The Ontological Argument for the Existence of God

[Simon Moon]

I have never seen this argument before, so I'm interested in some discussion of it. A philosopher by the name of Alvin Plantinga states it this way:

The Ontological Argument

1. It is possible that a maximally great being exists.
   
2. If it is possible that a maximally great being exists, then a maximally great being exists is some possible world.
   
3. If a maximally great being exists in some possible world, then it exists in every possible world.
   
4. If a maximally great being exists in every possible world, then it exists in the actual world.
   
5. If a maximally great being exists in the actual world, then a maximally great being exists.
   
6. Therefore, a maximally great being exists.
   

Thoughts?

Here's a variation of Plantinga's argument, that is just as valid as his, that proves that a god does not exist.

Another objection to the argument is also quite simple: one could change the possibility premise, and flip the argument on its head:

1. A being has maximal excellence in a given possible world W if and only if it is omnipotent, omniscient and wholly good in W; and
   
   
2. A being has maximal greatness if it has maximal excellence in every possible world.
   
   
3. It is possible that there isn’t a being that has maximal greatness. (Premise)
   
   
4. Therefore, possibly, it is necessarily true that an omniscient, omnipotent, and perfectly good being does not exist.
   
   
5. Therefore, it is necessarily true that an omniscient, omnipotent and perfectly good being does not exist. (axiom S5)
   
   
6. Therefore, an omniscient, omnipotent and perfectly good being does not exist.
   

The argument is completely valid, and suffers from the same flaw as Plantinga’s. There really is no reason to accept the formulation of the possibility premise (3), either – without additional arguments, at the very least.