Ontological arguments are fun. I don’t know of anybody who has ever been converted to theism on the basis of an ontological argument (although I had a professor once who was agnostic because he thought that the force of the ontological argument and the problem of evil balanced each other out) but they’re fun intellectual exercises. The modal version of the ontological argument developed by Plantinga is probably the most fun. While I’m not convinced by it, I’ve recently come to appreciate its rhetorical force in theism/atheism debates. Here is the argument:
P1. It is possible that God exists
P2. If it is possible that God exists, God exists in a possible world.
P3. If God exists in a possible world, then he exists in all possible worlds.
P4. If God exists in all possible worlds, then God exists in reality.
P5. Therefore, God exists in reality.
Conclusion: Therefore, God exists.
The crucial premise is premise 1. The other premises follow from axioms of modal logic. Granted, some have argued that an axiom of modal logic called S5 is controversial (it states that ‘if possibly necessarily p, then necessarily p’), but I’m going to avoid this controversy here because I have no expertise in modal logic. So assuming S5, premises 2 – 5 follow. Returning to premise 1, an atheist could deny that God’s existence is a possibility. There are a couple of options: 1) one could argue that the concept of God is logically incoherent, i.e. that some of the divine attributes entail a contradiction; 2) one could argue that the evil in the world makes God’s existence logically impossible. These strategies are possible, but I’m not sure how viable they are. They both seem to have fallen out of fashion lately. That’s not to say that they’re wrongheaded, but it is to say that both theists and atheists in the field have turned their attention to different atheistic arguments, usually probabilistic in structure.
But it is here that I feel the rhetorical force of the ontological argument. By ‘rhetorical force’ I don’t mean that I’m convinced by the argument. The atheist, when pressed, can bite the bullet and deny premise 1. But I think the atheist pays a high price for denying premise one: namely, he shoulders a heavier burden of proof. It’s not enough to show that God probably doesn’t exist; one has to show that God’s existence is logically impossible, impossible in every possible world. So in a rhetorical context, the ontological argument is quite a bit more useful to the theist than it’s often given credit for. It forces the atheist to defend a much stronger position. It makes the atheist ‘get off the fence’ about whether God’s existence is possible or not. One has to adopt a very strong form of atheism, not merely the inductive, probabilistic kind that I tend to favor. So what move should the inductive atheist make when faced with this argument? I’m not sure, which is why I feel the rhetorical force of the argument. Any thoughts?