Aquinas's First Way: Motion and Aristotelian Roots

Hello! Welcome to the fifth module of our course. We have spent the last few modules building up a detailed picture of Aristotle's metaphysical framework. Now, we will see how his ideas were adopted and developed by one of the most influential thinkers in Western philosophy, St. Thomas Aquinas.

This lesson focuses on Aquinas's famous "First Way," his argument for the existence of God from motion. Our goal is to reconstruct this argument and, crucially, to identify the specific Aristotelian premises it relies on. You'll see that a solid grasp of concepts like potentiality, actuality, and the nature of causal chains—which you've encountered in the previous modules—is essential for understanding Aquinas's reasoning.

The Five Ways and Common Misconceptions

In his monumental work, the Summa Theologiae, Aquinas presents five arguments, or "Ways" (Quinque Viae), to demonstrate the existence of God through reason. The First Way, the Argument from Motion, is perhaps the most famous and also the most frequently misinterpreted.

A common mistake is to read it as an argument for a "first domino" that started a chain reaction at the beginning of time. However, Aquinas, following Aristotle, did not believe one could philosophically prove that the universe had a beginning. The argument is not about a temporal sequence of causes but about the ultimate, simultaneous, sustaining cause of change here and now.

To get a clear perspective on why a careful reading is so important, please start by watching the first two minutes of this video.

Now that we've set the stage, let's reconstruct the argument step by step.

Step 1: The Reality of Change (Motion)

The argument begins with an empirical observation that is impossible to deny:

Premise 1: It is certain, and evident to our senses, that in the world some things are in motion.

As we know from our study of Aristotle, 'motion' (from the Greek kinesis) is used in a much broader sense than just change of location. It refers to any kind of change—the actualization of a potential.

To refresh this core Aristotelian idea, let's turn to a detailed explanation by philosopher Edward Feser.

This analysis of change as the reduction of potentiality ($\delta \acute{\upsilon }\nu \alpha \mu \iota \varsigma$, dynamis) to actuality ($ϵ\nu ϵ\rho \gamma ϵ\iota \alpha$, energeia) is the metaphysical foundation upon which the entire argument rests. For example, a cold cup of coffee is actually cold but potentially hot. Heating it is the process of actualizing that potential.

Step 2: The Principle of Motion's Causation

Having established that change occurs, Aquinas introduces his second premise, which is a direct application of Aristotelian causal principles:

Premise 2: Now whatever is in motion is put in motion by another (omne quod movetur ab alio movetur).

Why must this be so? A thing cannot move or change itself. The reasoning is as follows:

• Motion is the transition from a state of potentiality to a state of actuality.
• A thing can only cause change insofar as it is actual. For instance, a hot fire (which is actually hot) can make a log (which is potentially hot) become actually hot.
• It is impossible for something to be in the same respect both potential and actual. The log cannot be both potentially hot and actually hot simultaneously.
• Therefore, the thing undergoing change (which is in a state of potentiality with respect to that change) cannot be the source of the actuality that causes the change. Something already actual must be the cause.

The following image provides a simple visual for this step.

For a more rigorous breakdown of this crucial step, please read the designated sections from the "First Way" article on the Thomistic Philosophy Page.

Step 3: Two Kinds of Causal Series

If everything in motion is moved by another, we are faced with a causal chain: A is moved by B, which is moved by C, and so on. Aquinas, drawing on Aristotle, argues that we must distinguish between two fundamentally different types of causal series. This distinction is critical to avoiding the common misinterpretation of the argument.

1. Linear / Per Accidens Series (Accidentally Ordered): This is a temporal sequence where each cause can produce its effect and then cease to exist without stopping the chain. The classic example is a father begetting a son, who in turn begets his own son. The grandfather's existence is not required for his grandson to be born. Aquinas concedes that such a series could, in principle, go back in time infinitely.

2. Hierarchical / Per Se Series (Essentially Ordered): This is a simultaneous series where subordinate causes have no power of their own and act only as instruments of a higher cause. All members of the series must exist and act at the same time for the effect to occur. The classic example is a hand moving a stick that moves a stone. The stick only moves the stone because the hand is currently moving the stick. The causal power is derived.

Aquinas's argument is concerned only with the hierarchical (per se) series. He is seeking the ultimate explanation for why the change is happening right now.

Let's return to the Feser video for a clear explanation of this vital distinction.

Step 4: Rejecting an Infinite Hierarchical Regress

With the distinction between causal series in place, Aquinas adds his third premise:

Premise 3: This hierarchical series of movers cannot go on to infinity.

Why not? In a linear series, each member has its own causal power. But in a hierarchical series, the intermediate causes (like the stick) are mere instruments. They have no power to move anything on their own; they only transmit the power of the primary cause (the hand).

If the series of hierarchical causes were infinite, there would be no primary cause. You would have an infinite chain of instruments, none of which have any causal power to begin with. This would be like a train with an infinite number of carriages but no engine—it wouldn't go anywhere. Without a first mover in the hierarchical sense, there would be no motion here and now to be explained.

Feser explains why this kind of infinite series is incoherent.

Conclusion: The First Unmoved Mover

This leads directly to the conclusion of the argument:

Conclusion: Therefore, it is necessary to arrive at a first mover, put in motion by no other.

This First Mover is the ultimate source of the change we observe. It must be an unmoved mover because if it were itself moved, it would just be another instrumental link in the chain, requiring a further mover to explain its motion. It would not be the first or ultimate explanation.

Such a being must be pure actuality (actus purus), with no admixture of potentiality. If it had any potentiality, it would be capable of change, and that potential would need to be actualized by something else, contradicting its status as the First Mover.

Aquinas famously concludes this line of reasoning by stating:

...and this everyone understands to be God.

It's important to recognize, as Aquinas did, that this doesn't prove every attribute of God as understood in Christian theology (e.g., being all-loving). Rather, it argues for the existence of an ultimate, unchanging, immaterial reality that is the source of all change in the cosmos—a being that is Pure Actuality.

Summary of the First Way and its Aristotelian Core

Let's reconstruct the logical flow:

1. P1: We observe that some things are in a state of change (motion).
2. P2: Anything that changes is changed by another. (This is because change is the actualization of a potential, and a thing cannot actualize its own potential).
3. P3: The series of simultaneous, hierarchical (per se) movers cannot be infinite. (This is because instrumental causes require a principal cause to provide the causal power).
4. C: Therefore, there must be a First Unmoved Mover that is the source of all change. This being must be Pure Actuality.
5. Identification: This First Unmoved Mover is what we call God.

The specific Aristotelian premises undergirding the argument are:

• The definition of change (kinesis) as the actualization of potentiality.
• The metaphysical distinction between potentiality (dynamis) and actuality (energeia).
• The causal principle that whatever is moved is moved by another, which is a direct consequence of the potentiality/actuality distinction.
• The distinction between hierarchical (per se) and linear (per accidens) causal series.
• The principle that a hierarchical (per se) causal series requires a first member and cannot regress infinitely.

One final point of interest: Aquinas nuances Aristotle's position slightly. For Aristotle, the First Mover could be seen as "self-moving" in the sense that one part of it (its intellect) moves another (its will). Aquinas, concerned with the later theological doctrine of absolute divine simplicity, rejects any composition in the First Mover, arguing it must be absolutely unmoved, not just by another, but in any respect whatsoever. This is a subtle but important development we will revisit later.

Conclusion

In this lesson, we have reconstructed Aquinas's First Way and seen how it is not a simplistic argument about the beginning of time, but a sophisticated metaphysical argument for a sustaining cause of change based almost entirely on Aristotelian principles. The core moves rely on the concepts of potentiality and actuality and the crucial distinction between temporal and hierarchical causal chains.

In our next lesson, we will examine Aquinas's Second Way, the argument from efficient causation. You will find that it employs a very similar logical structure, but applies it to the existence of things rather than to their change.