Aristotle's Unmoved Mover and Celestial Motion

Hello! Welcome to the third module of our course.

Introduction

In our last lesson, we formalized the logical structure of the argument against an infinite regress of per se causes. We established that such a regress is logically impossible because a chain of purely instrumental causes, with no primary cause, has no power to produce an effect. This argument is a crucial tool in Aristotle's philosophical toolkit.

Today, we put that tool to work. We will reconstruct Aristotle's first major argument for a First Mover, as presented in his Physics. Specifically, our learning outcome is to reconstruct the argument from Physics VII-VIII for the necessity of a first unmoved mover to explain celestial motion.

We will trace Aristotle's line of reasoning, starting from the empirical observation of motion in the universe. We will see how he combines his analysis of motion with the logical principle we studied last time to arrive at a remarkable conclusion about the ultimate cause of the cosmos's eternal movement. This lesson focuses on the physical argument; in the next, we will explore the more abstract metaphysical version from his Metaphysics.

1. The Starting Point: An Eternal Universe

A common misconception is that the argument for a First Mover is about a cause that kicks things off at the beginning of time. For Aristotle, this is precisely the wrong way to think about it. His argument is not about a temporal beginning. In fact, he argued that motion, time, and the world itself are eternal—they have no beginning and no end.

This is a crucial premise. If motion is eternal, then the question is not "What started it?" but "What sustains it?". This shifts the focus from a temporal sequence of causes (per accidens) to a hierarchical series of simultaneous, sustaining causes (per se), which was the subject of our previous lessons.

Let's begin by reading a short text that outlines Aristotle's starting position on the eternality of motion and his method of inquiry in the Physics.

2. The Causal Principle and the Regress

Having established that motion is an eternal feature of the cosmos, Aristotle's next step is to analyze the mechanics of that motion. He begins with the principle we explored in Module 2: whatever is in motion is moved by something else.

Aristotle defends this by analyzing different kinds of moving things.

• Inanimate objects are clearly moved by external forces. A rock only moves if thrown or pushed.
• Living things (animals) appear to move themselves. However, Aristotle argues that even in a self-mover, we can distinguish between the part that initiates motion (the soul) and the part that is moved (the body). Furthermore, the animal as a whole is prompted to move by its environment (e.g., seeing food).

So, for any given motion, there is a mover. That mover is either itself moved or it is not. If it is moved, it too must have a mover. This initiates a causal chain. As we are talking about the cause sustaining the motion right now, this is a per se series.

The next reading selection details this step and introduces the necessity of a prime mover that is absolutely unmoved—not even accidentally.

From this, we can see the application of the logic from our last lesson. A chain of movers sustaining motion here and now is a hierarchical, per se series. Such a series cannot regress to infinity. It must terminate in a first member that moves other things but is not, itself, moved by anything. This is the Unmoved Mover.

3. The Effect: Eternal Circular Motion

We have established that the eternal motion of the cosmos requires an Unmoved Mover. But what, precisely, is this eternal motion? Aristotle observes various types of change in the world:

1. Substantial change: Generation and corruption (coming into being and passing away).
2. Quantitative change: Growth and decay.
3. Qualitative change: Alteration (e.g., a leaf changing color).
4. Locomotion: Change of place.

Aristotle argues that locomotion is the most fundamental type of change, as the others presuppose it (for agents to interact, they must come together).

He then asks: what kind of locomotion can be eternal and continuous?

• Rectilinear (straight-line) motion cannot be eternal. An object moving in a straight line must either travel infinitely (which Aristotle considered impossible within a finite cosmos) or it must stop and reverse direction. Reversing direction involves a stop and a change to a contrary motion, so it is not a single, continuous motion.
• Circular motion is the only candidate. An object moving in a circle can do so continuously, without beginning, end, or reversal. It perpetually returns to its starting point without ever ceasing its motion.

Aristotle identified this perfect, eternal motion with the observed revolutions of the celestial spheres—the "first heaven." Therefore, the primary task of the Unmoved Mover is to explain the eternal, circular motion of the outermost celestial sphere.

The next reading details this process of elimination.

4. Deducing the Mover's Properties

The argument has now established that an Unmoved Mover must exist to cause the eternal circular motion of the heavens. The final step in the Physics argument is to deduce the essential properties of this mover based on the effect it produces. The reasoning is as follows:

1. The Mover must cause motion for an infinite time. (Since the motion is eternal).
2. A finite power cannot produce an effect for an infinite time. A finite power would be exhausted. Therefore, the Mover must have infinite power.
3. An object with finite magnitude cannot possess infinite power.
4. An object with infinite magnitude is impossible. (Aristotle had separate arguments for why an actual infinite body cannot exist).
5. From (3) and (4), we have a dilemma. The Mover cannot have finite magnitude, and it cannot have infinite magnitude. The only remaining possibility is that it has no magnitude at all.
6. Something with no magnitude is indivisible and has no parts.

Thus, from the starting point of observed motion, Aristotle deduces that its ultimate cause must be an eternal, indivisible, immaterial being of infinite power.

This final reading from our resource lays out this last stage of the proof from Physics VIII 10.

Conclusion

Today we have reconstructed the argument for a First Unmoved Mover as presented in Aristotle's Physics. By applying the logical prohibition of an infinite per se regress to the specific problem of explaining eternal cosmic motion, Aristotle builds a powerful case.

Key Takeaways:

• The argument in the Physics is designed to explain the sustaining cause of eternal motion, not a temporal first cause of creation.
• It proceeds in steps:
  1. Establishing that motion is eternal.
  2. Applying the principle that every moving thing requires a mover, which leads to a causal series.
  3. Arguing this per se series must terminate in an Unmoved Mover.
  4. Identifying the primary eternal motion as the circular locomotion of the celestial spheres.
  5. Deducing that the Mover of this motion must have infinite power and therefore be without magnitude (i.e., be immaterial).

Preview of the Next Lesson:

The argument from the Physics is powerful, but it leaves a key question unanswered: how does an immaterial, unmoved being cause physical motion? The Physics establishes it as an efficient cause, but the mechanism is unclear. In our next lesson, we will turn to Aristotle's Metaphysics, Book XII (Lambda), where he presents a more profound and general version of the argument. There, we will see the Unmoved Mover's role shift from an efficient cause to a final cause—an object of desire and intellect that moves the world by attracting it.