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Suppose I agree with theists that "God exists" is a necessary proposition, and

Suppose I agree with theists that "God exists" is a necessary proposition, and so is either a tautology or contradiction. That seems to indicate that the probability of "God exists" is either 1 or 0. Suppose also that I don't know which it is, but I find the evidential argument from evil convincing, and so rate the probability of "God exists" at, say, 0.2. But if the probability of "God exists" is either 1 or 0, then it can't be 0.2 - that would be like saying that "God exists" is a contingent proposition, which I've accepted it isn't. How then can I apply probabilistic reasoning to "God exists" at all? If I can, then how should I explain the apparent conflict?

Interesting points. I take it that the most reasonable reply for a defender of the ontological argument to make is to claim that Prefoessor Smith's world is not in fact possible. If one can make a case for abstracta (properties or propositions necessarily existing) then there cannot be a world where only a single pencil exists. For a good case for such a Platonic position, see Roderick Chisholm's Person and Object. R.M. Adams also has a good discussion of the difficulty of imagining / conceiving of God's non-existence. I take this up in a modest book: Philosophy of Religion: A Beginner's Guide (Oneworld Press, Oxford) or in more detail in a discussion of Hume and necessity in Evidence and Faith: Philosophy and religion since the seventeenth century (Cambridge University Press).

I confess I don't understand the notion of "metaphysical necessity," if it does not entail that that there is no possible world in which the "metaphysically necessary" being does not exist. But only a pencil exists in world W. So I really don't see what is gained (or why the very question of God's existence is not simply begged) by the claim that God is a (metaphysically) necessary being.

If "God exists" is necessary, then the probability that God exists is 1. Full stop. It is not either 1 or 0, it is simply 1. It is also not 0.2 or any other number. Nothing like begging the question big-time, eh? On the other hand, I can't see why anyone serious about the question of God's existence (even theists, who would like the answer to be affirmative, but presumably not on foolish grounds) would accept the claim that "God exists" is necessary. If that were true than the could be no possible world (=a world that can be described without contradiction) in which God did not exist. But it seems obvious that there can be such a world. Consider this description: World W = a world in which only a single pencil exists. It's hard to spot the contradiction in that simple world! It would be a pretty boring place to be...but wait! If anyone were to be there, it would be a different world! Whew!