Analyzing Aquinas III: The Arguments from Motion and Cause

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After a short break, we will now continue along in our series on Thomas Aquinas, focusing today on the first two of his arguments for the existence of a First Cause, or God. These first two "ways of knowing God," can broadly be categorized as cosmological arguments. According to the philosopher Alexander Pruss, "a cosmological argument takes some cosmic feature of the universe -- such as the existence of contingent things or the fact of motion -- that calls out for an explanation and argues that this feature is to be explained in terms of the activity of a First Cause, which [is God.]"1 Aquinas' first two arguments attempt to do just that-- the first with respect to motion or change, and the second with respect to causation itself. Most of this post will focus on the first way, the so-called Argument from Motion, because the second way is very similar to the first. I apologize in advance because it will be quite impossible to do these arguments justice in a single blog post, and I encourage readers to check out Aquinas themselves along with some modern commentary for a better understanding.

One clarification that needs to be made in advance is that these arguments are not intended to provide an exhaustive defense for the God of Classical Theism or Christianity, nor is this an admittance that no such defense exists.  Often atheists will shrug off a cosmological argument because it does not defend every attribute of God that the defender holds: OK, so you have proven to me that there is a Prime Mover, or First Cause... so, what? That doesn't mean it is God-- it could be a computer, or the Flying Spaghetti Monster!  Pruss explains, and responds to, this point with the following:
Granted, there is a first cause, but does anything of religious interest follow? There is a gap between the statements that there is a first cause and that there is a God. Aquinas in his Five Ways proves the existence of an unmoved mover, and then says: “et hoc omnes intelligent Deum” (“and all understand this to be God”). Some critics have taken this to be his way of papering over the difficulty of moving from a first cause to God; however, that reading is mistaken in light of the fact that succeeding sections of the Summa Theologiae give careful and elaborate arguments that the first cause is wholly actual, unchanging, simple, one, immaterial, perfect, good, and intelligent. Rather, Aquinas is simply marking the fact that the theist will recognize the unmoved mover to be God. Aquinas recognizes that an argument that the first cause has at least some of the attributes of the God of Western monotheism is needed and offers such an argument.2
In other words, the argument is not meant to function independently as a proof for the God of Christianity, but rather a solid foundation on which to build.

The Argument from Motion

First off, let's look at a translation of Aquinas' actual words:

The first and more manifest way is the argument from motion. It is certain, and evident to our senses, that in the world some things are in motion. Now whatever is in motion is put in motion by another, for nothing can be in motion except it is in potentiality to that towards which it is in motion; whereas a thing moves inasmuch as it is in act. For motion is nothing else than the reduction of something from potentiality to actuality. But nothing can be reduced from potentiality to actuality, except by something in a state of actuality. Thus that which is actually hot, as fire, makes wood, which is potentially hot, to be actually hot, and thereby moves and changes it. Now it is not possible that the same thing should be at once in actuality and potentiality in the same respect, but only in different respects. For what is actually hot cannot simultaneously be potentially hot; but it is simultaneously potentially cold. It is therefore impossible that in the same respect and in the same way a thing should be both mover and moved, i.e. that it should move itself. Therefore, whatever is in motion must be put in motion by another. If that by which it is put in motion be itself put in motion, then this also must needs be put in motion by another, and that by another again. But this cannot go on to infinity, because then there would be no first mover, and, consequently, no other mover; seeing that subsequent movers move only inasmuch as they are put in motion by the first mover; as the staff moves only because it is put in motion by the hand. Therefore it is necessary to arrive at a first mover, put in motion by no other; and this everyone understands to be God.3

Some Caveats

There are several important points to be made before the argument itself can be understood (remember from a previous post that Summa Theologiae is merely an introductory textbook and not meant to be exhaustive). The first is that "motion" is used here in a very general sense which would be more accurately understood as change. This large nuance is often missed with modern critics, especially the more science minded ones, causing them to point to Newton's First Law of Motion as an alleged refutation. Aha, they might say, Aquinas' ignorance of science undermines his entire argument! Newton has since shown us that an object which is in motion will stay in motion by virtue of its own inertia, and needs nothing else to keep going! However, in understanding that "motion" actually means "change" here, the objection falls flat-- Aquinas would agree with Newton that for an object to change from moving to stopped, an external force would be required to reduce it from "potentially stopped" to "actuality stopped." (More specifically, Aquinas would say that any massive object has as part of its essential form, an inherent power to stay either at rest or in motion, depending on its current state).

A second point that needs to be made in advance, is that Aquinas held to a difference between types of causal chains. On the one hand, there are Essentially Ordered series', or causal chains per se, and on the other hand there are Accidentally Ordered series', or casual chains per accidens. The difference is fairly easy to illustrate: we could say that your parents are the cause of your existence (at least in a secondary sense, if you believe in God). Had they not conceived you, you would not be alive. Not only are your parents the cause of your existence, but their parents are the cause of their existence, and their parents are the cause of their existence, and so on and so forth. This is a causal chain, but notice something interesting here-- if your parents were to die, you would not somehow cease to exist. If your grandparents were to pass away, your parents would not cease to exist. That is what makes a series accidental-- "it is not essential to the continuation of the series that any earlier member of it remain in existence."4 In contrast, we have the essentially ordered series' of causation, in which every member of the series is completely dependent on every previous member. Consider my arm pushing a stick into a rock to move it. If my arm were to stop moving, the stick would not move, and if the stick stopped moving, the rock would not move. If any member of the series were to cease to exist, the entire causal chain would stop as well, because each member only has a causality that is derived from the rest of the chain. The chain could go back further too; my arm moves because my muscle fibers are being constricted, my muscle fibers are being constricted because they are being activated by motor neurons, etc etc. The case is still the same though: if any member of the causal chain were to cease to exist, the entire causal chain would end. This is what makes a causal series essentially ordered.

The Argument

Now, with a solid enough background, we can proceed to summarize his argument as follows:

  1. Some things in the universe are in motion [in a state of change].
  2. Motion [change] is the reduction from potentiality to actuality.
  3. Something cannot be reduced from potentiality to actuality, except by something already in a state of actuality.
  4. A thing cannot be in a state of potentiality and actuality in the same respect.
  5. Therefore, a thing cannot reduce itself from potentiality to actuality.
  6. Therefore, a thing cannot move [change] itself.
  7. Therefore, whatever is in motion [in a state of change] is moved [changed,] by another.
  8. But whatever moves [changes] that object, must itself have been moved [changed] by another.
  9. This reveals an essentially ordered causal series which could either be infinitely spanned or be terminated by a Prime Mover, who is Pure Actuality, or Actus Purus
  10. An essentially ordered causal series cannot be infinite.
  11. Therefore, it is necessary to arrive at a Prime Mover, who is Pure Actuality, or Actus Purus.
  12. Everyone understands this to be God. 

Potential Objections

The majority of the premises in this argument would be easily accepted by most people, provided that they are not simply trying to avoid the conclusion. Some premises, that may be hard to accept without any sort ulterior motives, can be more easily swallowed upon understanding the metaphysics behind Aquinas' thought (summarized in the previous post of this series). There are two objections, however, that carry some weight, and must be responded to:

Objection 1: While an object cannot move itself, that does not mean it requires anything to be moved.

This objection neglects to take something important into account: if nothing were required to reduce something from potency to act, there is no reason why it should happen at one particular time and not another. We see change constantly in every day life, and it is always accompanied by a specific action. A match is lit when it is struck on the box; if the act of striking it and creating friction were not required to light the match, there would be no reason why it should light when struck and not at some other time. A pleasant note sounds when a guitar string is plucked; if the act of plucking the guitar string were not required for the note to sound, there would be no reason why it should sound when plucked and not at some other time. It should be evident that "whatever is in motion must be put in motion by another."

Objection 2: Why couldn't the chain of motion be infinite?

One reason I spent clarifying the difference between an accidentally ordered series and an essentially ordered series in the first part of this post, is because it is extremely important for the argument. Aquinas believed that an accidentally ordered series could go on infinitely, while an essentially ordered series could not. Since each member of an accidentally ordered series can operate independently from each other member, it could technically continue on infinitely. For example, it was not illogical to Aquinas for someone to believe that the universe always existed. While he believed that the universe was created a finite number of years in the past, he did not think there was a way to know this beyond Special Revelation (ie. Biblical Revelation), and therefore did not think the idea to be illogical or absurd. The Argument from Motion is not intended, like the Kalam Cosmological Argument, to show that the universe must have a spatio-temporal first cause in the past. Rather, the Argument from Motion is intended to show that motion in the here and now, at any given moment, must be sustained by a Prime Mover. There cannot be an infinite number of members in an ordered series of "movers" because each member is dependent on all previous members, and therefore the chain could not exist at all without a Prime Mover.

The Argument from Efficient Causality

The second way is from the nature of the efficient cause. In the world of sense we find there is an order of efficient causes. There is no case known (neither is it, indeed, possible) in which a thing is found to be the efficient cause of itself; for so it would be prior to itself, which is impossible. Now in efficient causes it is not possible to go on to infinity because in all efficient causes following in order, the first is the cause of the intermediate cause, and the intermediate is the cause of the ultimate cause, whether the intermediate cause be several, or only one. Now to take away the cause is to take away the effect. Therefore, if there be no first cause among efficient causes, there will be no ultimate, nor any intermediate cause. But if in efficient causes it is possible to go on to infinity, there will be no first efficient cause, neither will there be an ultimate effect, nor any intermediate efficient causes; all of which is plainly false. Therefore it is necessary to admit a first efficient cause, to which everyone gives the name of God.5

Since this argument is so similar to the previous one, I will not spend effort elaborating on it, but let it stand on its own. In the following weeks, I will follow up with posts on the third, fourth and fifth causes, the arguments from Contingency of Material Objects, Perfection, and Teleology, respectively.


1. Pruss, 2012
2. Ibid.
3. Summa Theologiae, Question 2, Article 3
4.  Feser, 2010
5. Summa Theologiae, Question 2, Article 3
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