I have already tackled Alvin Plantinga's modal form of the ontological argument in a previous post, but I've never been entirely satisfied with my response. I was pondering it again today, and I think I have come up with a better rebuttal -- although it's more an "inoculation" than a rebuttal, as I will explain.
The most controversial aspect of Plantinga's argument is not the S5 axiom -- although, frankly, I apparently don't understand formal logic enough to see how the S5 axiom is possibly supportable, it seems to me to lead to obviously absurd results -- but rather, the acceptance of the possibility premise. To review, Plantinga's possibility premise is that "Maximal greatness is possibly exemplified", where maximal greatness is defined thusly: "It is proposed that a being has maximal greatness if it has maximal excellence in every possible world." (The definition of maximal excellence is not important for the purposes of this post) It has been argued that Plantinga is engaging in a form of begging the question here, in that when the listener agrees to this premise, she is agreeing to far more than she realizes.
Plantinga's response is that one could say this about any logical argument. And admittedly, he has a point: it is usually not fair to agree to a set of premises, and then when those premises lead to a result which one finds distasteful, to go back and revise the key premises until the result no longer holds. Otherwise, one could never mount any sort of logical argument whatsoever -- the premises would just be altered.
And thus I propose my inoculation against Plantinga's proof. This is a "proof" of sorts, which, via series of arguably objectionable premises, attempts to reach a result which is patently absurd. The point is that if a person rejects any of the premises in this proof, it makes it impossible for that person to agree to Plantinga's possibility premise. Thus, they are inoculated.
I start with a definition:
1. It is proposed that a proposition is perfectly cromulent if that proposition is true in all possible worlds.
And right off the bat, let's toss out our first premise:
2. There exist one or more propositions which are perfectly cromulent. (premise)
So what we are doing here is clarifying what we mean by "possible worlds". I would argue that if we want to take "possible worlds" at its face, then we must reject this premise. And I'm sure it is clear that if one rejects the premise at (2), one must also reject Plantinga's possibility premise -- for, if it maximal greatness were exemplified, then the statement "There exists a maximally excellent being" would be perfectly cromulent, and if (2) is false, there cannot be any perfectly cromulent statements.
But let's say we have a narrower definition of "possible worlds". Let's move on with another suggested premise, this time one we hope the listener will accept.
3a. If we can imagine a possible world in which proposition P is false, then P cannot be perfectly cromulent. (premise)
This really doesn't narrow the definition very much at all, but it does so in an important way. If a person agrees to premise (3a), then in regards Plantinga's argument, they have two choices: She can insist that she cannot even imagine a world without a maximally excellent being (in which case Plantinga's proof is superfluous!), or else she can admit that she can imagine such a world, in which case Plantinga's possibility premise once again breaks down: We can imagine a world with no maximally excellent being, therefore the proposition that "a maximally excellent being exists" is not perfectly cromulent, therefore the premise that "maximal greatness is exemplified" cannot be true.
Okay, so let's say the premise was rejected. Instead, let's substitute the negation.
3b. Even if we can imagine a possible world in which proposition P is false, P may still be perfectly cromulent. In other words, the imagined possible world is not actually a possible world.
I am having trouble imagining that a person might make this argument, but I want to cover as many bases as possible. It seems to me, however, that if one accepts premise (3b), it is rather hard to escape the next premise:
4. If a given proposition P is not proven to be false, then it is possibly perfectly cromulent.
If this premise is rejected, then I submit that the onus is now on the other person to say how we ever would determine whether a proposition is possibly perfectly cromulent. However, a person who has gotten this far and then rejects this premise is now free to agree with Plantinga's possibility premise if they wish.
Of course, if the premise is accepted, then we rapidly reach an absurd result:
5. For any given proposition P that has not been proven to be false, it is possibly necessarily true.
6. For any given proposition P that has not been proven to be false, it is necessarily true. (By that wily S5)
7. All propositions that have not been proven false are true.
Yes, one can still thread the gauntlet and agree with Plantinga's possibility premise. But most people won't -- and that's exactly the point. This forces people to clarify exactly what they are agreeing to before they accept Plantinga's premise.
Please, anyone who actually knows philosophy or logic, tell me where I went wrong!