Aristotle's Argument Against Infinite Concurrent Movers

Hello! Welcome back to our exploration of Aristotle's argument for an unmoved mover.

Introduction

In our last lesson, we established the crucial distinction between two types of causal series: the temporal, linear series (per accidens) and the simultaneous, hierarchical series (per se). We concluded that Aristotle's argument against an infinite regress of movers applies only to per se series—those responsible for sustaining a change or motion at this very moment. You can think of a per accidens series as explaining how a system came to be, while a per se series explains how that system is operating right now.

Today, we will reconstruct the core of that argument. Our learning outcome is to reconstruct Aristotle's argument for why a per se series of concurrent movers cannot be infinite. We will move from the intuitive examples of hands, sticks, and stones to the metaphysical principle that makes an infinite regress in such a series impossible. This will lay the final piece of groundwork before we can assemble the full argument for a first mover.

1. The Nature of Causality in a Per Se Series

Let's begin by deepening our understanding of the causal relationships within a per se series. The key concept is that the intermediate members are not just causes, but instrumental causes. Their power to cause is not their own; it is derived and dependent.

The philosopher Edward Feser provides a very clear explanation of this concept. Please watch the following segment from a discussion of his work.

As Feser explains, the argument hinges on the nature of the causality being exercised.

• The stick moves the stone, but its power to do so is entirely derived from the hand. It has secondary or derivative causal power. By itself, it is inert. It functions as a pure instrument.
• The hand (or more accurately, the person) moves the stick. It has primary or innate causal power. It does not need to be "picked up and moved" by something else in the same way the stick does to impart motion.

The crucial insight is that a per se series is, by definition, a chain of instrumental causes, where each member's ability to act as a cause is contingent upon the simultaneous action of the member before it.

2. Why the Chain of Instruments Cannot Be Infinite

Now we can assemble the argument. If we have a per se series causing an effect right now (e.g., a stone is moving), what can we deduce about the structure of that series?

The core logic is this:

1. In a per se series, any intermediate mover is an instrumental cause.
2. An instrumental cause has only derivative power; it causes only insofar as it is being caused to cause by a prior member of the series.
3. Therefore, if every member of the series were an instrumental cause, the entire series would have no causal power of its own. It would be a chain of pure passivity.
4. A chain with no causal power cannot produce an effect.
5. But we observe an effect (the stone is moving).
6. Therefore, the series cannot be composed only of instrumental causes. It cannot be an infinite chain of instruments.

The series must terminate in a cause that is not merely instrumental. It must end with a member that has primary, non-derived causal power—a "first" mover for that specific series.

This short video provides another excellent analogy using a swinging pocket watch to illustrate this exact point of dependency.

The watch chain is a perfect illustration of a per se series. Each link moves the next, but only because it is itself being moved. If the chain were infinite, with no hand to initiate and sustain the motion, the watch would be still. An infinite series of moved movers, with no prime mover, explains nothing. It is equivalent to saying the watch is moving because the last link is moving, which is moving because the link before it is moving, and so on, forever. This is not an explanation; it is an infinite deferral of an explanation.

3. Reconciling with an Eternal Universe

A common point of confusion is how Aristotle could argue against an infinite causal regress while simultaneously believing the universe was eternal, with no beginning in time. Your background in formal systems likely makes you sensitive to such apparent contradictions.

The resolution, as we've been building up to, lies in the distinction between the two types of series.

The article frames the problem perfectly. The solution, as later commentators clarified, is that:

• An infinite per accidens series (like a genealogy stretching into an infinite past) is possible. The causal power is passed on and becomes innate to the new member. A son's ability to procreate doesn't depend on his father being alive and actively procreating.
• An infinite per se series (a hierarchical stack of simultaneous, instrumental causes) is impossible. The causal power is merely borrowed. An infinite chain of borrowers with no original lender has nothing to lend.

The argument against infinite regress is not about a temporal beginning. It is about a hierarchical foundation for causation happening now. The first mover of a per se series is not the first domino in a temporal chain; it is the engine that is pulling the entire train of cars at this moment.

4. Aristotle's Physical Argument in Physics VIII

The argument we have reconstructed is primarily metaphysical, based on the nature of causality itself. It's worth noting that Aristotle also presents more physically-grounded versions of this argument. For instance, in Physics Book VIII, he argues that an infinite series of moved movers that are physical bodies is impossible.

He reasons that if the series of bodies is infinite and they are all moving each other simultaneously, they would have to compose a single, infinitely large or infinitely complex body. Aristotle had separate arguments (which we won't delve into here) for why such an entity is metaphysically impossible.

The handout below contains a key passage from Physics VIII where Aristotle makes a more conceptual version of this argument, distinguishing between instruments of motion and the source of motion.

In this passage, Aristotle identifies three categories:

1. The "last thing," which can be moved but has no power to move (the stone).
2. Things that are moved by themselves (a complex topic, but think of animals).
3. The necessary third thing: that which causes motion but is itself unmoved.

He argues it is "reasonable, not to say necessary," to posit this third class. Why? Because if you only have things that are moved by another (instruments), you can't explain where the motion originates.

Conclusion

We have now reconstructed the core of Aristotle's reasoning against an infinite regress of concurrent movers. The argument is subtle but powerful, and it rests entirely on the nature of the causal series in question.

Key Takeaways:

• The argument applies exclusively to per se (hierarchical, simultaneous) causal series.
• The intermediate members of a per se series are instrumental causes with purely derivative causal power.
• An infinite series composed entirely of instrumental causes is a metaphysical impossibility, as it would contain no actual source of causal power and thus could not produce an effect.
• Therefore, any per se series that is producing an effect now must terminate in a first mover—a cause that has primary, non-derived power.
• This "first" mover is first in the hierarchy of causation, not first in a temporal sequence.

Preview of the Next Lesson:

Having reconstructed the argument's content and reasoning, our next step will be to analyze its formal properties. We will identify the logical structure of Aristotle's argument against an infinite regress of movers, breaking it down into its precise premises and conclusion to evaluate its logical validity.