@Arjen,
I was thinking about this ontological argument for God's existence recently. I decided to reword it to put in logical terms;
Reason 1 is that God is the greatest possible thing (or the sum of all perfections).
Reason 2 is that it is greater to exist within both the understanding AND reality than just in the understanding
These two reasons follow logically to Conclusion 1, that if God exists in the understanding, he must also exist in reality.
Then Reason 3 is that for both the theist and atheist, whether or not they accept or reject God, they must have an understanding of what they are rejecting.
Reason 4 is the obvious fact that some people DO accept God and other people DO reject him.
These two reasons follow logically to Conclusion 2, that God exists in the understanding.
And then Conclusion 1 and Conclusion 2 both follow logically to the Final Conclusion that therefore God exists in reality.
I had a huge problem with this argument because it seemed to be absolute proof that God exists, and yet I was certain it could be doubted somehow. I've heard people try and argue against it by claiming that the first reason is invalid, as some people may have a different definition of what God is. That makes no difference, though. As long as the definition described in that argument exists, the argument follows; even if some people do mean something different when they say God.
I realised why the argument doesn't work though. I doubt I'm the first to think of this but as far as I know I hadn't read this anywhere else it just struck me.
Reasons 1 and 2 were the problem;
Reason 1:
God is the greatest possible thing (the sum of all perfections)
This is based on the assumption that the intrinsic maximum of all perfections must exist together. Of course the intrinsic maximum of each perfection exists as a concept (the Forms, perhaps), but for this argument to work, one must assume that
all the Forms exist together as one thing, with each Form as a 'property' of this concept of God.
Reason 2:
it is greater to exist within both the understanding AND reality than just in the understanding.
This is not necessarily so. This greatly assumes the existence of the material world, which is what is meant by 'reality'. I believe the material world is just a product of our mind, a way of understanding the information (sense data) given to us. In some ways it is a prison, a trap for our Souls (mind). Perhaps it is greater to escape this trap completely, to exist purely as mind and not as body. So then it is greater to exist only in the understanding (having rid itself of the material world) than to exist in this prison of 'reality' as well.
If either of those 2 reasons is successfully refuted, then Anselm's argument falls apart.