A key move in standard Leibnizian cosmological arguments is the claim that:
(UCB) The universe -- or (if the universe doesn't exhaust physical reality) all physical reality -- is a contingent being.
Now the primary means of support for UCB is a conceivability-possibility inference. Richard Taylor's use of such an inference is representative in this regard. Thus, he argues that for any object in the universe, we can imagine that it fails to exist (e.g., a six-foot-in-diameter translucent sphere). But if imaginability is evidence of possibility, then this is evidence that for any arbitrary object in the universe (whether a stamp or a solar system), it's possible for it not to exist. But we can just as easily imagine the whole universe failing to exist. Therefore, we can say with equal justification that the universe can fail to exist, in which case it's a contingent being.
Is the line of reasoning above for the contingency of the universe a good one? One might think not, on the grounds that Taylor conflates evidence for the possible non-existence of a material object (a stamp, a solar system, etc.) with evidence for the possible non-existence of the stuff of which it's composed (matter-energy).
William Lane Craig is aware of this sort of worry. However, he thinks he can get around it and make legitimate use of a conceivability-possibility inference to support UCB by cutting to the chase and asking us to imagine the most fundamental constituents of reality -- quarks (assuming the string theorists are wrong) -- failing to exist; alternatively, he asks us to imagine a universe composed of different quarks. Given this modification of the thought experiment, he assumes that we can adequately imagine this, and further that this is sufficient prima facie evidence that such things are possible.
I've raised worries for Craig's defense of the Leibnizian cosmological argument on other occasions. Here's another one: Craig's defense of UCB makes no real advance over Taylor's. For it seems to assume a "thing" ontology about fundamental reality, while a "stuff" ontology of fundamental reality is epistemically possible.
Let me explain a bit. For simplicity's sake, suppose there are just nine things (quarks, say), and these, in turn, are composed of a more fundamental "stuff" (say, matter-energy, or a relativistic quantum field). Suppose further that the latter is a metaphysically necessary stuff. Finally, suppose the stuff is capable of an unlimited number of modes of existing (e.g., as nine particles, as 18 smaller particles, as one big particle; as quarks to type T, as quarks of type T', etc.). If so, then while the particles -- the things -- are contingent beings, the "stuff" (viz., matter-energy, or the quantum field) is not. But if this scenario is epistemically possible, then as with Taylor's defense of UCB, Craig's defense of UCB fails to rule out that he's conflating the contingency of things within the universe with the contingency of the stuff of which it's composed. And if that's right, it doesn't seem to me that Craig's defense of UCB makes an advance over Taylor's.
Now one might try to push Craig's point here by saying that we can adequately imagine the universe (or a universe) as composed of different matter-energy, or of matter-energy ceasing to exist. But at the very least, this isn't clear. For even if one grants that imaginability can provide sufficient justification for very many possibility claims, one might yet sensibly worry that its justification-conferring ability does not extend to states of affairs as remote from ordinary experience as the non-existence of all matter-energy, or the existence of a different kind of matter-energy. In this regard, she may find such claims to be on a par with the controversial modal premise of (say) Plantinga’s modal ontological argument (Possibly, an Anselmian Being exists), or of conceivability arguments for dualism (Possibly, I exist apart from my body).
One might object that the previous 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 humdrum possibility claims as well. And since the sensible non-theist accepts the latter without argument, she should, to be consistent, accept the former. But this objection is less than persuasive. For a number of plausible accounts of the epistemology of modality have been proposed that provide a basis for distinguishing between justified humdrum possibility claims and unjustified exotic possibility claims. So, for example, it has been argued that our knowledge of metaphysical possibility is grounded in (i) our survival-conducive facility with counterfactual reasoning in ordinary contexts (e.g., Williamson, Nichols) and (ii) arguments from analogy/relevant similarity with the actual world (e.g., Hawke, yours truly). Such theories receive confirmation in a number of ways, such as their ability to explain the epistemic force of paradigm-case thought experiments (e.g., Gettier cases), as well as the lack of such conviction with respect to the more exotic modal thought experiments (involving, e.g., the possible existence of Anselmian Beings and disembodied existence). For the former thought experiment can be grounded in such accounts, while the latter cannot.
Most saliently for our purposes, it’s not at all clear how such accounts of our knowledge of metaphysical possibility could adequately support the possible non-existence of all matter-energy, or of the possible existence of a different kind of matter energy. Thus, it’s not clear how the evolutionary pressures that gave rise to our competence with counterfactual reasoning in daily life (e.g., reliable reasoning about what would happen if one tried to cross a busy intersection) would make us competent to determine something so remote from ordinary experience as the possible non-existence of all matter-energy, or of a different sort of matter-energy.
Nor are such possibility-candidates sufficiently similar to our experience and knowledge of the actual world so as to ground a solid analogical inference from the latter to the former. For the relevant sorts of experiences here would involve observations of the absolute origination and annihilation of matter-energy. But in all of our actual experiences, what we observe is relevantly dissimilar to this, viz., the mere rearrangement of preexisting materials, as well as their decomposition into simpler elements. It therefore appears that a non-theist could sensibly reject an imaginability-possibility inference in support of such exotic claims without thereby engaging in an unprincipled or arbitrarily selective form of modal skepticism.
In short, it seems to me that Craig's recent defense of UCB fails to rule out the epistemic possibility of a stuff ontology of fundamental physical reality. Because of this, his defense of UCB fails to rule out that he's conflating the contingency of things within the universe with the contingency of the stuff of which it's composed. And because this was the worry that Craig's defense of UCB was supposed to avoid, it fails to make a significant advance over Taylor's defense of it.
 Joshua Rasmussen has suggested to me that one can also imagine the universe as composed of more or fewer quarks, and that this in turn might confer sufficient justification on UCB. Below I'll raise an objection that challenges these thought experiments as well.
 A final worry: suppose we waive these concerns and grant that conceivability is a reliable guide to possibility, whether the possibilities are humdrum or exotic. Then the worry is that while we can conceive of a universe composed of different matter-energy (of different fundamental "stuff"), we can likewise conceive of a universe without God, or with a different god. But if so, then we have equal justification for thinking that God is no less a contingent being than fundamental physical reality.