Given an appropriate notion of maximal greatness, maximal greatness entails maximal greatness in all worlds. By the S5 axiom of modal logic, it follows that that if possibly there is a maximally great being, there is a maximally great being. Thus discussion of the modal ontological argument focuses on the possibility premise, the claim that possibly a maximally great being exists.
- If a belief that p is within the center of the motivational life of a person or community, and that life is generally flourishing, then, probably, p is possible. (Premise)
- There have been persons and communities that have led generally flourishing lives within the motivational center of which there was the belief that a maximally great being exists. (Premise)
- Possibly, a maximally great being exists. (By 1 and 2)
- A maximally great being exists. (By 3 and S5, as per opening remarks)
The idea behind (1) is that a life within whose motivational center there was an incoherent proposition would be unlikely to be flourishing. It would be a life that would, most likely, come apart. The flourishingness of a life is evidence of the coherence of the concepts by which that life is lived. And coherence is either identical with or evidence for possibility. In favor of (2), I cite such people as Francis of Assisi, Teresa of Calcutta, Jean Vanier or Thomas Aquinas, and perhaps such communities as the first generation Franciscans. And I even think the synchronic and diachronic Christian community over the ages, despite sins of many individuals, qualifies.
One might try to come up with parallel arguments for incompatible premises. For instance, deeply committed pantheists like Ghandhi appeared to live flourishing lives, and if pantheism is possibly true, it is true, a conclusion incompatible with (4). Two responses can be made. First, pantheists may well believe that a maximally great being exists, and think that the whole of existence is necessarily identical with it. So maybe what is at the motivational center of their lives is just the belief that a maximally great being exists, and the further claim that the whole of existence is necessarily identical with it is less central. This may not be very plausible, though. It seems that the divinity of everything is motivationally crucial to them.
A second response is to admit this. Yes, the existence of pantheists leading flourishing lives at whose motivational center pantheism was found give evidence for the truth of pantheism. So we have evidence for theism and evidence for pantheism. And maybe as far as this goes, the evidence is balanced. But, note that both the evidence for theism and the evidence for pantheism is evidence against atheism. We have evidence for theism and evidence for pantheism. Now we need to see what independent considerations we can bring against theism and against pantheism. (Note, for instance, that the problem of evil is more pressing given pantheism, if the world-deity is good.)
I doubt that there are people living flourishing lives at whose motivational center lies atheism. I do not here deny that atheists may lead flourishing lives, but I deny that if they do, their lives have atheism at their motivational center. For one (this suggestion is due to a grad student here), atheism is a negative doctrine, and lives centered on negative doctrines are unlikely to be flourishing. More likely, the motivational center is morality, love of fellow man, loyalty to friends, etc. In the committed theist or pantheist, these things are closely tied to the theism or pantheism. But in the atheist, these things are, I think, largely independent of the atheism, except maybe in the case of a kind of tragic loyalty in the face of the terrors of the unfeeling world, as in Russell, which I think is not going to lead a fully flourishing life, falling short in respect of the joyous aspects.