We can formulate Plantinga's modal ontological argument as follows:
- Possibly, a maximally great being exists.
- A maximally great being exhibits maximal excellence in all possible worlds.
- Therefore, there necessarily exists a being that exhibits maximal excellence in all possible worlds. (By 1, 2 and S5.)
Here is a surprising fact. Gaunilo's maximally great island objection doesn't work. For let's try to construct a parallel:
- Possibly, a maximally great island exists.
- A maximally great island would have insularity and the maximal excellence compatible with insularity (i.e., being an island) in all possible worlds.
- Therefore, there necessarily exists a being that exhibits the maximal excellence compatible with insularity in all possible worlds.
But premise (5) is unjustified. For while we have reason to think that a maximally great being would be maximally excellent in each world, we do not have reason to think that a maximally great island would have insularity in each world. Typical islands we know are not essentially insular. Indeed, there are pieces of ground that are islands at high tide but that are not islands at low tide, so insularity is not an essential property for them. In fact, I don't know that any island has insularity essentially. Any island could survive being joined to the mainland by a narrow land-bridge. And even if it were possible to have an island that is essentially insular, it would be unclear that a maximally great island would be essentially insular. Essential maximal excellence is very plausible a great-making property. But it is far from clear that essential insularity is a great-making property.
But without essential insularity, the most we can show in the place of (5) seems to be this:
- A maximally great island exhibits insularity in some worlds, in which it exhibits the maximal excellence compatible with insularity, and in all other words, if any, it exhibits maximal excellence.
- There necessarily exists an entity which in some worlds exhibits the maximal excellence compatible with insularity and in all other worlds, if any, it exhibits maximal excellence.
But isn't what (8) shows just as absurd? A being that is an island in some worlds and God-like in others? Actually, I suspect (8) is true. God is maximally excellent in all worlds and in some worlds he is literally an island. What do I mean? Well, as a Christian, I take it that it is possible for God to become incarnate as a human being. But surely not just as a human being. One might think that by the same token God could become incarnate as any sort of being, say an island, so in some worlds he is both God and an island, whereas in the actual world he is God and man. Indeed, wouldn't we expect that the maximally great island would be God, if that were possible? Now one might object that God can only become incarnate as the sort of being that can be a person. But an island can be a person. We can imagine all kinds of persons: persons of carbon and water and the like like us, persons of plasma, etc. Why not a person of earthy stuff? All kinds of complex computational phenomena could be instantiated by geological interactions within an island. Of course, such a person might end up functioning very slowly. But that's fine. And dualists like me will demand a soul for them. But that, too, is fine. Some possible islands have souls, then.
This strategy won't work for every possible parody. In particular, they won't work for parodies involving maximally great beings that exhibit some quality incompossible with divinity, like sinfulness. But it is not clear that it makes sense to talk of a maximally great among sinners, say. Sinfulness is defined by falling short of moral greatness, so a maximally great among sinners is going to be minimally sinful. But for any sin, there is a lesser possible sin, so there is no such thing as minimal sinfulness, probably.