The ontological argument restated by Alvin Plantinga

(29) There is a possible world in which maximal greatness is instantiated.

[…]

(30) Necessarily, a being is maximally great only if it has maximal excellence in every world

and

(31) Necessarily, a being has maximal excellence in every world only if it has omniscience, omnipotence, and moral perfection in every world.

[…]

But if (29) is true, then there is a possible world W such that if it had been actual, then there would have existed a being that was omnipotent, omniscient, and morally perfect; this being, furthermore, would have had these qualities in every possible world. So it follows that if W had been actual, it would have been impossible that there be no such being. That is, if W had been actual, it would have been impossible that there be no such being. That is, if W had been actual,

(33) There is no omnipotent, omniscient, and morally perfect being

would have been an impossible proposition. But if a proposition is impossible in at least one possible world, then it is impossible in every possible world; what is impossible does not vary from world to world. Accordingly (33) is impossible in the actual world, i.e., impossible simpliciter. But if it is impossible that there be not such being, then there actually exists a being that is omnipotent, omniscient, and morally perfect; this being, furthermore, has these qualities essentially and exists in every possible world.

What shall we say of this argument? It is certainly valid; given its premise, the conclusion follows. The only question of interest, it seems to me, is whether its main premise — that maximal greatness is possibly instantiated — is true. I think it is true; hence I think this version of the ontological argument is sound.

But here we must be careful; we must ask whether this argument is a successful piece of natural theology, whether it proves the existence of God. And the answer must be, I think, that it does not. An argument for God’s existence may be sound, after all, without in any useful sense proving God’s existence.* Since I believe in God, I think the following argument is sound:

Either God exists or 7 + 5 = 14
It is false that 7 + 5 = 14
Therefore God exists.

But obviously this isn’t a proof; no one who didn’t already accept the conclusion, would accept the first premise. The ontological argument we’ve been examining isn’t just like this one, of course, but it must be conceded that not everyone who understands and reflects on its central premise — that the existence of a maximally great being is possible — will accept it. Still, it is evident, I think, that there is nothing contrary to reason or irrational in accepting this premise.** What I claim for this argument, therefore, is that it establishes, not the truth of theism, but its rational acceptability. And hence it accomplishes at least one of the aims of the tradition of natural theology.
___________

* See George Mavrodes, Belief in God (New York: Macmillan Co.,, 1970), pp 22ff.
** For more on this see Plantinga, The Nature of Necessity, chapter 10, sec. 8.

The Analytical Theist, an Alvin Plantinga Reader

edited by James F. Sennett

pages 70-71

Eerdmans, 1998