A Modal Anti-Ontological Argument

I was just checking out Ichthus77, a philosophy blog that focuses on apologetics, because it is hosting the upcoming edition of the Philosopher's Carnival.  I noticed a recent entry on the modal ontological argument as developed by Alvin Plantinga and explained by William Lane Craig.  The modal ontological argument uses possible world semantics to recast the age-old ontological argument for the existence of God.  Here it is, as introduced by William Lane Craig (copied from Ichthus77):


Plantinga conceives of God as a being which is "maximally excellent" in every possible world. Plantinga takes maximal excellence to include such properties as omniscience, omnipotence, and moral perfection. A being which has maximal excellence in every possible world would have what Plantinga calls "maximal greatness." So Plantinga argues:
   
       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 in 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.


I don't want to challenge possible world semantics, at least not here.  Taking the logic of Plantinga's argument as given, then, it is claimed that the argument can only be defeated by denying its first premise.  We can deny the coherence of the very notion of a maximally great being or we can deny its possibility.  As it happens, I'm not convinced that the notion of a maximally great being is logically coherent, let alone possible.  However, what is more interesting to me at the moment is that the modal ontological argument can be defeated in another, much more damaging and interesting way.  All we need is the following defeater:  It is possible that a maximally great being does not exist.  Taking that defeater as a premise, we can formulate a modal anti-ontological argument:

1b.  It is possible that a maximally great being does not exist.
2b. If it is possible that a maximally great being does not exist, then a maximally great being does not exist in some possible world.
3b. If a maximally great being does not exist in some possible world, then it does not exist in every possible world.
4b. If a maximally great being does not exist in every possible world, then a maximally great being does not exist.
5b. Therefore, a maximally great being does not exist.

Each step in my argument mirrors a step in the modal ontological argument.  So my modal anti-ontological argument seems just as valid as the modal ontological argument.  Furthermore, the premises in my argument are at least as plausible as the premises in the modal ontological argument.  Premise 1 seems no more plausible than 1b.  The striking fact is that to argue against 1b would be to argue not for the mere possibility of a maximally great being, but for the very fact of its existence.  In other words, to overcome my defeater, you must argue that a maximally great being exists.  Yet, that is the conclusion which the modal ontological argument was supposed to establish.  So some other argument is required to establish the existence of a maximally great being.  The modal ontological argument cannot establish it, and thus fails.

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