alive, then he can’t possibly be dead. He must be immortal! Such are the logical perils of sticking a name on a definition.
So the ontological argument, in its classic Anselmian version, is unsuccessful. Even if existence is built into the very definition of God, it does not follow that there is a being that satisfies this definition. Is that the end of the matter?
As it happens, no. In recent decades, the ontological argument has been resurrected in an apparently more powerful form. The new version relies on a kind of logic undreamt of by Saint Anselm: modal logic. Modal logic outstrips the resources of ordinary logic. Whereas ordinary logic concerns itself with what is and is not the case, modal logic deals with what must be the case, what might be the case, and what could not possibly be the case—a far stronger set of notions.
Modal logic was developed by some of the greatest twentieth-century logicians, including Kurt Gödel and Saul Kripke. It was Gödel, the author of the notorious “incompleteness theorems,” who saw in modal logic a way of reviving the ontological argument in a strengthened form. The idea seems to have come to him in the early 1940s, but he did not divulge it until a few years before his death (by self-starvation) in 1978. Whether Gödel was convinced by his own version of the ontological argument is unclear. But he was certainly open to the existence of God, maintaining that it might be possible “purely rationally” to reconcile the theistic worldview “with all known facts.”
Gödel is not the only one to have noticed the theological uses of modal logic. Independently of him, several philosophers have come up with similar modalized updates of Anselm’s reasoning. The most prominent of them is Alvin Plantinga, a professor at the University of Notre Dame. Plantinga’s efforts to secure the existence of God by logic alone have even attracted the attention of Time magazine, which hailed his “tough-minded intellectualism” and called him “the leading philosopher of God.”
The modal ontological argument for God’s existence can look dauntingly technical. Gödel expressed the argument in a series of formal axioms and theorems, and Plantinga took the better part of his treatise The Nature of Necessity to lay out all the details. Still, the nub of it can be put in a fairly simple form.
A truly great being, the argument begins, is one whose greatness is robust in the face of chance. Such a being not only is great, but it would have been great even if events had turned out differently from the way they actually did. By this criterion, for example, Napoléon was not truly great, since he might have died of the flu as a child in Corsica instead of growing up to conquer Europe. Indeed, if his parents had arranged their schedule of sexual congress differently, Napoléon might not have existed at all.
Now, a maximally great being is one whose greatness is unexcelled in every possible world. Such a being would, if it existed, be omniscient, omnipotent, and perfectly good. And there would be no possible state of affairs in which these maximal qualities were in any way diminished. It follows that such a being could not be merely contingent, existing (like Napoléon) in some possible worlds and not others. If such a maximally great being existed at all, it would exist necessarily, in every possible world.
For brevity’s sake, let us call such a maximally great being “God.” So far, so good. Now comes the twist. Does God exist? “Almost certainly not,” an atheist like Richard Dawkins would say. But even Dawkins concedes that, improbable as God’s existence might be, it is at least possible that there is a God—just as it is possible (but highly unlikely) that there is a celestial teapot in orbit around the sun.
But this is a fatal concession for the atheist to make. To say it is possible that a celestial teapot is orbiting the sun is to say that in some possible world such a teapot is orbiting the sun. And to say it is possible that God exists is to say that in some possible world there is a God. But God is different from a teapot. He is by definition a maximally great being. Unlike a teapot, his greatness—and therefore his existence—is stable across different possibilities. So if God exists in some possible world, he must exist in every possible world—including the actual world. In other words, if it is even possible that God