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 exists, then it is necessary that he exists. That is the rather breathtaking conclusion of the modal ontological argument. And it is an entirely valid one, at least within the framework of modal logic. (To
