For one who plays the scientific 'game' and abides by its rules, there is no question but that the burden of proof lies on the one who asserts that there are miracles. No scientist worth his salt could hold that there is a presumption in favor of the existence of miracles. It is the other way around: there is an exceedingly strong, if not quite indefeasible, presumption in favor of their nonexistence, and indeed of the nonexistence of anything nonnatural. But this onus-assignment is relative to the scientific 'game' and partially constitutive of it.Two points. First, I don’t believe there is any scientific ‘game’. The burden of proof is simply to show that any event, or kind of event exists. The default position is to reject all existence claims – not just miracles.
Second, I don’t believe there is any such kind of thing as a miracle (or supernatural event). But there are kinds of accounts, which fall into several easily identifiable patterns. For example, if we define ‘miracle’ as the purported referent of an account which is inherently implausible and unsupported by any strong evidence (and usually and in addition there exists evidence that the claim is being made for reasons unrelated to scientific objectivity) of course the default position is to reject miracles, and the burden of proof is to supply the evidence that is conspicuously lacking.
It is not that scientists hold a strong presumption in favour of the nonexistence of a certain type of event (‘the nonnatural’), as Vallicella appears to suggest. Rather, that there is a strong presumption by scientists in favour of rejecting the existence of anything referred to by a certain type of account. The ‘burden of proof’ is merely the requirement to supply a certain kind of account.