God as the Ultimate Mover

Over the course of his career, Thomas Aquinas formulated many arguments for God’s existence. The most famous ones are the so-called “five ways”, which appear near the beginning of his Summa theologiae. The purpose of these brief arguments is not to establish the existence of a being with all the traditional divine attributes, but rather to plant a “seed” that will eventually produce the flower of “classical theism” as Aquinas’s account of God’s nature unfolds in the subsequent sections of the Summa.

In this article, we will take a look at the first way – “the argument from motion” – which Aquinas refers to as “the more manifest way” to God. Presumably, he thinks that the argument deserves this label because it starts from a datum that seems exceedingly hard to deny, namely that change (motus) occurs in the world. What Aquinas sets out to prove, however, is not the existence of a being who has kick-started the world’s machinery at some point in time and then left it running on its own fuel. Instead, his aim is to prove that all the changes we presently observe depend at every moment on the continuous causal activity of a changeless being we call God. Aquinas's aim is to prove that all the changes we presently observe depend at every moment on the continuous causal activity of a changeless being we call God. Share on X

The argument has three premises and a conclusion:
1) Some things are moved.
2) Anything that is moved, is moved by something else.
3) A causal series of moved movers cannot regress infinitely.
4) Therefore, there must be a first, unmoved mover (= God).

The secondary literature on this seemingly simple argument is immense, and there are many competing interpretations. What I will do here is to present one interpretation which, in my view, makes the argument rather compelling, provided that one accepts a Thomistic metaphysics.

What is motion? Potency and Act

When speaking about “motion” (motus), what Aquinas seems to have in mind is any kind of change – qualitative, quantitative or local – and he thinks of change in general as the actualization of a potential. In order to understand what this means, we must dwell for a moment on the ideas of “actuality” and “potentiality”. These concepts refer to two different ways in which something can be real. Presently, my skin is actually white. However, I know that if I were to spend a long time in the sun, it would become brown. This means that my skin is potentially brown. Potentials are real features of things, but they are real in a different way than actual properties. Importantly, if something is potentially X in a certain respect, it cannot at the same time be actually X in the same respect. For example, a particular spot of my skin cannot be both potentially brown and actually brown at the same time, since a potential, by definition, is a capacity for change.

Now, it is only in virtue of their actual properties that things in the world can cause other things to change. An actually brown thing, for example, can cause a person to have a visual experience of brown. Merely potential brownness, on the other hand, cannot make anyone experience brown, nor do anything else. This is because potentials are purely passive – they represent the power to be changed rather than the power to change. We may capture this by saying that potentials cannot give what they do not have, and what they do not have is actuality. Only what is actual can actualize something else.

Against the background of this analysis of actuality, potentiality, and change, we can now understand what it means when Aquinas claims that “anything that is moved [changed], is moved by something else”, which is Premise 2 of the argument. Since change is the actualization of a potential, and since potentials by their nature cannot do anything, it follows that a potential cannot actualize itself. It cannot, in other words, move itself from potentiality to actuality. This means that something else must do the job, and this “something else” must be actual in a relevant respect.

Motion and the Law of Non-Contradiction

Aquinas expresses this insight in slightly different terms than I have done here. He says that “it is impossible … that in the same respect and mode something is mover and moved, or that it moves itself”. Let us unpack this claim a bit. In its capacity as “mover”, a thing needs to be actual in some respect, because only what is actual can effect changes. As “moved”, on the other hand, a thing needs to be in potentiality in some respect, because only what has potentiality can be changed. Since nothing can be both in a state of actuality and in a state of potentiality at the same time and in the same respect, it follows that “it is impossible … that in the same respect and mode something is mover and moved”. The respect in which a thing is actually X cannot be exactly the same respect in which the thing is potentially X. Therefore, mover and moved must be non-identical.

The qualification “in the same respect and mode” is very important. Aquinas does not deny the commonsensical truth that – for example – an animal can “move itself” in the sense that it can walk around. However, if we look closer at such cases of purported self-movement, we find that what happens is that the animal is mover in one respect and moved in another respect. For example, it is the animal’s intention that causes the muscles to move, and the muscles cause the legs to move, and so on. What is impossible, however – and what Premise 2 excludes – is that the animal should be mover and moved in exactly the same respect.

No Infinite Regress: Where do “actualized actualizers” come from?

To recapitulate the argument so far: What Premise 1 says is that some things are moved – that is, some potentials are actualized. What Premise 2 says is that potentials can only be actualized by something that already has actuality in an appropriate respect. This means that potentials (which as such do not have any actuality at all) cannot actualize themselves. Now, what we are looking for is a complete explanation of the initial datum expressed by Premise 1. In other words, we want to know what ultimately explains the fact that change occurs in the world. This means that we cannot stop our inquiry as soon as we have discovered some mundane cause or causes that can actualize potentials. Instead, we must continue the inquiry by asking: From where do these mundane or worldly causes have their own actuality, in virtue of which they can change things?

There are two possible answers to this question. Either an “actualizer” – a cause that can actualize some potential – has received its own actuality from another cause, or it has the relevant actuality simply in virtue of what it is – its essence. In the first case, we must continue the inquiry and ask from where the previous “actualizer” has gotten its actuality, and so on. We hence embark on a causal regress, a series of “actualized actualizers” that have received their actuality from previous actualizers and that pass actuality on to subsequent links in the chain. Can a causal chain of this kind extend backwards to infinity? Premise 3 says that it cannot: “A causal series of moved movers [that is, “actualized actualizers”] cannot regress infinitely”.

Why not? The gist of Aquinas’s answer (which I here formulate in my own terms) is that the existence of things that have actuality in a dependent way cannot be fully explained except by reference to something that has actuality in a non-dependent way. Aquinas illustrates this principle by pointing to a mundane type of causal chain. Imagine a person who holds a stick in his hand and uses it to move a stone. In this scenario, the stick’s movement is the cause of the stone’s movement, but the stick only moves the stone because it in turn is moved by the person. The stick is hence a dependent mover – it does not have the power to move by itself but needs to receive the actuality of motion from somewhere else. A causal chain of this kind cannot regress backwards to infinity. It could not be the case, in other words, that the stick is moved by another stick, which in turn is moved by another stick, and so on ad infinitum. Since sticks do not have the actuality of motion in themselves – they are dependent movers – their motion must either remain unexplained or ultimately be explained by reference to a first link in the causal chain, which has the power to move in an independent way.

The lesson we learn from this example can be generalized and applied to the transmission of all forms of actuality, not just the actuality of motion. The most basic or fundamental form of actuality that a thing needs to have in order to be able to actualize potentials in other things is existence. The most basic or fundamental form of actuality that a thing needs to have in order to be able to actualize potentials in other things is existence. Share on X According to Aquinas, existence is the preeminent form of actuality – he calls existence “the act of all acts” – and things that do not exist obviously cannot actualize any potentials at all. This means that a full explanation of why there is change in the world – why potentials are actualized at all – requires us to posit a cause that has existential actuality in a wholly independent way. The causal chain of things that depend on other things for existential actuality cannot regress backwards to infinity. Instead, there must be a first cause whose inherent actuality explains the dependent actuality of the other members. Aquinas, following Aristotle, calls this first cause an “unmoved mover”. Perhaps a better name would be “unactualized actualizer”.

The Unmoved Mover as Pure-Act

What must an “unactualized actualizer” be like? According to Aquinas, it must be “pure actuality”. Only a being whose essence is identical to actuality as such can be actual in a wholly independent way. We can see that this claim is true (given a Thomistic metaphysics) by the following reasoning: Either an essence is identical to actuality or it is not. If it is not, then the essence in question must be actualized by something else. This means that the essence is an “actualized actualizer” and not the first cause of actuality in other things. Hence, only an essence that is identical to actuality can be a wholly independent cause of actuality in other things – an “unactualized actualizer” or “unmoved mover”.

According to Aquinas, such a being is “what everyone understands to be God”. The first way does not give us all these divine attributes explicitly, but it arguably contains them all in seed form. If we think carefully enough about what it means to be “pure actuality”, we will realize that it means to be God. Share on X In the context of the first way, Aquinas does not attempt to justify this identification, which may at first glance appear very controversial. In order to learn why Aquinas thinks that the unmoved mover must necessarily be God, we need to keep on reading in the Summa. If we do so, Aquinas will explain that since an unmoved mover is “pure actuality” (actus purus) and therefore lacks all potentiality, such a being must be unchangeable, eternal, non-material, non-composite, and unique. It must also be all-powerful, since it is the ultimate cause of the actualization of every potential that has been or ever will be actualized. The fact that actus purus itself has no unrealized potentials means that it is perfect and contains in some manner all the perfections we find in the world, including personal perfections such as intellect and will. This means that the unmoved mover must have intellect and will in a preeminent sense. The first way does not give us all these divine attributes explicitly, but it arguably contains them all in seed form. If we think carefully enough about what it means to be “pure actuality”, we will realize that it means to be God.

Image Credit: Werner Wittersheim | Flickr