The Ontological Argument - valid or debunked?

[SteveII]

I'm sorry, but it makes no sense that this defeats the argument. What is the difference between Premise 1 and Premise 1'?

Premise 1: It's possible that a 'maximally great being' exists.
Premise 1': It's possible that a 'maximally great being' does not exist. 

So the real difference is in P4' (P2' and P3' are definitionally true). 

Premise 4: If a maximally great being exists every possible world, then it exists in the actual world. 
Premise 4: If a maximally great being does not exist in every possible world, then it does not exist in the actual world. 

When you apply the modal logic "if something is necessary in one possible world then it is necessary in all possible worlds" to a negative such as P4', you are saying it is necessary that something does not exist. Isn't that saying the same thing as a maximally great being is logically impossible? So to support P4', you are back to having to show that a greatest conceivable being is not logically possible--which was the original challenge of the original argument. 

wiploc's argument shows that it can work both ways, that's the point. As you have yet to provide an argument that establishes the logical possibility of a maximally great being, then this argument is pretty much vacant at this point.

No. It does not work both ways because the meaning of P4' is very different from P4 because you cannot rely on modal logic of "necessary" as the original argument does.