Argument from Motion: Outline

Thomas Aquinas’ argument from motion begins with the empirical observation that things are in motion. What follows is a line of logic that establishes the existence of God on empirical grounds if it holds true. The argument is an a posteriori argument and thus the conclusion is not claimed to follow with certainty. For further information on the Five Ways, visit the introduction.

Argument from Motion: Explained

At first glance it appears as though Aquinas’ argument from motion relies on the idea of a causal sequence (one object causes another to move, etc.), but in reality, it rests on the notions of potentiality and actuality. It’s important to establish the difference between these two concepts because the discussion of Aquinas’ First Way could turn out very differently if it relied on the concept of a causal sequence. In truth, casual sequences are complex in nature and their complexity will play a more important role in the objections to Aquinas’ second way.

As discussed in the introduction, objects have two states of being: potential being and actual being. A piece of wood has the potentially to becoming a table, but until a carpenter carves it, its actuality is that of wood. Aquinas applied the same concept to that of motion and concluded that all motionless objects have the potentiality of motion whereas objects in motion have the actuality of motion. Given the law of non-contradiction, it’s impossible for something to be both potential and actual in the same respect at the same time. If a log of wood is potentially on fire, for example, we wouldn’t say that it is also actually on fire. The same line of logic holds true for motion. If we then come to the conclusion that nothing can move itself, we must conclude that potential motion becomes actualized when actual motion acts upon it. The next question then becomes, what started all of this motion in the first place? Aquinas rejects the idea of an infinite regressive chain of motion on that grounds that it's simply irrational to believe in such a regress; it's akin to seeing a line of dominos toppling each other over and being told that there is no starting point. In other words, there must be a first mover in the sequence of motion, and Aquinas concludes that such an entity is God.

Objections

Is the First Mover Even God?

One of the most common objections to the entirety of the Five Ways is that none of them seem to be actually proving the God that Aquinas believes in. Let us say, for a moment, that Aquinas' first way is completely sound in its premises and logic. Even if we come to the conclusion that there is a first mover, there's no apparent reason to believe that such a mover is Aquinas' Christian God; what reason is there for beliving that the first mover is responsible for bestowing the Ten Commandments onto Moses? In addition, there's no reason to endow the first mover with the qualities normally ascribed to God. Aquinas' argument from motion gives us no reason to believe that the first mover is omnipotent, omniscient, perfect, etc. On that note, the argument from motion doesn't even give us reason to believe that the first mover is even any sort of God in the first place; it could be the big bang or something else entirely.

A well-known refutation to this point is that the Five Ways aren't even intended to prove the existence of a Christian God, so attacking Aquinas' argument on these grounds seems to be a straw man. Instead, Aquinas' proof for a Christian God is developed throughout his Summa Theologica and Summa Contra Gentiles where he demonstrates the existence of God starting with his unmoved mover argument.

A Vicious Circle

The second sixth premise of Aquinas' argument states that all things in motion must've been moved by something else. Yet, when we invoke the idea of a first unmoved mover, are we not directly contradicting this key premise? In other words, who moved the unmoved mover? The obvious reconciliation to this contradiction might be to grant the first mover an exemption from the sixth premise, but then we are stating that God is pure actuality. If God is pure actuality then it follows that, in truth, God can't do anything because God is already everything that he could be. Thus there's no potential for God to be or do anything other than what God already is. If we subjugate the first mover to the sixth premise, we must answer the question of who moved the mover. If we give the first mover a special status and exempt him from this premise, we face the dilemma of pure actualization described above.

Does the Universe Need a Beginning?

Aquinas seems to believe that universe must have a beginning, but why? Why must we assume that the natural law of the universe is for there to exist a beginning, middle, and end? And in fact, there's no reason to believe that the natural processes of the universe behave in such a way. In addition, why must we assume that the natural state of things is to be at rest? Assuming that the universe has always existed and that objects have always been in motion is more in accordance with the principle of simplicity than the argument from motion. Such an assumption would also be in tandem with Newton's First Law, which states that objects in motion tend to stay in motion and objects at rest tend to stay at rest unless acted on by an outside force.

Outdated Physics

As mentioned in the introduction, Aristotelean physics greatly influenced Aquinas and his five ways. Aquinas didn't have the benefit of the knowledge of physics that we have today, and at the time, Aristotle's theories were the closest thing he had to explaining the natural universe.Of course, this doesn't change the fact that Aquinas' argument is based on outdated physics and that that in turn impacts the validity of his argument.

Sources

Dawkins, Richard. The God Delusion. Black Swan, 2016.