Anselm's ontological argument purports to be an a priori proof of God's existence. Anselm starts with premises that do not depend on experience for their justification and then proceeds by purely logical means to the conclusion that God exists. His aim is to refute the fool who says in his heart that there is no God (Psalms 14:1). This fool has two important features.

He understands the claim that God exists. He does not believe that God exists. Anselm's goal is to show that this combination is unstable. Anyone who understands what it means to say that God exists can be led to see that God does exist. On this view, the atheist is not just mistaken: his position is internally inconsistent.

What follows is an attempt to clarify the argument as it is presented in Chapter II of the Proslogium. The argument in Chapter III is rather different, and in some ways more interesting. After you have worked through this page, you might try to produce a similar gloss on the second argument. This will not be easy: the argument is notoriously complicated. But you might find it a useful exercise nonetheless.

A Running Paraphrase of the Argument

Let's work through the argument as Anselm presents it.

Anselm writes:

... we believe that thou art a being than which nothing greater can be conceived. This is Anselm's definition. We might paraphrase it as follows:

By "God" we mean an absolutely unsurpassable being, a being that cannot conceivably be improved upon. As we've stressed, you do not need to agree that this is what the word "God" ordinarily means. Treat it as a stipulation. Clearly, if Anselm can establish the existence of a being of this sort, his conclusion would be of immense philosophical and theological significance.

Or is there no such nature, since the Fool has said in his heart, there is no God? This puts the question: Is there in fact a being with the properties our definition assigns to God?

But, at any rate, this very fool, when he hears of this being of which I speak - a being than which nothing greater can be conceived - understands what he hears, and what he understands is in his understanding; although he does not understand it to exist. This begins and ends straightforwardly. The fool understands the definition of God but denies that God exists. The first hint of strangeness comes in what seems to be a parenthetical remark: "what he understands is in his understanding". Anselm apparently proposes to treat the understanding or the mind as if it were a place, and to speak of things existing "in the understanding". Anselm's assumption here is that if I understand claims about God, then we may say that God exists in my understanding or in my mind.

For it is one thing for an object to be in the understanding, another to understand that the object exists. For when a painter first conceives of what he will afterwards perform, he has it in his understanding be he does not yet understand it to be, because he has not yet performed it. But after he has made the painting, he both has it in his understanding and he understands that it exists, because he has made it. Anselm here explains a distinction. It is one thing for an object to exist in my understanding, and another for me to understand it to exist. This is a familiar distinction, even if the terms are not familiar. Ghosts, trolls, flying saucers and the like are all things I can think about. We might say that I have ideas of these things; Anselm says that they exist in the understanding. Anselm's point is that in general there is a difference between saying that something exists in my understanding and saying that I understand (or believe) it to exist. Trolls exist in my understanding; but I do not understand them to exist.

Hence even the fool is convinced that something exists in the understanding, at least, than which nothing greater can be conceived. For when he hears of this, he understands it. And whatever is understood, exists in the understanding. Here Anselm applies the distinction he has just drawn to the case of God. The fool understands claims about God. So God - a being than which none greater can be conceived - exists in his understanding. Anselm means this to be an entirely uncontroversial claim.

And assuredly, that than which nothing greater can be conceived cannot exist in the understanding alone: then it can be conceived to exist in reality, which is greater. Therefore, if that than which nothing greater can be conceived exists in the understanding alone, the very being than which nothing greater can be conceived is one than which a greater can be conceived. But obviously this is impossible. This is the heart of the argument. The trick is to show that God cannot possibly exist in the understanding alone. Anselm begins by contrasting existing in the understanding with existing in reality. This by itself is not problematic. Trolls exist in the understanding alone; Bill Clinton exists both in the understanding and in reality; and no doubt there are things that exist in reality that do not yet exist in the understanding because no human being has ever managed to frame a thought about them. The picture seems to be as follows:

In the area marked A we have things that exist in the understanding alone; in the area marked B we have things that exist both in the understanding and in reality; and in the area marked C we have things that exist in reality but not in the understanding. (For obvious reasons, we cannot give any concrete examples of the last category.)

At this stage the fool has conceded that God exists in the understanding: so God belongs either in A or in B. Anselm now argues that God cannot exist in the understanding alone. The argument seems to proceed as follows.

(1) Suppose (with the fool) that God exists in the understanding alone.

(2) Given our definition, this means that a being than which none greater can be conceived exists in the understanding alone.

(3) But this being can be conceived to exist in reality. That is, we can conceive of a circumstance in which theism is true, even if we do not believe that it actually obtains.

(4) But it is greater for a thing to exist in reality than for it to exist in the understanding alone.

(5) Hence we seem forced to conclude that a being than which none greater can be conceived can be conceived to be greater than it is.

(6) But that is absurd.

(7) So (1) must be false. God must exist in reality as well as in the understanding.

This reading of the argument is amply confirmed by the final paragraph: Therefore, if that than which nothing greater can be conceived exists in the understanding alone, the very being than which nothing greater can be conceived is one than which a greater can be conceived. But obviously this is impossible. Hence there is no doubt that there exists a being than which nothing greater can be conceived, and it exists both in the understanding and in reality.

A Reconstruction of the Argument

This is a useful first pass at the argument. Now let's go over it and try to isolate its most fundamental assumptions. (I'll highlight the premises of the reconstructed argument in red.) Remember the argument's dialectical context. The aim is to refute the fool - or less tendentiously, the rational atheist. So what we want to know about these premises is whether the fool should accept them. There is first the definition:

(a) By "God" we mean "a being than which no greater being can be conceived"

Then there are some assumptions about the Fool's understanding.

(b) We understand what it means to speak of a being than which no greater can be conceived. We understand what these words mean.

(c) We can conceive of such a being's existing in reality.

Anselm now assumes a principle that he clearly regards as trivial.

(d) If we understand what it means to speak of X, then X exists in the understanding.

From (a),(b) and (d) we may now infer:

(e) God exists in the understanding.

(Note: this is not a premise. It is an intermediate conclusion supported by a quick argument from premises we have already accepted.)

Anselm now employs a form of reasoning called reductio ad absurdum. This is a very useful technique. In a proof of this sort, we begin by assuming the opposite of what we want to prove. Then we derive a contradiction or an absurdity from this supposition. And from this we conclude that our original assumption was false. The general form of such an argument is as follows:

Suppose P From P it follows that Q But Q is absurd (self-contradictory). Therefore P is false. For Anselm the target of his reduction is the proposition that God exists in the understanding alone. So let us suppose that this is the case:

(f) Suppose that God exists in the understanding but not in reality.

From (f) and (c) we may now infer

(g) God in fact exists in the understanding alone, but he may be conceived to exist in reality as well as in the understanding.

At this point Anselm wields what is perhaps his most controversial premise. It is hard to know exactly how to formulate it. But something like the following seems to be what Anselm has in mind.

(h) If something exists in the understanding alone, but can be conceived to exist in reality, then that thing can be conceived to be greater than it actually is.

The idea seems to be: if we compare two things that are alike in all respects except that one exists in the understanding alone and the other exists in reality, then the one that exists in reality is clearly greater, better, more perfect. We will have to discuss the cogency of this assumption in class. But suppose for now that it is granted. We may then argue as follows. From (g) and (h) it follows that

(I) God can be conceived to be greater than it actually is.

But this is absurd. For given our definition (a), this just means that

(j) A being that cannot be conceived to be greater than it is can be conceived to be greater than it is.

From which it follows that our supposition (f) is false. We may therefore conclude

(k) God exists in reality.

Now you should ask: Is this a valid argument as it stands? Are the assumptions plausible? Would the fool be willing to grant them? Would you be willing to grant them? If you think that the argument is not a good one, you are under an obligation to say where it goes wrong. It might go wrong in several places. See how many "mistakes" you can find.