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Argument from Motion for God’s Existence

A step-by-step account of Aquinas’s first of five ways for proving God’s existence

Atheists and agnostics often try to justify their lack of belief in God by claiming there is no evidence for him. But for great thinkers such as St. Thomas Aquinas, the evidence is as plain as the motion we experience in our everyday lives. The first of Aquinas’s famous five ways for proving God’s existence in his Summa Theologiae uses motion as a starting point.

Although motion itself is plain and evident to our experience, the steps required to reason from motion to God’s existence are not. So, let’s unpack them. Since the argument uses motion as a starting point, we have to reflect on what motion is.

Understanding motion

For philosophers, there are various kinds of motion, which also is understood as change. There is local motion, such as when a stone moves from one place to another. This is what we normally think of when we think of motion.

There’s also qualitative motion, such as when wood becomes hot, or when the water temperature in a kettle on the stove rises. Quantitative motion is when something loses or gains quantity, such as when our bodies gain or lose weight.

The argument from motion can work with any type of motion, but for the sake of keeping it simple, we’re going to stick with local motion as our starting point. As to what motion is in and of itself—at least, the motion we experience in the physical world—it’s essentially the progressive actualization of a potential. Sounds complicated, but let’s look at it.

Consider a stone. As it rests on the ground in front of my left foot, it has the potential to rest in front of my right foot. If the stone were moved from my left foot to my right foot, then we’d say that the stone’s potential to be in front of my right foot was actualized.

Now, the stone isn’t in motion when it’s resting on the ground in front of my left foot. It’s not in motion when it’s resting in front of my right foot. The stone’s motion is the in-between stage of the stone’s potential to be in front of my right foot and the stone actually being in front of my right foot. Some of its potential is actualized, but there is yet still some potential to be actualized—it hasn’t reached the end point of its motion yet.

So, with Aquinas, who is following Aristotle, we can say that motion is the actualization of a potential. But, at least for motion in the physical world, we can add that the actualization of the potential is progressive, or gradual—that’s to say, it’s an actualization of a potential that’s ordered to further actualization. Or to put it simply, as we said a moment ago, it’s the in-between stage.

Motion requires a mover

With this understanding of motion, we can now consider that for Aquinas and supporters of this argument, at every moment something is in motion it must have its potential actualized by some actualizer outside itself. In other words, it can be in a state of motion only insofar as something outside itself is moving it.

Some philosophers see this principle as metaphysically necessary. Here’s one way they argue to support it.

Consider the stone as it moves from the front of my left foot to the front of my right foot. At every moment in the process, whether at the beginning when its potential is first actualized or during its journey toward my right foot, the stone acquires something new.

Because every time the stone’s potential to be put in a new spatial location is made actual, the stone acquires a new actual state, or a new reality about itself.

Suppose we freeze-frame the stone as it’s moving and call the space that it’s occupying space A. We would say that the stone has the potential to occupy another space, B.

We unfreeze the frame to let the stone move to space B and then freeze it again. We would now say that the stone’s potential to occupy space B was made actual—the stone acquired a new actual state or reality. This happens at every moment that the stone is in the process of moving to the front of my right foot.

Now, every new reality that the stone acquires while in motion will be due either to the stone itself, to nothing, or to a mover outside the stone. These are the only possibilities.

Not the stone itself

For Aquinas, and many others who’ve followed him, the motion of the stone can’t be due to the stone itself. The reason is because this entails a contradiction.

Consider that inasmuch as the stone is acquiring something new—namely, being located in space B—it must have been in potential to acquire that new reality. But if the stone itself were to account for the new reality, it would have to already have the new reality, in which case it wouldn’t be in potential to acquiring the new reality.

Therefore, to say that the stone itself accounts for the new reality acquired as it’s in motion would be to say that the stone is in potential to acquiring the new reality and not in potential to acquiring the new reality at the same time and in the same respect.

Since we can’t accept a contradiction, we must say that the stone’s new reality, or the actualization of the stone’s potential to be in front of my foot, whether in the beginning or while it’s on the journey, can’t be accounted for by the stone itself.

Nothing to do with nothing

Perhaps the new reality that the stone acquires at each moment in the process is due to nothing. The actualization of its potential might be just a brute fact. What do philosophers make of this?

Well, for starters, we could say that in this brute-fact scenario there would be nothing to make the potential of the stone to move to the front of my right foot different than what it is, in which case the potential would remain what it is—namely, potentiality and not actuality. But if that were true, then the stone wouldn’t be in motion, which is contrary to fact: the stone is moving.

We intuitively recognize that there must be something to make the potential different than what it is. But perhaps we can go a bit further.

Consider that if there were nothing to account for the stone’s motion, then there would be nothing to distinguish the state of the stone having its potential to move to the front of my right foot actualized and the state of the stone not having its potential to move to the front of my right foot actualized. In other words, there would be nothing to distinguish between what’s real and what’s not real.

But if there were nothing to distinguish between what’s real and what’s not real, then we would end up having to say what’s real (being) is identical to what’s not real (nonbeing), which is absurd.

Therefore, it can’t be that nothing accounts for why the stone’s potential to acquire something new, like occupying space B, is actualized. To put it simply, it can’t be that nothing accounts for why the stone is in the process of moving to the front of my right foot.

Since it’s impossible for the stone itself to account for every new reality that the stone acquires (occupying space B, C, D, etc.), and it’s impossible that nothing account for the stone acquiring its new reality, it necessarily follows that a mover outside the stone must account for the new reality that the stone acquires.

Therefore, whatever is in motion must have its potential actualized by some actualizer outside itself at every moment its potential is being actualized. In other words, it can only be in a state of motion insofar as something outside itself is moving it at every moment it’s in motion.

The nature of the mover

Now we need to think about the nature of the mover, or movers, that account(s) for the motion under consideration. For the purposes of our example, which involves the motion of a stone, let’s suppose the mover is a stick wielded by a man’s hand.

What we notice about the stick is that its act of being a mover is entirely derived from the hand. It can’t begin or continue to actualize the stone’s potential to move to the front of my right foot without the hand acting as a mover. At every moment the stick is moving the stone, or actualizing the stone’s potential, the stick requires the action of the hand to actualize its potential. Consequently, the stick can’t sufficiently account for why the stone is in motion rather than not.

The same is true for the hand. At every moment the hand moves the stick, or actualizes the stick’s potential to move the stone, it derives from the arm its act of being a mover.

Now, we have to ask an important question, “Can every mover in this type of series be like the hand and the stick?” In other words, can each mover in the series be such that at every moment it’s acting as a mover its power to move something else is derived from another?

Many philosophers answer no. And I think they’re right. Here’s how their reasoning goes.

They start with this idea: any mover that derives its power to move something else is a mover that doesn’t have its moving power within itself (by virtue of its own essence).

Take the stick again, for example. It can’t be a mover insofar as it’s a stick. In other words, the power to act as a mover is not identical to its essence—such power is separate or distinct. The stick is not a mover by virtue of the kind of thing it is. The stick is such that it’s in potential to receiving the power to act as a mover, but it’s not the moving power itself.

Now, if every mover in the series were such that its power to act as a mover was separate or distinct from its essence, and thus had to receive such power from an outside mover (such as the stick and the hand), then there would be no source from which the movers in the series could derive their power to act as a mover. Such a series of movers would be like a series of interlinked train cars without an engine. Because no train car has within itself the power to move itself or another car, there would be no source from which the cars could ever receive their power to act as movers.

If there were no source from which the movers in the series of movers moving the stone could derive their power to cause motion, then there would be nothing to account for why they are acting as movers rather than not, nothing to distinguish the real fact that they have their power to act as movers from the nonreal fact of the movers in the series not having their power to act as movers.

So the question is, “Can it be such that there’s nothing to account for why the movers in the series are acting as movers rather than not?”

As we reasoned above, if there were nothing to account for why one state of affairs is real rather than not, then we would have to say what’s real (being) is identical to what’s not real (non-being), which is absurd.

Therefore, there must exist a source from which the movers of the stone’s motion receive their power to act as movers.

From this it follows that there must exist at least one mover of the stone’s motion (as well as the moving power of every other mover of the stone’s motion) that has within itself its power to act as a mover—that’s to say, it has its moving power by virtue of its own essence. For such a mover, its power to move things is identical to its essence and thus has it in an underived or uncaused way. This type of mover is a first mover, what we could call God.

Pure actuality

Now, there’s one more thing we need to consider to complete our argument. We need to show that such a first mover has no potential to be actualized whatsoever and is pure act, or pure actuality, or, yet again, pure acting power.

So far, we’ve shown that the activity by which our first mover moves the stone is had by virtue of its own essence, and therefore it has it in an underived way.

But someone may object that even though the action by which our first mover ultimately moves the stone is had by essence and therefore in an underived way, it may have some other activity that is derived and thus not had by virtue of its own essence. On this scenario, such a being wouldn’t be purely actual (or pure acting power) because it would have a potential to that other activity.

How do we respond?

First, we have to understand yet another principle of philosophy, articulated by St. Thomas Aquinas: agere sequitur esse, or “action follows being” (Summa Contra Gentiles, 3.69). Something acts according to the way it exists. The mode of existence determines the mode of action.

For example, if something exists as a fish, it’s going have the capacity to breathe underwater. It’s not going to be able to think and love. If something exists as a human, it’s going to be able to think and love, but it’s not going to be able to breathe underwater (that’s to say, by its own powers, barring any equipment such as an oxygen tank).

With this principle in place, we can reason as follows: if the existence of our first mover were caused to exist, and the mode of existence determines the mode of action, then the activity by which our first mover moves the stone would be caused. You can’t have an action that is not caused and yet at the same time has existence from which the action flows that is caused.

But as we’ve already shown, the activity by which our first mover moves the stone is not caused, since it has its power to act as a mover by virtue of its own essence. Therefore, we must conclude that the existence of our first mover is not caused.

Now, whatever has existence that’s uncaused has no action whatsoever that is or can be caused. Remember, the mode of existence determines the mode of action. If the first mover’s existence is uncaused, then every action it performs must be uncaused.

We said our first mover has existence that’s uncaused. Therefore, our first mover has no activity whatsoever that is or can be caused, which means it has no potential to any action.

And for a being to have no action that’s in potential to being caused, or actualized by another, is to be purely actual, pure actuality void of any potentiality that could be actualized, pure acting power itself. That’s our first mover. And a purely actual being that’s ultimately responsible for every mover in a series having its power to act as a mover is what we call God.

Now, in order to say, “This first mover is God,” more work has to be done in unpacking what God as the purely actual mover entails. We would need to show that there can only be one such purely actual being, and that such a being is immaterial, eternal, omniscient, omnipotent, and so forth.

All these divine attributes are implied in the very idea of pure actuality. But that goes beyond the scope of this article.

Suffice it to say here that the argument from motion provides the evidence that atheists and agnostics are looking for. It’s evidence that’s right in front of their faces. They may just require a bit of help to see it.

Sidebar: The Argument in a Nutshell

For those who need to see things in a clear form, here is the syllogism that summarizes the argument from motion:

P1: At every moment something is in motion it must have its potential concurrently actualized by some actualizer, or mover, outside itself.

P2: The stone is in motion.

C1: Therefore, at every moment the stone is in motion it must have its potential concurrently actualized by some mover outside itself. Call it Mover 1.

P3: Mover 1 is either going to have its act of being a mover of itself (by virtue of its own essence), in which case it doesn’t have to derive its act of being a mover, or not.

P4: If Mover 1 has its act of being a mover of itself (and thus has it in a nonderived way and is purely actual), then we could call such a first mover God.

P5: If Mover 1 doesn’t have its act of being a mover of itself, then it must receive that act of being a mover from a mover outside itself. Call that mover Mover 2.

P6: The series of movers that accounts for the stone’s motion at every moment the stone is in motion either has a mover such that it has its act of being a mover by virtue of its own essence, or not.

P7: It can’t be that every mover in the series of movers that accounts for the stone’s motion at every moment it’s in motion is such that it doesn’t have its act of being a mover by virtue of its own essence.

C2: Therefore, there must exists at least one mover in the series of movers that accounts for the stone’s motion at every moment it’s in motion that has its act of being a mover of itself (and thus is uncaused in its act of being a mover and is purely actual), whether that mover is the immediate mover of the stone’s motion, Mover 1, or a remote primary mover in a series of movers that act simultaneously to account for why the stone is in motion rather than not.

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