In Defense of David Hume On Miracles, Part 2

J.L. Mackie’s argument against miracles in defense of David Hume, to be found in the first chapter of his classic book The Miracle of Theism, is stated very well:
The defender of a miracle…must in effect concede to Hume that the antecedent improbability of this event is as high as it could be, hence that, apart from the testimony, we have the strongest possible grounds for believing that the alleged event did not occur. This event must, by the miracle advocate’s own admission, be contrary to a genuine, not merely supposed, law of nature, and therefore maximally improbable. It is this maximal improbability that the weight of the testimony would have to overcome.” “Where there is some plausible testimony about the occurrence of what would appear to be a miracle, those who accept this as a miracle have the double burden of showing both that the event took place and that it violated the laws of nature. But it will be very hard to sustain this double burden. For whatever tends to show that it would have been a violation of a natural law tends for that very reason to make it most unlikely that is actually happened.
Mackie then distinguishes between two different contexts in which an alleged miracle might be considered a real one. First, there is the context where two parties
have accepted some general theistic doctrines and the point at issue is, whether a miracle has occurred which would enhance the authority of a specific sect or teacher. In this context supernatural intervention, though prima facie [“on the surface”] unlikely on any particular occasion is, generally speaking, on the cards: it is not altogether outside the range of reasonable expectation for these parties.
The second context is a very different matter when
the context is that of fundamental debate about the truth of theism itself. Here one party to the debate is initially at least agnostic, and does not yet concede that there is a supernatural power at all. From this point of view the intrinsic improbability of a genuine miracle…is very great, and that one or other of the alternative explanations…will always be much more likely—that is, either that the alleged event is not miraculous, or that it did not occur, or that the testimony is faulty in some way.
Mackie concludes by saying: “This entails that it is pretty well impossible that reported miracles should provide a worthwhile argument for theism addressed to those who are initially inclined to atheism or even to agnosticism.”