The Modal Ontological Argument

The ontological argument that is most touted these days is Alvin Plantinga’s modal version. There are several videos defending it on YouTube, and more than one caller to The Atheist Experience has used it to make his case. (One online defense of it can be seen here.)

The argument begins with the innocuous-sounding claim that it is possible that God exists. This is something most atheists would readily admit. After all, atheists — even positive atheists, who claim that God does not exist — don’t usually say that God couldn’t possibly exist. There may not be a God, but we can imagine a different reality in which there was one. And if it is logically possible for there to be a God, then there is a possible world in which God exists — even if in the actual world he doesn't.

But this of course depends on what is meant by “God.” Plantinga defines it as a being with “maximal greatness.” To have maximal greatness is to have every great-making property (power, goodness, and so on) to the greatest possible degree in every possible world. (After all, being perfect only in some possible worlds isn’t quite as outstanding as being perfect in every possible world.) In other words, a being that would be omnipotent, perfectly good, etc., in every possible world, would be maximally great.

But now, if it is possible for God (a maximally great being) to exist, then God exists in some possible world. But if God exists in some possible world, then it follows that he exists in every possible world — otherwise, he wouldn’t be maximally great in that one world. And if God exists in every possible world, then he exists in the actual world. Plantinga therefore contends that, given the possibility of God, you must accept that he exists.

This is a bit confusing, but perhaps what Plantinga’s claiming here can be better understood by means of an analogy. Consider Goldbach’s Conjecture, which is the most famous of all unsolved mathematical problems (it states that every even integer greater than two is the sum of two primes). Now, given that mathematical truths are necessary, it follows that if Goldbach’s Conjecture is true, then it is necessarily true — that is, it is true in every possible world. Suppose then that we claim it is possible that the conjecture is true — and that we mean, not merely epistemically possible (that for all we know it might be true), but logically possible. Suppose, in other words, that the conjecture is true in at least one possible world — say, that in World 435873, a mathematician has found a valid proof of it. If so, then, because what that mathematician has proved is a necessary truth — true for all possible worlds — Goldbach‘s Conjecture must be true in the actual world. Thus, the mere claim that the conjecture is logically possible implies that it is true. And that is what Plantinga is arguing for God’s existence. Given the way he defines things, the mere assertion that God is logically possible means that God exists.

As I said, the argument begins with an innocuous-sounding claim: that it is possible that God exists. However, as I also pointed out, the reasonableness of that claim depends on how “God” is defined. And so we need to ask whether as defined by Plantinga, God is possible.

In the YouTube video linked above, the presenter points out that one might argue against the possibility of God by claiming that the concept of God is contradictory — e.g., by raising the paradox of omnipotence. Can a God create a rock so heavy that even he couldn’t lift it? He then dismisses the paradox and concludes, at least provisionally, that God is after all possible — and therefore, given Plantinga’s argument, must exist. But that hardly touches upon the real problem. Plantinga defines God as maximally great — not merely as omnipotent — and the question is whether maximal greatness is possible.

Let’s consider Goldbach’s Conjecture once more. It may also seem innocuous to claim that it is possibly true — and it is, provided we mean epistemically possible. After all, it may be true. (In fact, there are good reasons for thinking it is.) But to claim that it is logically possible isn’t innocuous at all. For as we’ve seen, that is equivalent to claiming it is true. And yet that is exactly what Plantinga is doing with regards to the existence of God.

To claim that a maximally great being is logically possible is to claim that such a being actually exists. If a maximally great being doesn’t exist, then it isn’t even possible for it to exist (just as if Goldbach’s Conjecture isn’t true, then it isn’t even possible for it to be true). Thus, the question whether it is in fact possible cannot simply be ignored. Furthermore, unlike with Goldbach’s Conjecture, there are good reasons for claiming that it isn’t true that such a being exists. For it can only exist if in fact there is no possible world without an omnipotent, perfectly good being in it. And why would that be? Why isn’t there a possible world with nothing in it except, say, Alvin Plantinga’s beard?

The funny thing about all this is that Plantinga himself has admitted that his argument doesn’t prove there is a God. Even though the argument is valid — that is, the conclusion follows from its one premise — and Plantinga believes it is sound (since he believes the premise that God is possible is true), he admits that it is not a good argument. For, as we’ve just seen, an atheist who understands it is just going to deny the possibility of such a God. Plantinga has even compared it with arguing “Either 7+5 = 13 or God exists; 7+5 ≠ 13; therefore, God exists.” This, too, is a valid argument. In addition, Plantinga believes it is a sound argument (since he believes God exists, and thus regards both premises as true). But even if it is sound, it is not a good argument. After all, an atheist isn’t going to accept the first premise.

You might think that Plantinga himself admitting his argument doesn’t prove God’s existence would be enough to make theists stop using it. But you'd be wrong.

Franz Kiekeben is a former lecturer in philosophy and the author of two books on atheism, The Truth about God, and Atheism: Q & A. He has also written for Skeptic magazine and published academic articles on determinism and on time travel.