a certain potential possessed by an object, and it is this receiving potential that limits the act to the way that it is. On this point Aquinas writes:

Again, every act inhering in another is terminated by that in which it inheres, since what is in another is in it according to the mode of the receiver. Hence, an act that exists in nothing is terminated by nothing. Thus, if whiteness were self-existing, the perfection of whiteness in it would not be terminated so as not to have whatever can be had of the perfection of whiteness” [13].

Summa Contra Gentiles 1.43.5

And further on in the same article he writes: “Moreover, an act is all the more perfect by as much as it has less of potency mixed with it. Hence, every act with which potency is mixed is terminated in its perfection” [14].

Rev. Garrigou-Lagrange gives us several other examples: “The specific form of lion, a form which is indefinitely multipliable, is, by the matter in which it exists, limited to constitute this individual lion, this begotten and corruptible composite . . . .Act determines potency, actualizes potency, but is limited by that same potency. The figure of Apollo actualizes this portion of wax, but is also limited by it, enclosed in it, as content in vessel, and as such is thus no longer multipliable, though it can be multiplied in other portions of wax or marble . . . . Form, act, perfection, precisely by being received into a really containing capacity, is thereby necessarily limited (made captive) by that container” [15].

This brings us to the third point: Where act exists absolutely by itself, it will be entirely unlimited and unique. In our discussion so far we’ve already begun to see why act existing by itself would be entirely unlimited. But it would also be entirely unique. For instance, if there were such a thing as Brownness Itself, there could only be one such thing. Saint Thomas writes:

“Any form whatever, however material and low, if it be set down as abstract either in actual being or in the intellect, remains but one form in one species. For let “whiteness” be understood as something subsisting apart from every subject and it will not be possible to posit many whitenesses, since we see that “this whiteness” does not differ from “that whiteness” save through the fact that it is in this or in that subject. In similar fashion, if there were an abstract “human nature”, there would be but one only” [16].

De Spiritualibus Creaturis, Article 8

Different white things do not differ insofar as they are white, but insofar as they are different things, different subjects. Indeed, insofar as they are white they are all the same, at least formally (qualitatively). Their multiplicity is explained in that the same actuality of whiteness has been received in multiple different subjects with the capacity for that whiteness. So if there were two or more self-subsisting whitenesses, there could be absolutely nothing to distinguish them from each other; they would be identical.

And finally there is the fourth point: Where act exists in a manner that is limited and multiplex, it cannot be existing on its own, but must be existing in composition with some potency. This follows simply and directly from the previous point. If act existing by itself is unlimited and unique, then where act exists as limited and multiplex, it cannot be existing by itself but must be existing in composition with potency. So if we see that many different brown things exist, and if each instantiation of brownness exists in only a limited way, we know that we are not perceiving Brownness Itself but only a finite instantiation of brown in some composite subject which had the potency for receiving brownness.

This then, is the second of the twenty-four Thomistic theses. The third thesis builds from this to a consideration of the difference between God, who is pure act and self-subsisting being and therefore absolutely simple, and finite creatures, who only participate in being and therefore are composite.


[1]. A list with the 24 theses in Latin and English, along with brief commentary and references to passages in Aquinas, can be found here:

[2]. My own translation.

[3]. Klubertanz, Introduction to the Philosophy of Being, second edition, Wipf and Stock Publishers, 2005, page 88.

[4]. Summa Theologiae I, Q. 4, Art. 1. My own translation. Latin taken from:

[5]. Ibid. Translation by the Fathers of the English Domincan Province, accessed online here:

[6]. Ibid.

[7]. Aquinas. De Principiis Naturae. Translated as The Principles of Nature to Brother Sylvester by R. A. Kocourek. Html-edited by Joseph Kenny, O.P. 36. Accessed online:

[8]. Aquinas. Compendium Theologiae. Translated by Cyril Vollert, S.J. St. Louis & London: B. Herder Book Co., 1947. I, 18.1. Accessed online:

[9]. Aquinas. Summa Contra Gentiles. Translated by Anton C. Pegis. 1.28.6. Accessed online:

[10]. Aquinas. Summa Theologiae. 1.4.2. My own translation. Latin accessed here:

[11]. Aquinas. Summa Contra Gentiles. Translated by Anton C. Pegis. 1.28.2. Accessed online:

[12]. Aquinas. Summa Theologiae. 1.3.7. My own translation. Latin accessed here:

[13]. Aquinas. Summa Contra Gentiles. Translated by Anton C. Pegis. 1.43.5.

[14]. Ibid., 1.43.7

[15]. Garrigou-Lagrange, Reality: A Synthesis of Thomistic Thought, translated by Patrick Cummins O. S. B., Ex Fontibus Co., 2015, pages 37-38.

[16]. Aquinas, De Spiritualibus Creaturis. Translated by Mary C. Fitzpatrick and John J. Wellmuth. Milwaukee: Marquette University Press, 1949. Article 8. Accessed online:

6 thoughts on “Defending the 24 Thomistic Theses: The Second Thesis

    1. It’s very similar in a lot of ways. One main difference is that Aristotle and St. Thomas thought these perfections cannot actually exist in the mind-independent world, while Plato thought they did (or at least in some mind-independent realm). In the real world, according to Aristotle and St. Thomas, perfections only exist as limited and mixed with potency in things. As pure perfections, they only exist as mental abstractions.


      1. Perhaps I am encroaching on the next post, but are you saying that Aquinas would define these as mind-dependent abstractions even in relation to God?


      2. Yes, Aquinas calls them “divine ideas.” All forms pre-exist in God’s mind as divine ideas. So, in a sense, one might think of Aquinas as holding to a position somewhat midway between Plato and Aristotle. With Plato he holds that forms exist eternally, but he denies Plato’s belief that the forms exist by themselves, independently. With Aristotle he holds that forms only exist as limited instantiations *in* concrete things, or as abstractions in a mind. But because the divine mind exists eternally, the forms exist also eternally as ideas in the divine mind.


      3. Wouldn’t it also be true that “ideas in the divine mind” is a notional distinction due to our limited understanding? God is love, God is mighty, God is good, etc. He is not a composite of these things; He just IS what we as humans must categorize separately.


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