Anselm

The Ontological Argument: The ontological argument for the existence of God is not based on observation of the world, or on any form of external evidence, but simply on a particular definition of the meaning of 'God'. In other words, it says:

If you understand what God is, you understand that he must exist.

The argument was set out by Anselm (1033 - 1109), Archbishop of Canterbury, in the opening chapters of his Proslogion. He makes it clear that he is not putting forward this argument in order to be able to believe in God, but that his belief leads him to understand God's existence in this particular way - a way which leads him to the conclusion that God must exist.

Religious experience leads him to speak of God as 'the most real being, than which nothing greater can be thought'. [This does not mean something that just happens to be physically bigger, or better, than anything else - it is the idea of 'perfection', or 'the absolute', the most real thing.]

In the second chapter of Proslogion, the argument is presented in this way:

Now we believe that thou are a being than which none greater can be thought. Or can it be that there is no such being, since 'the fool hath said in his heart, "There is no God"'? [Psalm 14:1; 53:1] But when this same fool hears what I am saying - 'A being than which none greater can be thought' - he understands what he hears, and what he understands is in his understanding, even if he does not understand that it exists. For it is one thing for an object to be in the understanding, and another thing to understand that it exists ... But clearly that than which a greater cannot be thought cannot exist in the understanding alone, it can be thought of existing also in reality, and this is greater. Therefore, if that than which a greater cannot be thought is in the understanding alone, this same thing than which a greater cannot be thought is that than which a greater can be thought. But obviously this is impossible. Without doubt, therefore, there exists, both in the understanding and in reality, something than which a greater cannot be thought.

In other words, something is greater if it exists than if it doesn't. If God is the greatest thing imaginable, he must exist. One of the clearest criticisms of this argument was made by Kant (in his Critique of Pure Reason) in response to Descartes, who had maintained, in his version of the argument, that it was impossible to have a triangle without having its three sides and angles, and in the same way it was impossible to have God without having necessary existence. Kant's argument may be set out like this:

If you have a triangle
Then it must have three angles (i.e. a triangle without three angles is a contradiction)
But if you do not have the triangle, you do not have its three angles or sides either.

In the same way, Kant argued:

If you accept God, it is logical to accept his necessary existence
But you do not have to accept God.

To appreciate the force of Kant's argument, it is important to remember that he divided all statements into two categories - analytic and synthetic:

analytic statements are true by definition;
synthetic statements can only be proved true or false with reference to experience.

For Kant, statements about existence are synthetic; definitions are analytic. Therefore, the angles and sides of a triangle are necessary because they are part of the definition of a triangle. But that says nothing about the actual existence of a triangle - necessity (for Kant) is not a feature of the world, but only of logic and definiton.

Kant gives another way of expressing the same idea. He says that existence is not a predicate. In other words, if you describe something completely, you add nothing to that description by then saying 'and it has existence'. Existence is not an extra quality - it is just a way of saying that there is the thing itself, with all the qualities already given.