by Scott F. Aikin and Robert B. Talisse
God is by definition is the greatest possible thing.
If God is the greatest possible thing, then He cannot fail to manifest any perfection — otherwise, there would be a possible thing greater than He.
Existence is a perfection; that which does not exist lacks something that would improve it.
Therefore, God must exist.
The conclusion can be strengthened, further, with the thought that necessary existence is a greater perfection than contingent existence, and so it is necessary that God necessarily exists. Now, that's a pretty heavy conclusion derived only from some strikingly lightweight premises. This is what makes the Ontological Argument so interesting – it seems clear that something's gone wrong, but it turns out that it's very hard to explain what it is.
In our Reasonable Atheism and elsewhere, we've held that the Ontological Argument is a kind of litmus test for intellectual seriousness concerning God's existence. We've claimed further that atheists in particular had better grapple with it. Here's why: Every atheist thinks the argument goes wrong; moreover, they think it's obvious that it fails. But saying that the argument's failure is obvious is not yet to identify what the failure consists in. Yet very few atheists offer much more than simple derision of the argument. Now, that's not intellectually serious – especially if the whole point of any argument is to articulate reasons for the sake of guiding belief. Saying that an argument is obviously wrong and then not having anything substantive to say about its failure is contrary to what honest argument is all about. Smug dismissals of the Ontological Argument as insipid or mere wordplay are themselves mere blather. On top of that, it is exactly the sort of thing anyone devoted to the Enlightenment project should avoid. If you're committed to reason and think the Ontological Argument isn't any good, you've got to wrestle with it and devise an account of its flaws. And while you're at it, you had better bother to consult the most sophisticated versions of the argument available. Otherwise, you're just a poser and a hypocrite.
Now, the Ontological Argument has its critics, and some of the more trenchant objections have been devised by theists. One longstanding objection has been that the Ontological Argument proves too much – specifically, that it overpopulates the world with strange but necessarily existing entities. And so, to St. Anselm's version of the Ontological Argument, the Catholic monk Gaunilo ran the counterargument that the same reasoning could prove that there is a Perfect Island. Atheists have gotten in on the game too. Michael Martin has argued that the Ontological Argument can prove that there must be a perfectly evil being (1990: 93). Richard Dawkins claims that by identical reasoning he can prove that pigs can fly (2006: 84), and Christopher Hitchens argued that it allowed him to prove that there are dragons (2007: 265). We joined the game in our Reasonable Atheism, where we argued that the same reasoning at work in the Ontological Argument can be extended to prove that God can't be the thing that necessarily exists (2011: 88).
Here's another run at the “proves too much”critique, one that takes the existence is a perfection premise in a quite different direction. Here, we are not concerned to show that the Ontological Arugment overpopulates the Christian's world, but rather that it underpopulates it in a crucial respect.
Call it the Ontological Proof for the Impossibility of Satan. To start, we employ the similar definitional setup as the Theistic Ontological Argument. Let's say that Satan is, by definition, the worst possible thing. If something is the worst possible thing, then it not only must have lots of bad properties, but it must not have any perfections; it must be the kind of thing that could not be made any worse than it already is. If it had a perfection, it would be better, not worse, than a thing that lacked that perfection, and thus would not be the worst possible thing. Next, we adopt the Ontological Argument's premise that existence is a perfection. And the conclusion swiftly follows: Satan must lack existence. Further, assuming that a possibly existing thing is better than necessarily not existing thing, it must follow that it is necessary that Satan necessarily does not exist. The Christian's world just got a whole lot smaller.
At first blush, this argument might be excellent news for theists and atheists alike. That there's no Satan is morally speaking an excellent outcome. It is a proposition that atheists already knew, but it will also be one that will relieve the theists of the threat of all-encompassing eternal torture.
But now consider a troubling consequence of the argument. If one accepts the Ontological Argument for the Impossibility of Satan, one must hold that there are some evils that, in virtue of not existing, are worse than evils that do exist. Consider a case of evil – say, the kidnappings in Cleveland, Ohio. An implication of our Ontological Argument for the Impossibility of Satan is that the morally identical copy of those kidnappings that might have happened in Pittsburgh but did not,are worse than the ones that occurred in Cleveland. This looks twisted. The implication is that the world is made better when evils actually occur, as existing evils are less bad than nonexistent ones. How could that be? The culprit is the premise that existence is a perfection. And that's the premise driving our Ontological Argument for the Impossibility of Satan, and some version of this premise features in all versions of the Ontological Argument for God's Existence that we know of. Indeed, it strikes us that some such premise is a sine qua non of Ontological Arguments as such. Alas, this premise must be rejected if the theist wants a world populated by both a God and Satan. Perhaps the better course for the theist would be to just abandon the Ontological Argument.