The Principle of Motion: "Whatever is Moved is Moved by Another"

Hello! Welcome back to our course on Aristotle's theory of the immovable mover.

Introduction

In our previous lessons, we established Aristotle's foundational metaphysical concepts. We defined change (kinēsis) as the actualization of a potentiality, as such, and we categorized the different types of change: generation/corruption (of substance), alteration (of quality), increase/diminution (of quantity), and locomotion (of place).

Today, we move from defining change to analyzing its fundamental causal principle. This lesson addresses one of the most crucial premises in Aristotle's entire argument: the principle that 'whatever is moved is moved by another' (omne quod movetur ab alio movetur). This isn't just an empirical observation for Aristotle; it's a metaphysical necessity. Understanding its justification is the key that unlocks his argument against an infinite regress of movers, which we will explore in subsequent lessons.

Our goal is to analyze this principle, understand its logical foundation, and examine how it withstands significant objections, particularly from the perspective of modern physics.

1. The Metaphysical Foundation of the Principle

Why did Aristotle believe that nothing can move itself? The answer lies in the concepts of potentiality (dynamis) and actuality (energeia) that we've already covered. The argument is a direct logical consequence of these ideas.

Let's begin with a clear explanation from philosopher Edward Feser, who will connect the actuality/potentiality framework directly to this causal principle.

Feser's explanation highlights the core intuition: a mere potentiality is, in that respect, a privation. It lacks the actuality it could have. To cause something is an action, and action requires actuality. Therefore, a potential cannot cause its own actualization.

We can formalize this argument, as Aquinas does in his Summa Theologiae, drawing directly from Aristotle's Physics. For this, let's turn to a more technical reading.

Let's break down the logical structure you just read. It's a deductive argument that should be familiar in its form, given your background in formal reasoning.

1. To be moved is to be in the process of changing, which means going from a state of potentiality to a state of actuality with respect to some property (e.g., location, temperature).
2. To move (or to cause change) is to be an agent that brings something from potentiality to actuality. To do this, the mover must already possess the actuality it is imparting (or an analogous actuality). For example, a fire, which is actually hot, makes wood, which is potentially hot, become actually hot.
3. A crucial premise: It is impossible for the same thing to be in actuality and potentiality in the same respect at the same time. A thing cannot simultaneously be actually hot and merely potentially hot.
4. Therefore, it is impossible for a thing to be both mover and moved in the same respect. It cannot actualize its own potentiality. It cannot give itself what it does not yet have.

This principle applies to all the types of change we discussed. A green leaf is only potentially yellow; it requires an external agent (e.g., biochemical processes triggered by changes in sunlight) to actualize that yellowness. An object at rest is only potentially at another location; it requires an external mover to actualize that new location.

2. Illustrating the Principle in a Causal Chain

The principle omne quod movetur ab alio movetur becomes most powerful when we see it applied to a series of movers. Let's look at a simple, intuitive example.

This example of the watch, chain, and hand is a classic illustration of what Aristotle calls a hierarchical or per se causal series. In this type of series, the causal power of each intermediate member is entirely derived from and dependent on the preceding member at the very moment of its operation. The clasp only moves the watch because the chain is moving it, and the chain only moves the clasp because the hand is moving it.

We will analyze this type of causal series in detail in our next lesson, as it is the key to arguing against an infinite regress. For now, the takeaway is simply to see the principle "whatever is moved is moved by another" in concrete action.

3. A Major Challenge: Inertia and Projectile Motion

Your physics background will immediately raise a powerful objection: Newton's first law of motion. An object in motion stays in motion unless acted upon by an external force. A thrown spear or a satellite in orbit seems to continue moving without a continuous, conjoined mover pushing it. Doesn't this directly contradict omne quod movetur ab alio movetur?

This is a formidable objection, and addressing it requires two levels of clarification.

A. Clarifying the Principle: "Is Moved" vs. "Is Being Moved"

First, there's a crucial subtlety in the Latin phrase omne quod movetur, ab alio movetur. The verb movetur is ambiguous. It can be translated with a progressive aspect ("is being moved") or as a simple statement of fact ("is moved").

Dr. Michael Augros, in a talk on Aquinas's arguments, provides an excellent analysis of this point.

As Augros explains, Aristotle's argument is that for any motion, there must be a cause for that motion. The cause could be the thing that initiated it (the generator, or the remover of an impediment) or a thing that sustains it. The principle does not, by itself, demand that the mover be conjoined and acting throughout the entire motion. A thrown spear "is moved" by the thrower, even if the thrower is no longer in contact with it. This interpretation already weakens the objection from inertia.

B. The Metaphysical Status of Inertial Motion

The second, deeper response comes from analyzing what inertial motion is from an Aristotelian perspective. Is it a "change" in the same sense as a ball heating up? McLaughlin's paper, which we looked at earlier, provides a sophisticated answer.

This is a complex argument, so let's synthesize the key points:

• Inertial Motion is a State, Not a Process: From a Newtonian viewpoint, uniform rectilinear motion is considered a state, dynamically equivalent to the state of rest. It is not a change or process. For Aristotle, change is the actualization of a potential. Since inertial motion involves no ongoing actualization of potential (the object is not becoming more or less anything), it does not require a continuous mover to sustain it.
• Motion per se vs. per accidens: The "motion" of an inertially moving body is a change in its relation to other objects (per accidens), not an intrinsic change in the body itself (per se). The mover principle, omne quod movetur ab alio movetur, applies to per se changes—the actualization of an intrinsic potential.
• Real Motions are Composite: As McLaughlin and physicists like Feynman point out, purely inertial motion is an idealization. In the real universe, every moving object is subject to forces (gravity, friction, etc.). These forces do cause an ongoing actualization of potential (e.g., a change in velocity vector). Thus, any real motion is a composite of its inertial state and the per se changes being caused in it by external forces.

In summary, the Aristotelian principle is not violated. The inertial aspect of motion requires an initial mover to establish that state, but not a sustaining one. The non-inertial aspects of any real motion (acceleration, change of direction) do require a continuous, concurrent mover, just as the principle states.

Conclusion

Today we have dissected the principle that 'whatever is moved is moved by another'. We've seen that it is not a naive, pre-scientific observation but a robust metaphysical conclusion derived from the concepts of actuality and potentiality.

Key Takeaways:

• The principle omne quod movetur ab alio movetur is based on the axiom that a thing cannot be in potentiality and actuality in the same respect at the same time. Therefore, nothing can actualize its own potential.
• The principle is better understood as "everything that is moved has a mover," not necessarily that it "is being moved" by a conjoined mover at every instant.
• Apparent counterexamples from physics, like inertia, are resolved by understanding that purely inertial motion is a state, not a process of actualizing a potential. It is a motion per accidens (relational) rather than per se (intrinsic).
• All real motions are composites, and any aspect of them that involves the actualization of a potential (e.g., acceleration due to gravity) does require a concurrent mover, in full agreement with the principle.

Preview of the Next Lesson:

Now that we have established that every change requires a mover, the next logical question is: what happens when we trace this chain of movers? Can it go on forever? Aristotle's answer is a firm "no," but only for a specific kind of causal chain. In our next lesson, we will distinguish between per se (essential, hierarchical) and per accidens (incidental, temporal) causal series. This distinction is the final piece of the puzzle needed to construct the argument against an infinite regress of movers.