Evaluating Aquinas’s argument
1 Why does the first cause have to be God?
Here it’s worth remembering my dictionary-style definition of ‘God’ given at the beginning of this chapter: ‘the supreme personal being existing beyond the world, creator and ruler of the universe’. Does Aquinas think that he’s shown, just by the argument that I outline above, that the first cause which gets going the world’s order of causes is bound to be God, rather than, say, Mickey Mouse?
The answer to this question (though you can’t tell this just from the extract above) is ‘no, he doesn’t’. Aquinas is not trying to prove that the first cause of all the other causes is what ‘everyone calls “God”’ (and not, for example, what ‘everyone calls Mickey Mouse’). He would agree that we need separate arguments to prove this (and elsewhere in his works, he tries to provide them). Aquinas’s argument here is intended simply to direct our thinking to the idea of something, or someone, that explains the world’s existence. The ‘Second Way’ is not supposed to prove, all on its own, that God is omnipotent, omniscient, and has all the other qualities that Aquinas and other Christians believe he has.
2 If we need God to explain the existence of the world, then don’t we need something else to explain the existence of God?
The answer to this is simple: no, because Aquinas’s argument is not an attempt ‘to explain the existence of the world’. Aquinas is not presenting a cosmological argument here (that is, an argument of the (7b) kind, which deduces God’s existence just from the world’s existence). Rather, Aquinas is looking for an explanation of a specific feature of what exists in the world, namely the order of causes. His argument is that there can’t be an order of causes in the world unless something outside the world got the order of causes going in the first place. We won’t need an explanation of the same sort for what Aquinas calls ‘God’ unless this God is also an order of causes. But Aquinas does not think that God is such an order of causes.
This question would be a good criticism, if Aquinas’s argument was a form of the cosmological argument (7b) which asks, ‘Why is there something rather than nothing?’ But that is not Aquinas’s argument, so this question does not apply to it.
3 Why couldn’t an order of causes have no start, but go backwards all the way to infinity?
Aquinas argues that there can’t be an order of causes that doesn’t begin somewhere. To use the model I’ve suggested, the row of dominoes can’t stretch back into infinity without any beginning. If there are to be any subsequent domino-falls, there must be a first domino-fall. Something outside the series of domino-falls has to act on the row of standing dominoes to get them to start falling in the first place. Just likewise, says Aquinas, something outside the order of causes that we see in the world has to act on them to get them going. As we saw, Aquinas thinks that this outside intervener is God.
So what about this claim that the order of causes must be started by the intervention of something outside it? The trouble with this claim, it seems to me, is not that we have reason to think that it’s false. It’s rather that we don’t have any good reason to think that it’s true or that it’s false. We can know – at least I think we can! – that there couldn’t be a series of falling dominoes without a first falling domino. But how can we possibly generalise from that simple case, which just concerns one simple sort of cause–effect relation, to a conclusion about the whole causal order? In the words of the eighteenth-century thinker David Hume (1711–1776):
When we look beyond human affairs and the properties of the surrounding bodies: when we carry our speculations into the two eternities, before and after the present state of things; into the creation and formation of the universe; the existence and properties of spirits; the powers and operations of one universal spirit, existing without beginning and without end; omnipotent, omniscient, immutable, infinite, and incomprehensible: we must be … apprehensive, that we have here got quite beyond the reach of our faculties.
So it’s very hard to tell whether Aquinas is right that the order of causes must have a beginning outside itself. And that suggests an answer to our earlier question whether this argument of Aquinas for God’s existence is a good one. Take a moment to think again about how you would answer that.
I think the answer has to be that, at any rate, Aquinas’s argument can’t make us sure that God exists. For his argument to give us certainty about God’s existence, we would have to be certain that the order of causes that we see in the world cannot stretch back to infinity, but must have had a beginning. But apparently, we can’t be certain of that. This uncertainty about one of the argument’s premises transmits to its conclusion.