***NOTE: A substantially revised and expanded reply can be found here.***
As previously noted, someone recently put some of my criticisms of the Leibnizian cosmological argument to William Lane Craig, and Craig offered a reply. I offer a rejoinder below:
Craig's most important reply is his charge that my key criticisms entail an untenable modal skepticism:
We generally trust our modal intuitions on other familiar matters (for example, our sense that the planet Earth exists contingently, not necessarily, even though we have no experience of its non-existence). If we are to do otherwise with respect to the universe’s contingency, then the non-theist needs to provide some reason for his scepticism other than his desire to avoid theism.
My reply to this objection is similar to the one I offer in a paper I'm currently working on (with some modifications. See below):
"One might object that my criticism relies on an arbitrarily selective form of modal skepticism, on the grounds that the demand for justification for exotic possibility claims should then apply to the humdrum possibility claims as well. And since we accept the latter without argument, we should, to be consistent, accept the former. But this objection is unpersuasive. For a number of plausible accounts of our knowledge of possibility have been proposed that allow for knowledge of humdrum metaphysical possibilities, while leaving exotic possibility claims unjustified. For example, it has been proposed that our knowledge of metaphysical possibilities is grounded in (i) our facility with counterfactual reasoning in ordinary contexts, (ii) our folk theory of how the world works, and (iii) arguments from analogy/relevant similarity with the actual world. Such accounts can nicely explain the epistemic force of relatively uncontroversial thought experiments involving humdrum metaphysical possibilities (e.g., the Gettier cases), while leaving the more "far out" or exotic modal claims unjustified (e.g., "it is possible that I exist apart from my body", "it is possible that an Anselmian Being exists", etc.). Most saliently for our purposes, it appears that such accounts leave Craig's modal claim involving the possible non-existence of the fundamental constituents of reality unjustified: it's not clear how such a claim could be justified via relevant similarity with the actual world, our folk theory of how the actual world works, or our facility with counterfactual reasoning in ordinary contexts.