Aquinas, Summa Theologica



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© Michael Lacewing

Cosmological arguments from causation
The question at the heart of the cosmological argument is ‘why does anything exist?’ The argument is that unless God exists, this question is unanswerable.

Aquinas, Summa Theologica, Pt 1, Q. 2, Art. 3


The argument from motion We can summarise Aquinas’s argument from motion like this:

  1. Some things in the world are in motion.

  2. Whatever is in motion is put into motion by something else. Nothing can move itself.

  3. If A is put into motion by B, and B is also in motion, then B must have been put into motion by something else again.

  4. If this goes on to infinity, then there is no first mover.

  5. If there is no first mover, then there is no other mover, and so nothing is in motion.

  6. Therefore, there must be a first mover.

  7. The first mover is God.

We can raise questions about (2) and (5) and the move from (6) to (7). It turns out, however, that the argument from motion is – on one interpretation at least – just a special case of Aquinas’ argument from causation. Premise (2) of the argument from motion states that things are always put in motion by something else. We can understand this as a claim that the motion of A is an effect of some action of B, for instance B’s motion, which causes A’s motion. The first mover will, therefore, also be the first cause of movement. If the argument from causation fails, the argument from motion will fail as well. So we shall discuss objections to both versions together.



The argument from causation

  1. We find, in the world, causes and effects.

  2. Nothing can be the cause of itself. (If it were, it would have to exist before itself, which is impossible.)

  3. Causes follow in order: the first causes the second which causes the third, etc.

  4. If you remove a cause, you remove its effect.

  5. Therefore, if there is no first cause, there will be no later causes.

  6. Therefore, given that there are causes, there cannot be an infinite regress of causes.

  7. Therefore, there must be a first cause, which is not itself caused.

  8. God is the first cause.


Discussion The argument from causation is relatively straightforward. Premise (6) may cause some confusion, however. The idea of an infinite regress of causes is difficult to understand. The important point is this: infinity is not a very large number. If there is literally an infinite chain of causes, that chain of causes never has a starting point. We never reach an account, therefore, of how the process gets started – it never gets started, it has always been.

We know more about the universe now than people did when Aquinas was writing. Our best scientific theory suggests that the universe began just under 14 billion years ago as a Big Bang. But the Big Bang doesn’t necessarily mark the beginning of the chain of causation – what caused the Big Bang?

If we can infer the existence of a first cause, why think that first cause is God? Aquinas is here assuming a number of things about our concepts of God and of natural things. It is part of our concept of God that God is uncaused; it follows, for example, from his omnipotence. By contrast, our concept of natural things is that they are part of a causal chain, each thing being caused by something before it. So God obviously fits the bill as a first cause; nothing else does.

The Kalam argument


The Kalam argument is also an argument from causation, but it differs from Aquinas’ version by focusing specifically on things being caused to exist. (‘Kalam’ was medieval Islamic theoretical theology.)

  1. Of anything that begins to exist, something causes it to exist.

  2. The universe began to exist.

  3. Therefore, there is a cause of the existence of the universe.

To get to the conclusion that God is the cause of the universe, we have to add further premises to the Kalam argument.

Premise (1) assumes that something can’t come out of nothing. Premise (2) is based on the claim that the universe cannot have always existed. If the universe has always existed, it is infinite in time. This is impossible, so the universe must have a beginning. Why?

Infinity Infinite time is not a ‘very long time’. It means, quite literally, that there was no beginning, ever. Because the universe exists, to claim that the universe has always existed is to claim that an actual infinity – something that is in fact infinite – exists. This is quite different from talking about the idea of infinity. The idea of infinity makes sense; but does it make sense to think that something infinite actually exists?

The universe gets older as time passes, we naturally think. But this couldn’t happen if the universe were infinitely old, because you cannot add any number to infinity and get a bigger number: ∞ + 1 = ∞. So if the universe is infinitely old, it is not getting any older as time passes! Or again, to have reached the present, an infinite amount of time would need to have passed. But it is not possible for an infinite amount of time to have passed, since infinity is not an amount. So if the universe was infinitely old, it could never have reached the present.



The same point applies to an infinite series of causes (in support of Aquinas’ argument from causation). Each new cause doesn’t add one more cause to the series, since ∞ + 1 = ∞. And we would never have reached the point in the series of causes at which we are now if it were an infinite series.

Descartes, Meditation III, pp. 15–16


In the Meditations, Descartes tries to bring into doubt all his beliefs. He doubts whether an external, physical world exists, but is certain that he exists. In Meditation III, he offers a cosmological argument. It is unusual because the only thing that Descartes knows to exist, at this point in the Meditations, is himself. So Descartes asks what causes his existence, rather than the existence of the universe. As the argument is long and complicated, I have divided it into sections.

  1. If I cause my own existence, I would give myself all perfections (omnipotence, omniscience, etc.).

  2. I do not have all perfections.

  3. Therefore, I am not the cause of my existence.




  1. A lifespan is composed of independent parts, such that my existing at one time does not entail or cause my existing later.

  2. Therefore, some cause is needed to keep me in existence. My existence is not uncaused.

  3. I do not have the power to cause my continued existence through time.




  1. Therefore, I depend on something else to exist.

  2. I am a thinking thing and I have the idea of God.

  3. There must be as much reality in the cause as in the effect.




  1. Therefore, what caused me must be a thinking thing and have the idea of God.




  1. Either what caused me is the cause of its own existence or its existence is caused by another cause.

  2. If its existence is caused by another cause, then its cause is in turn either the cause of its own existence or its existence is caused by another cause.

  3. There cannot be an infinite sequence of causes.

  4. Therefore, some cause must be the cause of its own existence.

  5. What is the cause of its own existence (and so, directly or indirectly, the cause of my existence) is God.

Descartes adds a further argument, picking up premise (5).

  1. Some cause is needed to keep me in existence.

  2. There cannot be an infinite chain of causes because what caused me also causes my continued existence in the present.

  3. My parents, or any other supposed cause of my existence, do not keep me in existence.

  4. The only cause that could keep me in existence is God.

Discussion We will, in the next section, discuss objections that apply to all three versions of the cosmological argument. However, it is worth briefly evaluating Descartes’ unique addition of the claim that his continued existence through time also needs to be caused. There are two misunderstandings that need to be countered to make the claim clear.

One might object that my continued existence doesn’t require a cause, because nothing changes – I simply continue to exist. If I cease to exist, that requires a cause. But this misunderstands both causation and continued existence. I am sitting on a chair – nothing is changing. But there is a cause of this continued state of affairs, namely gravity and the rigidity of the chair. Should either of those standing conditions change, then I would no longer be sitting on the chair. I’d either be floating (no gravity) or sitting on the ground (collapsed chair). That people don’t die at any given instant is the result of whatever it is that keeps them alive. Therefore, we should accept that my continued existence does require a cause. It is worth noting that what causes my continued existence must itself continue to exist – it can’t be a cause in the past, since my continued existence must be caused from moment to moment.

We might object, however, that my continued existence is simply dependent on the immediately preceding state of affairs, and so we don’t need to say that what caused me to exist in the first place also keeps me in existence. For instance, my bodily processes keep me alive at any moment, but they didn’t give me life. But this forgets that Descartes is talking about his self, which is his mind, not his body. Descartes has argued that he, his mind, is an entirely separate substance from the body. So what keeps a mind in existence through time? If it was something in his mind itself, he would know, he claims. And it can’t be his parents – they only gave existence to his body (if he has one). The only explanation he can think of is God.

It is worth noting, then, that we could argue that Descartes is wrong to think that minds are separate substances from bodies. If we are bodies, then our continued existence could be caused by the ever-changing physical conditions of our bodies and environment.


Issues concerning arguments from causation


Hume raises three objections to cosmological arguments from causation. The first attacks the assumption that everything (apart from the first cause) has a cause. The second attacks the claim that there must be a first cause. And the third attacks the claim that the first cause is God.

The causal principle Must everything have a cause? Hume argues that this claim is not analytic; we can deny it without contradicting ourselves (A Treatise of Human Nature, Bk 1, Pt 3, Ch. 3). (Every effect has a cause is analytic. But is everything an effect?) Likewise,

‘Something cannot come out of nothing’ is also not analytic. That means that these claims are not certain. Now, our experience supports these claims, but experience cannot establish that a claim holds universally. And in particular, we may argue, we have no experience of such things as the beginnings of the universe (nor perhaps, the cause of the continued existence of a mind as a separate substance). Furthermore, the beginning of the universe is not an event like events that happen within the universe. For instance, it doesn’t take place in space or time, since both come into existence with the universe. We cannot apply principles we have developed for events within the universe, such as ‘everything has a cause’, to the universe as a whole.



Is a first cause necessary? Cosmological arguments reject the possibility of an infinity of causes to infer the existence of a first cause. But do we know that an infinity of causes is impossible? Again, it is not an analytic truth, nor can we have experience of this matter. It seems conceivable, therefore, that something has always existed, and each thing has in turn caused the next. This universe – what exists as we know it now – came into existence just less than 14 billion years ago. But even if this universe has a beginning, perhaps it was caused by a previous (or another) universe, and so on, infinitely.

However, we saw above that an actual infinity of causes leads to paradoxes. If these paradoxes cannot be resolved, then they are genuine self-contradictions (e.g. that each new cause adds to the number of causes and that it does not). Anything that entails a contradiction must be false. So, if he cannot solve the paradoxes, Hume is wrong: we can deduce that there cannot be an infinite series of causes.

To this response, we can add another. Science tells us that time came into existence with the universe. Time itself ‘began’ with the beginning of the universe just under 14 billion years ago. That means that whatever caused the universe (if it has a cause) cannot exist ‘before’ the universe – there is no ‘before’ the universe! Instead, the cause of the universe must exist outside time (which is fine if the cause is God). We think incorrectly then if we think that another universe, one that existed before this universe, caused this universe. If there is an infinite series of causes, this cannot be how it takes place. Hume might respond that we simply can’t know the answer here. So we should draw no conclusions.

A third response claims that if there is not a first cause, we cannot explain the whole series of causes. We can explain one cause in terms of the one before, but not why there is a series of causes at all. Hume dismisses this – ‘uniting’ the individual causes into a series is ‘an arbitrary act of the mind’. The series doesn’t have any separate existence that needs explanation (Dialogues concerning Natural Religion, §9, p. 39).



Is the first cause God? The causal arguments from Aquinas and the Kalam say little to defend the claim that the first cause is God. Descartes, however, argues that the cause of his existence must be a mind which itself has the idea of God (10). This depends on premise (9), that a cause must have as much ‘reality’ as its effect. Hume argues that this is neither an analytic truth nor is it established by experience. Rather, ‘anything may produce anything’ (A Treatise of Human Nature, Bk 1, Pt 3, Ch. 15, §1). There is no a priori reason to think that matter cannot produce thought, and experience would indicate that matter does indeed produce thought. So we cannot infer that either the first cause or what sustains Descartes’ continued existence as a mind must itself be a mind, let alone one that has the perfections attributed to God.

An inductive argument


The three versions of the cosmological argument above are intended to be deductive. However, Richard Swinburne claims that the cosmological argument is better understood as an inference to the best explanation (The Coherence of Theism). If we look over the three cosmological arguments above, it is apparent that we can’t deduce God’s existence. But – at least for the versions from Aquinas and the Kalam – the premises are plausible, and the inferences are intuitive. God’s existence isn’t logically proven, but it is probable, given the premises, because God’s existence is the best explanation for why the universe exists.

For example, although it is not an analytic truth that everything that begins to exist has a cause, Hume agrees that it is probable – our experience supports it. And the theory of the Big Bang and the problems with actual infinities make it more plausible that the universe (or matter/energy) has not existed without beginning, but came into existence. If we reject God as an explanation for the existence of the universe, we need another explanation that is better.



Scientific explanation must always assume that something exists. If we explain this universe in terms of another universe, we then have to explain the existence of that universe. This other universe is completely inaccessible to us, and we have little evidence for its existence. Furthermore, this method will lead to regress, because we will have to explain the cause of that universe just as we explain the cause of this universe.

But we can object that introducing God is not the best explanation, e.g. because it is not the simplest. We can also object that even this version of the cosmological argument doesn’t show that the cause of the universe has the traditional attributes of God.


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