http://www.princeton.edu/~grosen/puc/phi203/ontological.html
http://www.trinity.edu/cbrown/intro/ont ... ument.html
Therefore, Lord, who grant understanding to faith, grant me that, in so far as you know it beneficial, I understand that you are as we believe and you are that which we believe. Now we believe that you are something than which nothing greater can be imagined.
Then is there no such nature, since the fool has said in his heart: God is not? But certainly this same fool, when he hears this very thing that I am saying - something than which nothing greater can be imagined - understands what he hears; and what he understands is in his understanding, even if he does not understand that it is. For it is one thing for a thing to be in the understanding and another to understand that a thing is.
For when a painter imagines beforehand what he is going to make, he has in his undertanding what he has not yet made but he does not yet understand that it is. But when he has already painted it, he both has in his understanding what he has already painted and understands that it is.
Therefore even the fool is bound to agree that there is at least in the understanding something than which nothing greater can be imagined, because when he hears this he understands it, and whatever is understood is in the understanding.
And certainly that than which a greater cannot be imagined cannot be in the understanding alone. For if it is at least in the understanding alone, it can be imagined to be in reality too, which is greater. Therefore if that than which a greater cannot be imagined is in the understanding alone, that very thing than which a greater cannot be imagined is something than which a greater can be imagined. But certainly this cannot be. There exists, therefore, beyond doubt something than which a greater cannot be imagined, both in the understanding and in reality.
1. God is something than which nothing greater can be conceived. (definition of "God")
2. If someone understands the concept of God (i.e. the concept of something than which nothing greater can be conceived) then God "exists in the understanding" of that person. (definition of "exists in the understanding")
3. It is greater to exist in reality than in the understanding alone. (More precisely: if x exists in the understanding but not in reality, and y is exactly like x except that y also exists in reality, then y is greater than x.)
4. The fool understands the concept of God (= the concept of something than which nothing greater can be conceived).
5. Therefore (from 2 and 4) God exists in the understanding of the fool.
6. Suppose for the sake of argument that God exists only in the understanding of the fool (i.e. not in reality as well). (This assumption will form the basis of a reductio ad absurdum.)
7. Then we could conceive of something exactly like what exists in the fool's understanding except that it also exists in reality.
8. The entity that we conceived in 7 would be greater than the entity that exists only in the fool's understanding (by 3)
9. But in that case what the fool conceived was not after all something than which nothing greater can be conceived (after all, we've just conceived of something greater).
10. So we have a contradiction! (Between 5 and 9)
11. So the assumption we made in 6 must be mistaken (since it led to a contradiction).
12. So God exists in reality. (6 was the assumption that God does not exist in reality; since 6 is mistaken, God does exist in reality.)
It turns out that it is formally valid, so the controversy lies in the soundness of the argument. Which of the premises would you argue against in order to prove the argument unsound? Otherwise I view it to be a fairly convincing argument in a Cartesian sort of way.
