What is Existence? Aquinas’s Argument for Existence as Act

The question of what existence is/means is a controversial one in philosophy. In the last few centuries, the debate has largely centered around the question of whether existence is a predicate or property, especially in relation to ontological arguments for the existence of God. Kant famously denied that existence is a predicate, arguing that it adds nothing determinate to a thing.

In the entire history of philosophy, St. Thomas Aquinas’s position on existence is rather unique. Aquinas held that existence is an act of things, what he calls the actus essendi (“act of being”). Here is a good overview of his position:

“Existence [esse] itself is the most perfect of all things, for it is compared to all things as act [actus]. For nothing has actuality, except inasmuch as it is. Hence existence itself is the actuality of all things, and even of forms themselves. Hence it is not compared to others things as the receiver to the received; but more as the received to the receiver. For when I speak of the existence of man, or horse, or whatever else, existence itself is considered as formal and received, not however as that to which existence happens” [1].

Saint Thomas’s inclusion of et etiam ipsarum formarum (“and even of form themselves”) was arguably philosophically revolutionary, as it signifies a departure from (or development of) Aristotle’s position that it is form which is the fundamental actuality of things. Here Saint Thomas suggests that even form must have a further actuality: existence.

But what is the reason for this position? In this post I will try to explicate and develop the line of argumentation contained in the above passage. The conclusion of this passage, i.e. what Saint Thomas is trying to establish overall, is that existence is a perfection and not an imperfection. The passage is the reply to the third objection in Summa Theologiae I, Q. 4, Art. 1. This article considers utrum Deus sit perfectus, “whether God is perfect?” The third objection argues that God is not perfect, because in Q. 3, Art 4 it had already been concluded that the essence of God is existence itself, but “existence itself seems to be most imperfect, since it is most common, and is receiving of all additions.” Saint Thomas responds, as we have seen, by stating that far from being most imperfect, existence is in fact perfectissimum omnium, “the most perfect of all things.” The explicit justification of this assertion is that existence is analogous to act; and in the body of the article Aquinas had explained that “something is perfect according as it is in act.” In other words, to the extent that something is actual, it is perfect. Since existence is a kind of act, it is a kind of perfection. Indeed, he goes on, existence is the most fundamental act, the actuality of all things; and such merits for it the title of perfectissimum.

The above argument is clearly valid: if existence is an act, and if act is perfection, then existence is perfection. But what we are interested in is the first premise of this argument: why think that existence is an act? The heart of Saint Thomas’s reasoning is contained in a single line: nihil enim habet actualitatem nisi inquantum est. Nothing has actuality except inasmuch as it is.

Of course, this line on its own is not a complete argument; there must be some hidden implicit premise(s). The argument as it stands now seems to be something like this:

  1. If nothing has actuality except inasmuch as it is [exists], then existence is an act
  2. Nothing has actuality except inasmuch as it is
  3. Therefore, existence is an act

But what is the justification for the first premise? How does it follow from nothing having actuality except inasmuch as it is, that existence is an act? To answer this, we should perhaps take a step back and examine what Saint Thomas means by act in the first place.

In his commentary on Aristotle’s Metaphysics, Saint Thomas explains that actus is incapable of being properly defined, since it is such a fundamental notion. What can be done, however, is to point out inductively examples of actus. This can especially be done by comparing actus with potentia, since these are “proportional” terms and are defined in relation to each other. So Saint Thomas writes: “Thus proportionally from particular examples we are able to come to recognize what actus and potentia are” [2]. Some examples Saint Thomas (following Aristotle) uses are: one who is building is to actus as one who is capable of building is to potentia; one who is awake is to actus as one who is asleep is to potentia; one who sees is to actus as one whose eyes are closed but has the power of sight is to potentia; carved wood is to actus as uncarved wood is to potentia.

Because of their proportion to each other, Aristotle “defines” actus as “the existence of a thing [but] not in the sense in which we say that a thing exists potentially” [3]. Aquinas renders this as follows: “[Actus is] when a thing is, but is not however thus as when it is in potentia” [4]. In his De Principiis Naturae, Saint Thomas states in the opening lines: “That which is able to be is said to be in potency; that which is now is said to be in act” [5]. So we might say: (1) S is actually P [or is P in act] if and only if S is now P, and is not now potentially P.

But Aquinas’s account runs deeper. For Saint Thomas, actus and potentia are real, intrinsic principles. Actus is that principle by which S is actually P. Potentia is that principle by which S is potentially P. Take, for instance, whistling. Suppose at time t I am actually whistling. What makes it the case that I am actually whistling is the very act of whistling itself, i.e. the act of making a small hole with my lips and forcing air through them. If an apple is actually red, then redness (or the act of redness) is that by which it is red. (Of course, we don’t normally think or speak of redness as an “act.” Another way to express it could be “actuality.” Redness is the actuality by which the apple is actually red).

On this basis, we can say that (2) A is an act (or an actuality) if and only if A is that by which S is actually P (or A is that which makes S actually P). With this understanding of actus, we can see why, for Saint Thomas, existence is an act. If, as he states, nothing has actuality except inasmuch as it exists, then existence is that by which any actuality is actual. Just as the actuality of redness is that by which the apple is actually red, so existence is that by which the actuality of redness itself actually is. Existence is that by which I actually am, and you are. Hence “existence itself is the actuality of all things, and even of forms themselves.” Existence is the act by which all acts act.

This, then, is one argument from the Thomistic corpus for the existence-as-act thesis. It is certainly not the only argument; and in future posts I may explicate others, as well as engage with positions such as that of Kant.


[1]. Summa Theologiae I, Q. 4, Art. 1. Translation my own.

[2]. Commentary on the Metaphysics, Book IX, 5, 1827

[3]. Metaphysics, IX, Chapter 6

[4]. Commentary on the Metaphysics, Book IX, 5, 1825

[5]. De Principiis Naturae 1

7 thoughts on “What is Existence? Aquinas’s Argument for Existence as Act

  1. This was a great article. I recently watched Joe Schmid’s critique of the First way and in his critique he says that the terminus of a per se series could receive its casual power from a member in a per accidens chain, which is true. In the context of the First way, however, the casual power operative in the per se series is act, esse, so the per se chain would terminate in pure esse. Have you seen that video?

    Liked by 1 person

    1. I haven’t watched that video from Joe yet, though I’ve read some of his other critiques of the first way. I actually disagree with Joe that any per se causal chain could terminate in a first member that is derivative on a per accidens series (at least if we’re talking efficient causality). The reason being that I think *every* being that is not pure esse is necessarily per se dependent on a sustaining cause, so the first member of a per se chain by definition would have to be pure esse.

      Liked by 1 person

      1. I see. I think what Joe is getting at is that there could be a per se chain where the type of causation is not esse and in that scenario the terminus of the per se chain could receive its casual power from a member of a per accidens chain, but I definitely see your point. Also, what do you think of his work promoting existential inertia? Do you know of any responses or readings to combat his theory?


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