Re: Aquinas’ Argument from Motion

A reader has asked me to comment on another of Mike D’s posts at The A-Unicornist blog:

Aquinas’ Argument from Motion

Some day (in the distant future) I’d like to write more on this site about Aquinas’s philosophy but today we shall merely focus on why Mike’s criticisms of the argument from motion are unsuccessful. I’ll briefly summarize some of Aquinas’s philosophy, but what follows may not make sense to those without some background in said philosophy.

He summarizes the argument as follows:

  1. Evident to our senses is motion — the movement from potentiality to actuality. Things are acted on.
  2. Whatever is moved is moved by something else. Potentiality is only moved by actuality.
  3. Unless there is a First Mover, there can be no motions. To take away the actual is to take away the potential.
  4. Thus, a First Mover exists.

This argument, which Mike admits popped up in a quick Google search (not the best way to present an argument), needs to be explained in more detail because many of the terms are foreign to the modern, lay reader.

(1a) By “motion” Aquinas is basically referring to change, not solely motion from one location to another.

(1b) Metaphysically speaking, change is the reduction of something from potentiality to actuality. Suppose we have some thing (it doesn’t matter what because we are describing change in general) that can be in states S1, S2, or S3 (but, for simplicity, never two or more of these states at once). If at time t1 the thing is in state S1 then we say it is actually S1 and potentially S2 or S3. Now suppose the thing changes to be in state S2 at time t2. What has happened is that the potentiality at t1 (states S2 and S3) has been reduced to an actuality (state S2) at t2.

(2) Aquinas is saying change only occurs because of an actual cause, not a potential cause. If we go back to (1b), at time t1 state S2, which is merely a potential state of the thing, cannot be the (efficient) cause of any change. State S2 can only be the (efficient) cause of a change when it is actual, such as at time t2.

(3a) Aquinas distinguishes between accidentally-ordered causal series and essentially-ordered causal series. An example of an accidentally-ordered causal series is a human lineage: the grandfather causes the father to come into existence and the father causes the son to come into existence. Why is this an accidentally-ordered causal series? Because each cause in the series has the power to produce its effect independently of its own cause. For instance, once the father comes into existence he has the power to produce a son independently of the grandfather’s existence or reproductive powers.

An example of an essentially-ordered causal series is a man moving a staff to move a stone. Why is this an essentially-ordered causal series? Because the staff is dependent on the man in order to move the stone. The staff does not have the ability on its own to move the stone. In order to move the stone the staff has to be moved by another.

For the purpose of this argument, Aquinas denies the existence of an infinite number of causes in an essentially-ordered causal series. There has to be a first cause to an essentially-ordered causal series because the intermediate causes cannot bring about change on their own.

(4) The First Mover is pure actuality. This means it does not undergo change itself (think the divine attribute of immutability) for it does not have the potential to do so (hence, pure actuality as opposed to a thing that is a composite of actuality and potentiality). It brings about changes in (or creates) other things.

It is understandable if what I have written so far does not make perfect sense to you. I’ve tried to summarize many parts of Aquinas’s metaphysics in a few paragraphs. This is not the ideal way to learn about the subject but hopefully it provides some basis for the criticisms that follow.

Mike begins by complaining that the terms actuality and potentiality are obscure:

These terms are obscure in the extreme. If they’re supposed to describe cause and effect, then there’s no reason to take them as axiomatic.

But the terms are necessary to explain change, not cause and effect (which is not to say change and causality are unrelated to each other). If we grasp what change is and agree that change occurs then we must believe actuality and potentiality exist. To deny actuality is to deny that anything exists at all. But Mike believes something exists. To deny that potentiality exists is to deny that change is possible at all. But Mike appears to believe that change occurs. Actuality and potentiality are axiomatic given the reality of change.

Mike’s next argument appeals to the law of conservation of energy and the laws of motion:

But in this case, the law of conservation of energy and the laws of motion eliminate the need for an endless well of actuality to exist, because energy is always conserved and the causal chain can persist without external influence. In other words, the total sum of energy in the universe, even if it is finite, is sufficient to describe all instances of potentiality coming to actuality. We don’t need to posit a “First Mover” that acts as an infinite source of actuality – the laws of motion and conservation of energy sustain the chain of interaction on their own.

Unfortunately, Mike’s reasoning here is unclear. His talk of “an endless well/source of actuality” suggests he may be incorrectly viewing actuality as analogous to energy in the sense that it is something that can be quantified by measurement. The assertion that “the causal chain can persist without external influence” is begging the question. No formal contradiction between Aquinas’s metaphysics and the laws of nature is identified.

The claim that “the laws of motion and conservation of energy sustain the chain of interaction on their own” is false because scientific laws are human descriptions of how nature operates (or appears to operate given our current state of knowledge). The laws themselves do nothing. For instance, it is not a scientific law that throws a baseball, it is a pitcher, say, who throws a baseball. The fact that you can describe a thrown baseball with scientific laws in no way means the baseball was not thrown by the pitcher.

Further still is the fact that actuality and potentiality need not be separate interacting mechanisms; in quantum mechanics, what Aquinas would call a particle’s “potentiality” could be described as an intrinsic property of the particle, and it becomes “actualized” through quantum indeterminacy at some probabilistically determined point – as in the case with virtual particles, quantum entanglement, or nuclear decay.

In this paragraph we see how hard it is to truly deny actuality and potentiality even by those who claim to be doing just that. Ignore Mike’s notion that actuality and potentiality are mechanisms and notice that he appears to be equating potentiality with an intrinsic property of a particle. That looks like an admission that potentiality exists but just giving it a different label.

Mike moves on to criticize the principle of motion: whatever is undergoing real change is being changed by another being in act.

The problematic implication here is that potentiality and actuality must both always exist. Actuality cannot in itself bring potentiality into being, for that potential for change would mean that actuality has some intrinsic potentiality (is this confusing yet?). But the argument terminates in the idea that God is “pure actuality”. But if God is pure actuality, God cannot do anything. All concepts we use to describe action – causality, creation, etc. – imply the existence of a potential change.

A couple key distinctions can defuse this argument. First, we need to make a distinction between change and creation. Change involves some substrate remaining throughout the change. If a canvas was white and is now blue because someone painted it we would say that the canvas has undergone change and the canvas is the substrate remaining throughout the change. But the divine creation of some thing does not involve some thing undergoing change, rather, it involves a new thing coming into being.

Second, when Aquinas says God is pure actuality he means that God has no passive potency, he cannot undergo change Himself. But Aquinas does say God has active potency, the ability to create or change other things.

With these distinctions in mind we can see that something in act can create a new thing without actualizing a pre-existing potential and that God, according to Aquinas, does have the ability to change or create other things.

Mike’s final main point is that perhaps potentiality can actualize itself. He claims that’s just what happens in quantum mechanics:

The wave function of a particle can only be determined by probability; there is no classical mechanism that can explain why the particle changes at one time rather than another.

But our inability to know why a particle changes at one time and not another in no way demonstrates that the particle’s potentiality actualizes itself. Nor does it explain how the merely potential can actualize itself when it doesn’t yet exist in act.


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