Thomas Aquinas’ Arguments From Motion and Efficient Causality: A Socratic Dialogue

St. Thomas Aquinas
St. Thomas Aquinas

Over the coming weeks I’d like to explore each of St. Thomas’ five proofs for the existence of God by way of socratic dialogues (read my other dialogues here).

In today’s dialogue Brad asks Sarah for arguments for God’s existence. Sarah responds by summing up the first two ways of Aquinas, both of which are based upon the idea that an infinite regress is impossible. If you’re unfamiliar with Aquinas’ five proofs for the existence of God, read his summaries of them here from the Summa Thealogica.

Brad: Sarah the last few times we’ve met you’ve tried to show why my reasons for being an atheist aren’t good, but do you have any positive reasons for thinking God exists? You do believe God can be proven, right?




Sarah: It depends what you mean by prove, Brad. I don’t think proving God’s existence is like proving 2+2=4, only in mathematics can you get 100% proof about anything. But I do think that we can logically move from what we observe to a conclusion that God exists.

Brad: So you admit that you can’t be certain.

Sarah: Can you be certain about something without having 100% of it proved? Look, I’m just saying that if we believe most thing in life, like the reality of the world, and that we’re not just bodies in the matrix, that our parents love us and aren’t just pretending to because of some sinister plan, that Yemen exists even though we’ve never been there. You’ve haven’t been to Yemen, right?

Brad: No.

Sarah: Almost everything we believe cannot be proven with mathematical certainty, and why should we expect them to? We believe most things based on good reasons, and if we have good reasons to believe in God, then we should believe in God just like we believe in those other things.

Brad: Okay, well, lay them on me; these reasons of yours. I’m an open-minded guy, what I’m after is the truth.

Sarah: Okay, there are lots of different reasons to believe in God but I think the most powerful reason is that only God can explain fundamental features of reality. I’m not saying that God explains things science hasn’t figured out.

Brad: Glad to hear it, that would be God-of-the-gaps.


Thomas’ Five Ways


Sarah: Right. I’m saying that certain features can in principle only be explained by God. Have you ever heard of the Five Ways of St. Thomas Aquinas?

Brad: Funny you should mention them, yes. I’ve just been reading about them. Surely you can do better than them!

Sarah: What makes you think these aren’t good arguments?

Brad: I thought you told a while back that you’d read The God Delusion. Dawkins completely demolishes Aquinas’ arguments. I actually have a copy of it here. Here, listen to this. He writes, “The five ‘proofs’ asserted by Thomas Aquinas in the thirteenth century don’t prove anything, and are easily – though I hesitate to say so, given his eminence – exposed as vacuous.” And then he goes through them one by one, knocking them down.


Argument From Motion


Sarah: Yes, I have read The God delusion. I think Dawkins “misunderstands” rather than “demolishes” Aquinas’ arguments. Dawkins commits the straw man fallacy, that is, he refutes a weaker version of his opponents argument. I don’t think he did this intentionally, but as a biologist, and not a philosopher, I think he just misunderstood Aquinas as many atheists are prone to do. Let’s start with the first way Thomas proposes, or the argument from motion. What do you take that argument to mean?

Brad: Why don’t you refresh my memory.

Sarah: Alright. Everything we observe is a combination of the potential and the actual. Water is actually wet and potentially solid. Wood is actually hard and potentially flammable. Whenever something goes from potential to actual it must be activated by something else.

Brad: Sorry to interrupt. When you use the words potential and actual—I want to make sure I’m understanding you correctly—you mean by potential, the ability of something to be different than it is, right? And by actual you mean the state something is in?

Sarah: Yes, potency refers to what something can be while actuality refers to what it currently is.

Brad: K, great, keep going.

Sarah: Oh that’s fine, thanks for clarifying. So when something goes from potential to actual it must be activated by something else. Water doesn’t freeze itself, wood doesn’t light itself on fire. But this chain can’t regress forever. If we keep saying that what actualized the potential in one thing was something else then we keep shifting the explanation backwards without explaining anything.

Brad: Okay. Give me an analogy.

Sarah: Alright, consider a train with an infinite number of boxcars, is it moving or standing still. Boxcars can’t move themselves so it must be still. A moving train, however, requires a car that moves itself and all others. Likewise, our universe, being one of change and motion, requires something that is pure actuality and has no potential whatsoever. This is God.

Brad: Okay, I have two responses. First, even if you’re right and an infinite regress is impossible, claiming that whatever stops the regress is “God” is unhelpful at best. You’ve proven very little about the God you believe exists.

Sarah: We can call the solution to our infinite regress “The First Cause” or “Pure Actuality” and then see later if these terms refer to the classical definition of “God” so you don’t have an objection against the argument so much as an application of its conclusion.

Brad: What do you mean by “pure actuality?”

Sarah: Pure act refers to something that doesn’t lack anything, something that doesn’t wait for another to give it something it already does not possess.


An Infinite Regress


Brad: Okay, well my second point is that I have no problem with an infinite regress. What’s the issue? Mathematicians in the 21st century don’t fully comprehend infinity, why assume a medieval monk got it right?

Sarah: it’s not enough to say we don’t fully comprehend infinity, one could say we are always learning about many concepts. What mistake does Thomas make in his argument? Isn’t it possible that the more we’ve learned about infinity since Thomas has confirmed rather than refuted his position?

Brad: look, I’m not a mathematician any more than you are. I just don’t see the problem in having causes that stretch back forever. You ask what mistake does Thomas make? He too quickly rules out the possibility of an infinite regress of “movers.” If you want to hang your hat on Thomas’ medieval views on reality, fine, I’m not convinced.

Sarah: let’s try a simple thought experiment. Imagine a chandelier that’s one link short of reaching the ceiling. If you let it go it falls, right?

Brad: Um, sure, yes, it would fall to the ground.

Sarah: Okay. Supposse the ceiling is thousands of feet high. So you add ten thousand more links to the chain. But it’s still one chain short so, and this is important, the chain is only being held up by other links, with ten thousand links it still falls.

Brad: I’m following.

Sarah: Let’s think of a higher ceiling. How about a million more links or a billion, if it doesn’t reach the ceiling it still falls. Now, suppose there are an infinite number of links in the chain, after every link there is another, but the ceiling is always one link away.

Brad: You’d have an infinitely high A/C bill for this ridiculously big home? Is that your point?

Sarah: Let’s be serious.

Brad: Sorry, continue.

Sarah: Even with an infinite number of links, those are the only things holding up the chandelier. If we agreed that billions or trillions of links in this chain can’t by themselves hold up the chandelier, then how could an infinite number of them do that? We’ve already seen that adding chains doesn’t help solve the problem. To keep the chandelier up we need something that doesn’t depden on anything else to stay up. Chains by they nature can’t do that, but a ceiling can.

Brad: Okay.

Sarah: The same is true in the universe. an infinite number of movers doesn’t explain why there is motion, only an unmoved mover, something that grants motion to all but receives it from none can explain that. So at the end of the day you either have to accept an infinite regress, which I think there are good reasons to think is not possible, things moving themselves for no reason, or an unmoved mover. I’m happy to take your thoughts on that and then maybe we can talk about the other ways.

Brad: Okay, I don’t mean to sound flippant, but I’m not buying it. First, using a mundane example like a chandelier is misleading—I’m not saying your intending to be misleading—but I think it is misleading. Yeah, we understand how everyday things like chain links, and chandeliers, and even gravity work, but to say that we therefore can conclude that the universe must be like a ceiling seems a little trite. We know so very little about this universe, I’m not going to be convinced by these sorts of examples, maybe there could be an infinite number movers, and if there is, whether or not that makes sense to you, you’ll have to learn to deal with it because that’s reality. You said you had five proofs? Why don’t move on?




Sarah: Before we do that, you said you were open-minded and willing to be proven wrong. Yet, when confronted with evidence for God you say there’s lots we don’t understand. So here’s a question: What would be something, in theory, that would show God exists that you couldn’t respond to with, “We don’t understand how everything works so maybe it’s not God”? It just seems to me like your atheism has something in common with certain religious beliefs, it can’t be falsified.

Brad: Oh that’s easy. God could appear in the sky right now, and with a big booming voice say, “Brad, I exist! You were wrong to be an atheist, Sarah is right, listen to her you doofus!” I’d also be convinced by one shred of evidence. Look, five thousand years ago there were no doubt people who had arguments for why the earth was flat, or, more recently, that the sun revolved around the earth. In both cases, though their arguments may have sounded compelling, they were wrong. Evidence is what should change our beliefs, not thought experiments and word games.

Sarah: Really? A Booming Voice? How do you know it’s God and not aliens, the government, or an eccentric billionaire? Saying “it’s God” doesn’t really tell us anything. Also, do you think we understand everything about sound or how the brain processes sound? Finally, shouldn’t God provide reasons that show he exists that all people can access, such as the natural world around us? But let’s say an infinite past universe of movers is possible, I still don’t think that disproves Thomas’s arguments because, while the second way relies on this principle, the third does not.

Brad: I think I’d just believe it were God because . . . Well, I was going to say that would be more plausible than aliens, but I don’t think that. Well, let’s say there were no signs of aliens and I had no signs of being mentally deranged. I think I’d just accept that it were true, whether or not you think I should, It think I just would. You say God should provide reasons that he exist that everyone can access, ah, yeah! That’s another great reason to think he doesn’t exist, since if he did, surely he’d know what would convince people and do that. But okay, what’s this third proof?

Sarah: I hate to be the one to flee the battle field, but I have to go. Think about what I said, okay?  And may I suggest a book?

Brad: Sure.

Sarah: You can borrow this one if you want. It’s called Answering Atheism. I think you’ll like it. I did. He’s very respectful and fair to atheists.

Brad: Dude, he’s another Catholic!

Sarah: What can I say, sometimes they get it right!

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11 thoughts on “Thomas Aquinas’ Arguments From Motion and Efficient Causality: A Socratic Dialogue

  1. You don’t address what to me is the simplest argument against Aquinas’ infinite regress proposition: if God started the Universe, what started God? If you can be happy with “God always was”, why can’t the Universe always have been? This is where most debates seem to end. Atheists are ok with “we don’t understand where the Universe came from”, Theists with “We don’t understand where God came from”.

    1. That’s because God existence do not depend upon some external cause, which seems the case for the universe. For us saying something like “we don’t understand where God came from” is simply nonsense.

      1. Exactly… the WHOLE POINT of the argument FOR God is that at the end of everything, there has to be SOMETHING/SOMEONE who does NOT come from anything. HE is the origin of everything, HE is outside of space, outside of time, HE, quite simply, IS. Which is how he revealed Himself to Moses. The name he gave is I AM. Now if God started from somewhere or something, He quite simply wouldn’t be God would he? Hence, “what started God?” is non-sense.

    2. God doesn’t “start” the universe on Aquinas’s account. He conserves it in existence. It is crucial to understand the act/potency distinction that lies at the foundation of Aristotelian and Thomastic metaphysics if you want to understand what Aquinas is driving at. Combinations of actuality and potentiality (aka all the things we observe around us everyday) are not able to serve as ultimate explanations for why there is change (called motion by Aristotle) in the first place. An ultimate explanation for the reality of change can only lie in the existence of something that is Pure Actuality, for reasons laid out by Thomas and his commentators. If you are interested, check out the work of Edward Feser. He is a great resource for modern Thomism.

    3. Thanks Paul, if God is a metaphysically necessary being, then his non-existence is impossible. If his non-existence is impossible then the objection “who created God?” is, to use Dawkins’ word, “vacuous,” since one is essentially asking, “who created the uncreated creator?” I think Aquinas gives us good reason to think that there must be a unmoved mover, an uncaused cause, an explanation that is not itself explained by something outside of itself, *and* that this thing exist beyond the universe. And, to quote Aquinas, “this all men speak of as God.”

  2. Matt,

    Great examples!

    A lot of confusion over infinite regresses in Thomas has arisen with the popular resurgence of the Kalam-style arguments which are based on the impossibility of an actually infinite series (e.g., Craig). A lot of people think that was Thomas’s argument as well when it is actually the opposite. Thomas had no problem with an infinite series coming into existence, but would still argue that even it needed a First Cause!

    (Oh and I think this should say “mean” : “—I’m not accusing you of being meaning”).

    1. Thanks Doug. I changed the spelling error, thanks for that. You’re right. Aquinas’ proofs do note depend upon the universe having a beginning. He disagreed with Bonaventure (and Craig) that the finitude of the world is philosophically demonstrable, though he obviously accepted it as a fact of revelation. I’m not sure if I side with Bonaventure or Aquinas. What are your thoughts?

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