Descartes’ argument for God’s existence in the 3rd Meditation

Sam asked:

Lay out the structure of Descartes argument for God’s existence in Meditation 3. What is the crucial premise in the argument, and what evidence does Descartes provide for it? How might we object to the argument?

Answer by Craig Skinner

Descartes doesn’t set out his arguments explicitly listing premises and conclusions. He antedates predicate logic and was no fan of syllogistic logic. He seems to think that God’s existence is pretty well self-evident, and arguments mere heuristic devices for the more slow-witted meditators.

He may therefore have been disappointed that his arguments were criticised rather than acclaimed by theologians (and others), as detailed in the Objections together with his Replies which Descartes published along with his Meditations.

His Causality Argument in Meditation 3 is a bit different from the Design Argument or the usual Cosmological Arguments; and from the Ontological Argument, his version of which is in Meditation 5. But we need to dip our toe into the waters of scholastic philosophy to grasp it.

A fair reconstruction is:

P1: I have the clear and distinct idea of God (a most perfect being: infinite, eternal, omnipotent, benevolent).

P2: A cause must be at least as great (real) as its effect.

C: This idea of God (P1) can’t be from (imperfect) me (P2). Its cause must be God or (impossibly) greater. So God exists.

The argument is valid. But it is sound only if P1 and P2 are true. Both can be challenged.

You ask what is the crucial premise.

All premises in an argument are crucial in the sense that if any one is false, the argument is unsound. Let’s count P2 as the crucial one because it looks a bit obscure, and was the one more criticized at the time. I will say something about P1 later.

Premise 2:

The relevant discussion is couched in technical scholastic terms. Two types of reality (being) are distinguished regarding ideas. The existence of an idea (its formal reality) and the content of an idea (its objective reality). Here ‘objective’ refers to the object contained in the idea, rather like the modern use of ‘subjective’ – it refers to the tree (say) in my mind, not the tree in the garden. The notion of degrees of reality is the introduced. Ideas all have the same degree of formal reality, all being states of mind, but differ in degrees of objective reality – lowest is a ‘mode’ (a property of a thing eg shape), intermediate is a finite substance, highest is an infinite substance.

P2 therefore expresses the ‘Causal Principle’: the degree of formal reality of the cause must be at least as great as the objective reality of the effect. Hence an idea whose content (objective reality) is infinite (my idea of God) can’t have its cause in a finite being (with less than infinite formal reality) such as me, only in God, so God exists.

What evidence does Descartes provide for P2?

None. It is simply an assertion. Philosophers like to call something a Principle — Principle of Sufficient Reason, Principle of Identity of Indiscernibles, Final Anthropic Principle etc — when they want us to swallow an idea without good argumentative support.

Objections to the argument:

P1 is questionable:

1. Descartes assumes we all have the same (God-given) innate ideas. But I can simply deny having the P1 idea. Gassendi (5th Objections) says our finite minds can’t have an idea of infinity. Descartes replies that we can – it’s our understanding that is limited, not the thing of which we have (limited) understanding. I side with Descartes here. But it doesn’t follow that I have a clear and distinct idea of God. Descartes gives no criteria for, or definition of, clear and distinct ideas, no guidance as to recognition of slightly unclear or somewhat indistinct ideas which are thereby unreliable.

2. The argument from clear and distinct ideas is viciously circular. The conclusion that God exists is based on a clear and distinct idea, but the truth of these ideas is only guaranteed by assuming the existence of God.

P2 is false: two objections (both by Mersenne, 2nd Objections)

1. The idea of God CAN come from me. Having some degree of perfection, I can posit higher and higher degrees of it.

Descartes replies that the idea does in a sense come from me. It is innate, planted by God. We couldn’t form the idea of God if God didn’t exist. But here, Descartes simply repeats the Causal Principle and begs the question.

2. Animals and plants (greater) come from inanimate causes (lesser). Descartes replies that animals lack reason and so have no perfection not found in inanimate matter, or, if they do, it comes from some other source. I find Descartes unpersuasive here, and would generalize Mersenne’s point to say that simple things plus simple rules can yield complexity eg simple initial conditions in the cosmos plus laws of nature allow atoms, compounds, galaxies, life and mind.

To say that finite minds need an infinite cause simply begs the question as to God’s existence.

In short, Descartes’ argument is unsound.

Of course neither Descartes nor anybody else has proved the existence of God. Belief in God is a matter of faith and revelation. If God existed he could show himself unequivocally (alleged revelations to date being highly dubious), but in the absence of this, no proof is possible in my view. Hence those of us unconvinced by revelation and unwilling to make leaps of faith should be, at least, agnostic.


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