To: Pearl K.
From: Geoffrey Klempner
Subject: Why Descartes needs to prove God's existence
Date: 1 February 2007 12:05

Dear Sachiko,

Thank you for your email of 25 January with your University of London Modern Philosophy essay in response to the question, 'Why was it important for Descartes to prove the existence of God and how well does he succeed?'

This is an excellent piece of work and represents a big step up from anything you have sent me before. In fact, I can't remember another example of an essay on this topic from any of my students which goes into the 'objections and replies' in the way that you have. Well done for that, and for explaining Descartes arguments, especially the trademark argument, with care and attention to detail.

The question is in two parts. As a matter of strategy, my feeling is that if this occurred in an exam you should try to spend as much time answering the question, 'Why is it important to prove the existence of God?' as you do on the arguments for God's existence.

By coincidence, the current issue (Issue 124) of Philosophy Pathways has a very good article by Alfredo Lucero-Montano which, to my mind, provides an interesting new angle on why God is needed by Descartes.

The usual answer - which is the one which you give - is along the lines of:

1. If there's any chance I am being deceived by an evil demon, then I don't know anything other than the fact that I exist.

2. Only the existence of a non-deceiving God rules out the possibility that I am being deceived by an evil demon.

This logically leaves open the possibility that I'm not being deceived by an evil demon but STILL don't know anything for other reasons. So Descartes has to argue for something stronger, to the effect that not only does the existence of God rule out the possibility that I am being deceived by an evil demon, but also God has so arranged the world that when I use my faculties of judgement and perception responsibly, I am able to gain knowledge. Descartes goes to some lengths to explain how it is that we do sometimes form false judgments, despite this favourable arrangement.

More can be said on this, for example, how it is that our sense of what constitutes a 'good explanation' or what makes an acceptable basis for an inductive generalisation corresponds to the way the world in fact is: i.e. the regularities we respond to are the right regularities for gaining a grip on the laws of nature. This is not something that Descartes explicitly discusses in the Meditations but it is part of what he means by a 'world made by a God who is not a deceiver'.

You do refer to the Cartesian Circle, which pads this answer out somewhat. However, that discussion is an answer to the question how well Descartes succeeds in his attempt to prove the existence of God, rather than the question why he needs God in the first place.

The argument in Alfredo Lucero-Montano's article seems to be that, without God, the cogito is not knowledge of an objective truth but merely a subjective state of certainty. I cannot doubt that I exist. But if the possibility of an evil demon cannot be ruled out, then this state of certainty does not amount to any real knowledge of myself qua thinking substance.

This makes the need to prove the existence of God even more urgent. The notion of a self, or thinking subject is without any metaphysical foundation in the absence of a proof of God's existence. That is a big claim, but it helps us to see what Descartes' response would be to the usual objections raised against the cogito, e.g. that it 'only proves the existence of a subject at this point in time', or 'only proves the existence of the thought, not the thinker'. All Descartes needs is a starting point for his argument for the existence of God, nothing more. That is what the 'cogito' (allegedly) provides. Only when God's existence is established is he able to say, with well-grounded confidence, 'I am a thinking substance'.

Regarding the second argument for the existence of God, the ontological argument, what do you think of the claim that Descartes is not guilty of 'making existence a predicate' because he is concerned with 'necessary existence'?

Forget about the cartesian circle for a moment. It does seem not incredible that someone would say, 'I have a clear idea of God as a being which possesses the property of necessary existence.' Surely, if the idea in question is clear, not self-contradictory or incoherent, then that proves the existence of God. Therefore the ontological argument is valid!

Kant's example of thinking of 50 Thales and 50 existing Thalers - there being no difference between the two thoughts - is irrelevant because the 50 Thalers do not necessarily exist. I might have 50 Thalers in my pocket or not. But a being which possesses the property of necessary existence must exist. It's as simple as that.

So, if you are looking to say everything that needs to be said on this topic, you would have to say something about the dubiousness of Descartes' claim that he does, in fact, have the idea of God that he thinks he has. The objection can't merely be to 'necessary existence' as a concept because we want to say that some things do necessarily exist: numbers and sets, or in general pure abstract objects. If you can coherently think of the concept of the null set or the number 0, then the null set or the number 0 'exist'. So why not God?

All the best,

Geoffrey