Theism and Explanation - Appeared-to-Blogly

What Would They Explain? 75
how might we estimate its prior probability? Hume’s discussion suggests
that we ask how often the alleged miracle has occurred in the past. But in
the case of most alleged miracles, the answer will be “never.” An event of
this kind will not have occurred before. If we make this the basis of our
reasoning, the result will be a prior probability of zero. The alleged miracle
in question is simply impossible.
At times, Hume writes as if this were true, as if miracles were indeed
impossible. In one passage, for instance, he writes that
there must . . . be a uniform experience against every miraculous event,
otherwise the event would not merit that appellation. And as uniform
experience amounts to a proof, there is here a direct and full proof,
from the nature of the fact, against the existence of any miracle. 56
The problem is that he immediately speaks of that proof being “destroyed”
and the miracle “rendered credible” by an opposite, more powerful proof.
And he argues that the testimony to a miracle can never provide such proof.
But this last argument seems redundant. If the prior probability of the miracle
is zero, no amount of testimony could outweigh it.
Hume’s mistake lies in the way he assesses prior probability. To do so
by counting the number of previous instances seems wrong-headed. It certainly
fl ies in the face of scientifi c practice. As John Earman writes,
scientists not uncommonly spend many hours and many dollars searching
for events of a type that past experience tells us have never occurred
(e.g. proton decay). Such practice is hard to understand if the probability
of such an event is fl atly zero, and the probability of the putative law
asserting the non-occurrence of this type of event is unity. 557
Yet Earman’s criticism also indicates a way forward. On what basis do scientists
search for previously unobserved events? Presumably because they
have a promising theory which predicts that this event will sometime occur,
in these circumstances. (As I write, scientists have just switched on the
Large Hadron Collider near Geneva, in the hope of [inter alia] discovering
the Higgs boson, a hitherto undiscovered particle whose existence is
predicted by the standard model of particle physics.) The likelihood of the
event occurring is dependent on the strength of the theory that predicts it.
Applied to reports of miracles, and adopting the probabilistic framework
of Hume’s argument, this suggests that the prior probability of a miracle
is dependent on the prior probability of the theistic hypothesis. To put this
another way, it would be reasonable to accept a hypothesis that involved a
miracle only if we already had reason to believe that there existed a being
capable of working miracles. What reasons might we have for believing
this? I shall be addressing that question in the chapters that follow.