This is indeed one of my fundamental suspicions about this sort of argumentation, as you may have already noticed from my questioning of the feasibility of Bayesian reasoning re: this particular matter, from both subjective and objective perspectives. I might not have articulated it very clearly. Anyway, I recently found
this blog post to be pretty helpful. It consists of some combox discussion that grapples with points highly significant for our topic, that could perhaps be behind my doubts, and that I would probably want to stand by. The following captures its basic reasoning.
Let us assume that our concept of God is the concept of a being that has a certain modal property, the property of being such that, if existent, then necessarily existent, and if nonexistent, then necessarily nonexistent. Call this the Anselmian conception of deity. It follows that
God exists, if true, is necessarily true, and if false, necessarily false. Simply put, the proposition in question is either necessary or impossible, and thus necessarily noncontingent.
. . . [H]ow can such a noncontingent proposition have its probability either raised or lowered by any empirical consideration? In particular, how can considerations about the kinds and amounts of natural and moral evil in the world lower the probability of
God exists? If true, then necessarily true; if false, then necessarily false. . . [E]vidential considerations are simply irrelevant to the probability of noncontingent propositions.
(
This post is also not too bad.)
I’ve heard numerous perspectives on this, I’d like to outline my own, particularly because I may or may not be using terms in a quaint sense:
I would distinguish between metaphysical and epistemic modality here (not saying this distinction hasn’t already been drawn, just that more attention to it seems helpful).
I take metaphysical modality to concern whether a thing could be
real or
actual, or whether a proposition could
really be true, or false. Here, ‘real’ and ‘actual’ are intentionally left undefined as it seems we have a fairly well understood prephilosophical notion behind these words. Just in case though, here’s an example:
Could it ever
really be the case that I run faster than the speed of light, unaided? No. We can
imagine me doing so, but that couldn’t ever
actually happen.
Epistemic modality concerns whether, given
your beliefs and knowledge, a thing could be real or actual.
For example, some may say the state of affairs consisting in God creating all human persons immediately in hell is a scenario which is able to be
real. Others, perhaps in virtue of their understanding of perfect goodness, may disagree.
As you know a proposition is necessarily true if it’s true in all possible worlds and impossible if it’s true in none.
It seems to me that we conceive of possible worlds, but do not imagine them. ‘Conceiving’, here, refers to that intensional cognitive act, the object of which is a particular scenario obtaining in a world, described by complete and compossible propositions. The world is complete in that every proposition or its negation describe the world such that no aspect of the world is left undescribed by a proposition. These proposition are compossible in that the conjunction of all of them is able to be
real. Whereas ‘imagining’ refers to that intensional cognitive act, the object of which is a particular scenario which isn’t able to be real.
This is why, for example, we can mentally ‘picture’ impossible scenarios like me running faster than the speed of light without assistance.
Now, I take the relevant probability to be a measure of our confidence that certain propositions bare particular truth-values. Because we may be ignorant of certain things, we can be unsure of the truth-values certain propositions may bare.
So, it makes sense to me that we can be unsure of whether we’re ‘conceiving’ or ‘imagining.’ In so far as we are, it makes sense that we’re unsure of whether a proposition is necessarily true or impossible: we’re ‘picturing’ the relevant scenario, but don’t know if it’s obtaining in a possible world or not. All the probability is doing is numerically representing the ignorance we actually have, or variant degrees of confidence perhaps. Without such numerical representation, our confidence would still exemplify in shades.
I hope none of this sounded patronizing, I wouldn’t feel comfortable explaining my position unless I covered the bases lol.