Motion as Actualized Potentiality

Hello! Welcome to the second module of our course.

Introduction

In our previous lessons, we established the foundational concepts of Aristotle's metaphysics: substance, potentiality, actuality, and hylomorphism. We concluded that the physical things we observe are hylomorphic composites of matter (potentiality) and form (actuality), and that change involves the actualization of a potential.

Today, we move from this metaphysical toolkit into the realm of Physics. Our goal is to arrive at a precise, formal definition of change, or what Aristotle calls kinēsis (often translated as 'motion'). This is the specific learning outcome for this lesson: to define motion (kinesis) as the actualization of a potentiality as such.

This definition might seem abstract, but it is the analytical engine that drives Aristotle's entire argument for a First Mover. By defining what a process of change is at its most fundamental level, he lays the groundwork for asking what is required to explain its existence.

1. Motion as Change, Change as Actualization

First, it's important to understand that when Aristotle speaks of 'motion' (kinēsis), he means something much broader than our modern usage. For him, motion is synonymous with change of any kind.

To get a feel for this broader scope and the core idea that all change is an actualization of potential, let's start with a brief overview from the philosopher Edward Feser.

As Feser explains, Aristotle identifies several types of change:

• Locomotion: Change of place (what we typically call motion).
• Alteration: Change of quality (e.g., a leaf changing color).
• Growth/Diminution: Change of quantity (e.g., an organism growing).
• Generation/Corruption: Substantial change (e.g., an acorn becoming an oak tree, or a log burning to ash).

The unifying insight is that every instance of change is a movement from a potential state to an actual one. The green leaf had the potential to be yellow; the process of changing color is that potential being actualized. This is the intuitive starting point for Aristotle's definition.

2. The Problem with Simpler Definitions

You might wonder why we need more than this. Why not just define change as "a thing having a property at one time that it lacked at an earlier time"? Or, for locomotion, "being at place P1 at time T1 and at place P2 at time T2"?

This type of definition, sometimes called the "at-at" theory of motion, is insufficient for Aristotle. While it correctly identifies the start and end points (the termini) of a change, it completely ignores the process itself. It describes a state of affairs, not the continuous passage from one state to another. From your background in mathematics, you can see this is analogous to defining a function by its endpoints while ignoring its behavior in between. Aristotle wants to define the curve, not just the points.

To understand why this is a problem, let's read a short explanation.

The key takeaway is that the "at-at" theory leaves out the very thing we are trying to explain: the process of getting from the potential state to the actual state. For Aristotle, this process is a real entity in its own right that requires a definition.

3. Unpacking Aristotle's Definition

This brings us to Aristotle's famous and notoriously difficult definition of motion from Physics III.1:

Motion is the actuality (entelecheia) of that which exists in potentiality (dynamei), in so far as it is such (hêi toiouton).

Let's break this down piece by piece. It's a dense philosophical statement that requires careful analysis.

The Core Puzzle: Process vs. Product

At first glance, the definition seems puzzling. If we consider a pile of bricks that has the potential to be a house, the actuality of that potential is the finished house. But a house is a product, not a process. Motion, however, is a process (e.g., house-building). So how can motion be an actuality?

This is where the final clause—"in so far as it is such" or "qua potential"—becomes absolutely critical. It is the key that unlocks the entire definition.

The "Qua" Clause: The Key to the Definition

The "qua" clause tells us how to consider the potentiality that is being actualized. We are not talking about the actuality that replaces the potentiality (the finished house), but the actuality of the potentiality itself while it is still a potentiality.

Professor Cohen's notes provide an exceptionally clear way to understand this. He distinguishes between two ways an imperfection can be "perfected": deprivative and constitutive.

Let's summarize the argument you just read:

1. Motion is not the final product. The actuality of "the buildable" qua buildable cannot be the finished house, because once the house exists, the materials are no longer "buildable" in the same way; they have been built. The potential has vanished.
2. Motion is the actuality of the potential itself. The process of building is the actuality of the buildable materials as buildable. It is the potentiality for being a house being active or manifest.
3. Levels of Actuality. This makes sense when we think of levels.
   • P1 (First Potentiality): Bricks and wood sitting in a yard. They have the potential to become a house.
   • A1/P2 (First Actuality / Second Potentiality): The process of house-building is underway. The potentiality of the bricks and wood is now actual, but it is the actuality of a potential. It is an "incomplete actuality." It is also a potentiality (P2) for the finished house.
   • A2 (Second Actuality): The finished house. The process is complete, and the potentiality (qua buildable) is gone.

Therefore, kinēsis is that intermediate state, A1/P2. It is the "constitutive perfection of an imperfection," as Cohen puts it. It is the state of being on the way.

The Stanford Encyclopedia of Philosophy entry on Aristotle's Natural Philosophy puts it with formal precision.

4. Synthesis and Summary

We have taken a deep dive into a very subtle definition. Let's use a simple, visual resource to consolidate the main idea.

For the pot of water:

• P1: Cold water that is potentially hot.
• A1/P2: The process of heating. The potential to be hot is being actualized. This is the kinēsis.
• A2: The water is now actually hot. The kinēsis has ended.

This precise understanding of change as an incomplete actuality—a process that exists only while a potentiality is being actualized—is the essential premise for the next step in Aristotle's argument.

Conclusion

This lesson was dedicated to unpacking one of the most important and subtle definitions in Aristotle's philosophy.

Key Takeaways:

• For Aristotle, 'motion' (kinēsis) refers to any kind of change, not just change of place.
• A simple "at-at" definition of change is insufficient because it describes only the start and end points, not the continuous process of change.
• Aristotle's formal definition is: the actuality of a potentiality, as such (qua potential).
• The "as such" clause is crucial. It specifies that motion is not the final state that replaces the potential, but the very process of the potential being active and on its way to fulfillment.
• Therefore, motion is a unique kind of being: an incomplete actuality. It exists only so long as the process is ongoing.

Preview of the Next Lesson:

Now that we have a rigorous definition of what motion is, we will move on to the next learning outcome: distinguishing between the different types of change in Aristotle's physics: generation/corruption, alteration, and locomotion. We will explore the specific characteristics of these categories of motion, which will prepare us to analyze the causal principles that govern them, leading directly to the famous principle that "whatever is moved is moved by another."