Thomas Aquinas on the existence of God

Fred asked:

In the ‘Summa Contra Gentiles’ by Thomas Aquinas, Aquinas argues to reasonably proof the existence of God. His arguments are based on a series of inferences that existed when he was alive. However, I do not think, as a philosophy student, I should repeat Aquinas’ views to answer exam questions.

My question now is: If the above statement is right than what are techniques I can use to answer philosophical questions?

Answer by Jürgen Lawrenz

What you should do is to focus on the word ‘proof’ and ask yourself, what does it mean?

The exercise of Aquinas is called a ‘proof’, but in reality it is no such thing.

It is simply an engagement with the problem of infinite regress and supposes it is reasonable (as Aristotle suggests, on whom Aquinas leans) that the infinite chain of causes must stop somewhere. If one is so disposed, one can call this ‘God’, and Aquinas does so, but Aristotle doesn’t.

In fact Aristotle came to a recognition that a single cause is unachievable, since the sum of all processes that regress to infinity cannot be shown to converge. There could be a plurality of prime movers. But Aquinas evades this problem by bringing angels into the argument, which is hardly a reasonable supposition and would be rejected by people who don’t believe in angels.

It is not even as good as the reasonable supposition that occurs quite frequently in Court, that if a lot of circumstantial evidence points to a person as a murderer, then we are likely to have a case of ‘proof beyond reasonable doubt’. We know that errors have been made in such cases; but this proves nothing more than that humans may still err when the most rigorously compiled evidence is to hand.

Aquinas ‘proof’ is therefore nothing better, as you say, than a series of inferences. To amount to a proof, he should be able to marshal evidence and count on universally acceptable logical premises.

When you inspect his procedure in that light, you will find mere conjectures and hypotheses standing behind it. They in turn are not based on factual evidence, but on further conjectures, hypotheses, hearsay, articles of belief and so on. This is considerably less than a Court would encourage any jury to regard as ‘proof beyond reasonable doubt’. Unfortunately for Aquinas, the kind of logic that was acceptable in his era is no longer adequate.

It is well-known, but worth repeating in such cases, that if your major premise is that the moon is made of green cheese, then the finest inference on any astronomical issue can only result in a nonsense conclusion.

We come to the point, then, that a ‘reasonable proof’ of God’s existence is not possible. We have no data which could serve as premises that would satisfy factual, let alone scientific, criteria.

Accordingly the best you can ask for, is that a ‘reasonable hypothesis’ can be framed and achieve consent in some circles. But to stick with the truth, hypothesis is still aiming too high, because even an hypothesis must be made on the basis of fairly strong factual evidence.

So that leaves you with ‘conjecture’; and indeed what Anselm, Aquinas, Descartes and other thinkers broadcast as an ‘ontological proof’ is nothing more than a conjecture, based on certain logical question-and-answer games that are suggestive, but a million miles away from ‘proofs’.

You should be aware, finally, that both Leibniz and Kant took the premises of such ‘ontological proofs’ and demonstrated by logical inferences the opposite result. Since this is self-contradictory, it means, of course, that there is something wrong with the premises.

You should recall finally that Kant made the very convincing claim that anything related to God cannot be a topic on which we can have knowledge. He expressly wrote in his Critique of Pure Reason that one of his purposes was to settle the question of what we can know, so as to ‘make room for faith’.

So our conclusion here must be that the word ‘proof’ is too often misunderstood and misused, even by philosophers. ‘Evidence’ is not proof, although it may end in a proof. Hypotheses, conjectures, beliefs and all sort of presuppositions are frequently taken as ‘evidence’, which all too often they are not. But clearly they are already a step down from the possibility of use as premises in proofs.

Accordingly you cannot have a ‘reasonable proof’. You either have a proof or you have none.


Ashley asked:

In the original version of his ‘Third Way’ Aquinas makes an obviously invalid inference. Say as clearly as you can what that inference is, and explain why it is invalid.

Answer by Martin Jenkins

Here is the alleged ‘proof’:

1. Things observed are subject to the possibilities of existence and non-existence.

2. What exists is generated from non-existence by something existing.

3. What generates must itself not exist at some time.

4. If not existing is possible for things, then at sometime, there was nothing really existing.

5. If there was nothing really existing, there would be nothing now. This is false as there is something.

6. Therefore, there must be something which necessarily exists and is not subject to the possibility of non-existence. This thing is God.

The inference made is from the possibility/ impossibility of particular existing things to that of an absolute state of non-existence. It doesn’t follow as it is possible that there is always some thing existing even if it is also possible for it to not exist. This chain of contingent existence can reach infinitly back in time removing the need for a necessary being and first cause.

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