I first thought it was amazing, then I thought it was bad, but now I’m convinced it’s valid, sound, and slightly question-begging, though far from useless.

We can put the modal ontological argument something like this:

1. It is possible that a Maximally Great Being exists.
2. If it is possible that a MGB exists, then a MGB necessarily exists.
3. If a MGB necessarily exists, then a MGB exists.
4. Therefore, a MGB exists.

Here a Maximally Great Being is defined as a being with all great-making properties: omniscience, omnipotence, omnibenevolence, necessity, etc.

When I first heard the argument I laughed. Not because I thought it was bad—no, on the contrary, it was because I though it was very clever. Like a trick. Like Abbott and Costello proving 7x13=28.

I thought about it awhile more, and I believed there was reason to doubt the argument.

My Objections

For one thing, I didn’t understand (2) above.

For another, I thought parodies of the argument succeeded. For example, take a maximally great unicorn. Can’t we use the argument to prove one of those exists?

I was aware of two objections to these parodies.

The first is that there don’t seem to be objectively great qualities that apply to physical objects like unicorns. My thought was that this really doesn’t matter. As far as I could tell, the Modal Ontological Argument (MOA) doesn’t rely on any notion of objectively great qualities. If it did, I thought, one could easily enough deny the existence of such an objective greatness. The MOA doesn’t need such a thing. All we have to do is pick out qualities, and use the term “Maximally Great Being” to refer the the entity which has those properties.

The second is that a unicorn can’t be necessary because there are possible worlds in which it couldn’t exist. For example, a maximally great unicorn couldn’t exist in a world 3 cm long, or a world uniformly 3000°K. My question was why we should conclude that a unicorn can’t be necessary, rather than that there is no world 3000°K or 3 cm long. And can’t we use the same argument against a MGB? A MGB wouldn’t exist in a world without MGBs.

My Responses

I finally understood (2) when I thought of

The Principle of Necessary Necessity (or so I’ll call it): That which is necessary is necessary necessarily.

It is not possible that a necessary thing, for example, exists in the actual world but not other possible worlds. That would contradict the very definition of being necessary.

We can take the equivalent principle from this: Any given entity described as having the property of necessity is either impossible or exists necessarily.

I haven’t learn’t modal logic (I’ve barely learnt regular formal logic). I’m pretty sure formal modal logic validates everything I just said, which is why philosophers all agree on every premise but (1).

I still think the “no objectively great qualities” response to parodies is a bad one. I might even add that since we only need one objectively great quality, necessity will do.

I sort of retract my objections to the “doesn’t exist in such worlds” objection to parodies, because claiming there’s a world without x seems quite different from saying there’s a world that’s 3000°K.

I think we best base our confidence in premise (1) on intuition. A MGB seems perfectly possible, though a necessary unicorn doesn’t. Space itself seems contingent, but it must be necessary if a necessary unicorn is to exist.

So, while parodies require us to accept a strong claim—“space is necessary”—the MOA requires a very weak claim—that it is possible that a MGB exists.