Aquinas' Second Way: Proof of God's Existence as the Uncaused First Cause, Study notes of History

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Aquinas’ Five Ways

Today we begin our discussion of the existence of God. The main philosophical problem about the existence of God can be put like this: is it possible to provide good arguments either for or against the existence of God? We will be considering a few traditional arguments for the existence of God, and the main argument against the existence of God, the ‘argument from evil.’ The main positions on the question of the existence of God are three:

• Theism, the belief that God exists.
• Atheism, the belief that God does not exist.
• Agnosticism, not believing that God exists and not believing that God does not exist.

But there is one related topic that is worth taking up at the outset. Some people are inclined to object to considering arguments for and against the existence of God on the following grounds: “Belief in God is a matter of faith, not of reason. So it is not the sort of thing that we should be arguing about -- the whole point of belief in God is that you should believe it without needing any evidence or arguments!” There are a few things to be said about this line of thought. First, it is not obvious that faith and reason have to be opposed in the way that the objection assumes. In particular, it might be the case that one ought to believe in God whether or not one understands any good arguments for God’s existence, but that it is still worthwhile to look for good arguments. So, for example, many medieval philosophers took as the motto of their philosophical investigations “faith seeking understanding”: not as giving up faith in God, but as seeking a deeper understanding of the object of that faith. Second, whether you are an atheist or a theist, you might think that you will stick to what you believe even if you cannot back it up with arguments. But even if this is so, you seem to have an obligation to be able to respond to arguments for the contrary view. Third, you might have none of these views. You might think: I’d like to decide for myself whether or not God exists, and I want to know why I should believe one thing or the other. And if this is your attitude, then it seems that the only place for you to look for help are the arguments that have been offered for and against God’s existence.

The first three arguments for the existence of God which we’ll be discussing are all among the proofs of God’s existence offered by St. Thomas Aquinas. Thomas offered 5 proofs for God’s existence; of the two which we will not be discussing, one is the topic of your first paper. Thomas was born in 1225 and, while his works were extremely controversial in their time -- some were condemned as heretical by the bishop of Paris -- he has since come to be regarded as the greatest theologian and philosopher in the history of the Church. His Summa Theologiae -- from which the arguments we will be discussing were taken -- is regarded by many as the definitive exposition of the Catholic faith.

We begin with Aquinas “second way” -- his second argument for the existence of God. In order to evaluate this argument, our first task is to identify Aquinas’s premises. One key premise seems to be his idea that nothing can be the efficient cause of itself -- i.e., that Nothing can be the cause of its own existence.

We begin with Aquinas “second way” -- his second argument for the existence of God. In order to evaluate this argument, our first task is to identify Aquinas’s premises. One key premise seems to be his idea that nothing can be the efficient cause of itself -- i.e., that Nothing can be the cause of its own existence. A second idea, which Thomas expresses in the passage immediately following, is that there can be no infinite chain of causes: The chain of causes of things coming into existence cannot be infinite.

So far, then we have two premises, and a pair of conclusions. The pair of conclusions was: A. There is a cause of the existence of some things which was not itself caused to exist. B. This “first cause” is God. A first question: is how are these two conclusions related? A plausible thought in this case seems to be that what Aquinas is really after is a proof of the existence of God. So conclusion A above is really just a kind of preliminary conclusion; the idea, Aquinas seems to think, is that if we can give an argument for A, we will then be in a position to get from there to the conclusion, B. So let’s focus first on how we could get to conclusion A.

A natural first thought is that perhaps this conclusion simply follows from the two premises which we have already identified, namely Nothing can be the cause of its own existence. The chain of causes of things coming into existence cannot be infinite. If this is right, then the following argument should be valid: Nothing can be the cause of its own existence. The chain of causes of things coming into existence cannot be infinite.

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There is a cause of the existence of some things which was not itself caused to exist. Is this argument valid? It seems that this argument is not valid. After all, we can imagine that nothing has ever been caused to come into existence. If that were true, then both premises would be true, and yet the conclusion would be false. How can we fix this problem with Thomas’ argument?

Here’s one way in which that could happen. Let x y mean that x brought y into existence. Then it seems that the chain of causation could, for all we have said, look like this: In this diagram, some things are caused to exist (premise 1), nothing is the cause of its own existence (2), and there are no infinite chains of causation (3). However, if this were the way things were, then our conclusion would be false: there would be no cause of the existence of things which was not itself caused to exist. This suggests that, as it stands, our interpretation of Aquinas’ argument is invalid. How might Aquinas fix this problem with the argument?

A natural idea is to modify premise 2:

2. Nothing can be the cause of its own existence. Plausibly, with this premise, Aquinas didn’t just want to rule out things being the immediate cause of their own existence; he also wanted to rule out the possibility of something being one of a series of causes one of whose effects is its own existence. One way to express this idea is as follows: let’s say that if A causes B and B causes C, then A, even though not the direct cause of C’s existence, is still causally responsible for C’s existence. And let’s extend this notion of causal responsibility to cover causal chains of arbitrary length, so that if A1 causes A2, and A2 causes A3, and ..... Ax causes Ay, A1 is causally responsible for Ay, no matter how long this series of causes is. 2. Nothing can be the cause of its own existence, or be causally responsible for its own existence. This seems like a plausible claim, and if we replace 2 with 2, it looks like Thomas’ argument is valid:
3. At least one thing has been caused to come into existence. 2*. Nothing can be the cause of its own existence, or be causally responsible for its own existence.
4. The chain of causes of things coming into existence cannot be infinite.

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C. There is a cause of the existence of some things which was not itself caused to exist.

Aquinas’ 2nd way

1. At least one thing has been caused to come into existence. 2*. Nothing can be the cause of its own existence, or be causally responsible for its own existence.
2. The chain of causes of things coming into existence cannot be infinite.
3. There is a cause of the existence of some things which was not itself caused to exist. (1,2,3)
4. If there is a cause of the existence of some things which was not itself caused to exist, then God exists.

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C. God exists. (4,5) This shows how much work is often required just to come up with a valid version of an author’s argument. And, of course, doing this is just half of our job: we want to know whether the argument is not just valid, but also sound. To figure this out, we have to ask: are any of Aquinas’ premises false? If not, then we will have a sound argument for the existence of God.

Aquinas’ 2nd way

1. At least one thing has been caused to come into existence. 2*. Nothing can be the cause of its own existence, or be causally responsible for its own existence.
2. The chain of causes of things coming into existence cannot be infinite.
3. There is a cause of the existence of some things which was not itself caused to exist. (1,2,3)
4. If there is a cause of the existence of some things which was not itself caused to exist, then God exists.

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C. God exists. (4,5) This shows how much work is often required just to come up with a valid version of an author’s argument. And, of course, doing this is just half of our job: we want to know whether the argument is not just valid, but also sound. To figure this out, we have to ask: are any of Aquinas’ premises false? If not, then we will have a sound argument for the existence of God. We can immediately eliminate premise (4) from consideration. After all, premise (4) is supposed to follow from 1, 2, and 3, and we have already agreed that the argument is valid; so, it seems that if the first three premises are true, premise 4 must be as well.

Let’s turn to premise 3 first:

3. The chain of causes of things coming into existence cannot be infinite. One might wonder how Aquinas could know this. After all, the idea of an infinite series of things is perfectly coherent -- just think of the series of natural numbers (or, for a series which is infinite in both directions, the set of integers). As noted above, Aquinas does provide an argument for this premise. His argument is contained in the highlighted portion of the argument: What is the argument here? It is difficult (for me, at least) to see. Aquinas seems to be thinking of an infinite series as a series from which a first cause is “taken away.” And he’s right that if you take away a cause, you often take away all of its effects, whether direct or indirect. But this seems to be the wrong conception of an infinite series; an infinite series is not one whose first member was taken away, but a series which is such that, for each member of the series, there is one which comes before it.

Can we give any other defense of the assumption that the chain of causes of things coming into existence must be finite in length? One way to argue for this would be to show that there is some sort of absurdity in the idea of an infinite chain of causes of events. One attempt to show this is the example of “Thomson’s lamp”: a lamp which is turned on and off an infinite number of times between 3:00 and 4:00 one afternoon. The infinite series of events then can be represented as follows: on, off, on, off, on, off .... and so on, without end. Because there is no end to the series, every “on” is followed by an “off”, and every “off” is followed by an “on.” So it seems that at 4:01 the lamp can be neither on nor off. But this is absurd; there is no other state for the lamp to be in. If this shows anything, it shows that there cannot be an infinite series of events in a finite time. Can you see why? What does this tell us about premise 3?