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Answering Two Objections to Aquinas


I'd like to share an excerpt from my new book Answering Atheism, which answers two common objections to Thomas Aquinas’s first way of proving the existence of God. This way is usually called the argument from motion.

Objection #1: What moved God?

The “new atheists,” like Richard Dawkins, sometimes make a caricature of Aquinas and claim he is arguing that, “Everything needs a cause. Therefore, there had to be a first cause called God. God is the exception and has no cause of his own.”1 But Aquinas never makes such a simplistic argument.

He says that everything in motion only moves because it has a potential to move. Since nothing can move itself, an object can only move if its potential to move is activated by something outside of it. For example, water has the potential to freeze if the air temperature lowers enough to act upon the water and freeze it. But water can’t just turn into ice by itself. Likewise, the air that freezes water has a potential to become colder if something else acts upon it (like changes in air pressure).

But what makes God different from anything else is that he has no potential to move or change. God is pure motion itself (or what Aquinas calls act). Nothing changes God in order to bring about effects in the world. Instead, God brings about effects by his own power. God can act on his own because, once again, there is nothing potential about God.

Aquinas is not saying everything has a cause except for God, as if God were one object among many in the universe. Instead, he logically arrives at the conclusion that since all things have potential in them and musty be moved by another there must be something which has no potential that moves everything. God is not an “exception to the rule,” he’s an entirely different “rule” or kind of being.

This refutes Dawkins’s claim that there is no good reason to say God does not require a mover of his own. Asking the question, “If everything needs to be moved, then what moved God?” is like asking the question, “If every car on a train needs to be pulled, then what is pulling the locomotive?” It is because God is not acted upon by anything else that he is able to be pure being itself and cause everything else to exist and move in the world.

Objection #2: Why not an infinite number of movers?

Another popular objection goes like this: “Aquinas begs the question when he says that there cannot be an infinite number of movers and so there must be an unmoved mover that sets everything in motion. But why can’t there just be movers or causes going back infinitely in time?2

To this objection Aquinas makes a distinction between causes that are sequential and causes that are simultaneous.3 Sequential causation is like a chain of dominoes. After you knock over the first domino you start a chain reaction of dominoes hitting other dominoes. In fact, you could destroy the first domino after you’ve pushed it since it is no longer needed to keep the whole set of dominoes falling. Aquinas believed that sequential causes in the past, like a set of dominoes, could have occurred for all eternity.4

Aquinas argues that God explains the existence of simultaneous causation. An example of this kind of causation would be a golfer hitting a golf ball. The act of the golfer hitting the ball is not as simple as we might think. The golf ball is moved by the golf clubhead, but the clubhead is simultaneously moved by the swing of the shaft, which is moved from the handle, and the handle is simultaneously moved by the flexing of the golfer’s muscles, which cannot flex without nerve signals from the golfer’s brain stimulating them, and so on and so on.

Aquinas argues that such a chain of causes cannot be infinite because the causation is all happening at the same time. This is like a train with an infinite number of boxcars that are all moving at the same time. In that case, there has to be an unmoved mover (the locomotive) that causes all the simultaneous motion to occur at all. Likewise, there must be an “uncaused cause” that sustains the elaborate chain of simultaneous causes in our universe. We call this uncaused cause God.

Even though Aquinas’s arguments are very powerful, we must be careful when using or attempting to refute them because they are easily misunderstood. The philosopher Edward Feser has written in his excellent book Aquinas that the five ways are only summaries that were originally written for beginning theology students. Feser says that:

"Aquinas never intended them to stand alone, and would probably have reacted with horror if told that future generations of students would be studying them in isolation, removed from their original immediate context in the Summa Theologica and the larger context of his work as a whole."5

A careful student of Aquinas will also see that his arguments only make sense if you understand all the terms he uses, such as “act” and “potency” and different types of “causes.” This doesn’t make them bad arguments; they just require more understanding in order to be adequately communicated to a non-believer. For those who are interested in truly understanding these arguments in depth, I recommend Feser’s book Aquinas: A Beginner’s Guide.
Originally appeared on the Catholic Answers Blog. Used with permission.


  1. A paraphrase of Dawkins’ objection found in The God Delusion (Houghton Mifflin Company, New York: 2006), 101.
  2. Michael Martin, Atheism: A Philosophical Justification (Temple University Press, Philadelphia, 1990), 99.
  3. The formal terms are per se causation (simultaneous causation) and per accidens (sequential) causation. See Feser 69-70.
  4. See Fr. Robert Spitzer, New Proofs for the Existence of God: Contributions of Contemporary Physics and Philosophy (Wm. B. Eerdmans, Grand Rapids, 2010), 177-180.
  5. Edward Feser, Aquinas: A Beginner’s Guide (OneWorld, Oxford, 2009), 62-63.
Trent Horn

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Trent Horn holds a Master’s degree in Theology from the Franciscan University of Steubenville and is currently an apologist and speaker for Catholic Answers. He specializes in training pro-lifers to intelligently and compassionately engage pro-choice advocates in genuine dialogue. He recently released his first book, titled Answering Atheism: How to Make the Case for God with Logic and Charity. Follow Trent at his blog, TrentHorn.com.

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