I’ve discussed this issue on more than one occasion, but I want to revisit it. There is a Humean standard of evidence, popularized by Carl Sagan, according to which extraordinary claims demand extraordinary evidence.
That’s a catchy slogan. Many unbelievers find it compelling. Even self-evident.
But what does the slogan amount to, and is it sound?
1.The superficial appeal of the slogan lies in its compact symmetry. The principle seems to be that like requires like. Yet, at a general level, it’s hard to take that principle seriously. Suppose we said it takes a cow to eat a cow? Would that be compelling?
2.What does it mean to say that extraordinary claims demand extraordinary evidence?
i) Does it mean the evidence for an extraordinary claim must be the same kind of thing as the event it attests? Supernatural claims demand supernatural evidence? Paranormal claims demand paranormal evidence? Where both evidence and event belong to the same class or category of thing? Is that what this rule of evidence amounts to? The nature of the evidence must correspond to the nature of the event?
Yet that seems to be viciously regressive. After all, the objection to miracles (to take a specific example) is that miracles are inherently implausible. And that is why we need a special kind of evidence to overcome the presumption of their nonoccurrence.
But if the sceptic is demanding the same kind of evidence, if a miraculous report demands miraculous evidence, then the evidence would suffer from the same (alleged) implausibility as the event it attests.
If you say a miraculous event is implausible because it’s miraculous, then miraculous evidence for a miraculous event would be equally implausible.
Yet the slogan seems to concede that a miracle is credible as long as you can furnish the right kind of evidence. On the fact of it, the slogan doesn’t say that no quality or quantity evidence would ever count as probative evidence for an extraordinary claim.
ii) And if, in fact, this is what the slogan really amounts to, then is that a sound standard of evidence? How is the sceptic in any position to rule out the possibility of a miracle? Isn’t his own worldview based on a preponderance of the evidence? If so, then his worldview must make allowance for counterevidence. The evidentiary standard cuts both ways. If he can’t make allowance for any possible evidence to the contrary, then is worldview isn’t based on the state of the evidence.
iii) But what is the alternative? If it doesn’t mean that an extraordinary claim requires the same kind of evidence to attest the event, then it would require a different kind of evidence. But, by definition, a different kind of evidence would be ordinary evidence.
3.It’s also ambiguous to say an extraordinary claim demands extraordinary evidence. This can mean either of two things:
a) It requires extraordinary evidence to attest the occurrence of an extraordinary event.
b) It requires extraordinary evidence to attest the extraordinary nature of the event in question.
i) But (a) seems circular. Unless you can already recognize the extraordinary (e.g. miraculous, supernatural, paranormal) nature of a reported event, why would you demand special evidence to attest that claim? You would only demand extraordinary evidence if you already classified the event in question as an extraordinary event.
For unless the event already fell within your preconception of an extraordinary event, then ordinary evidence would suffice to attest its occurrence.
ii) So that leaves us with (b). But the problem with that interpretation is that sceptics don’t think you need extraordinary evidence to identify a miracle (to take one example) as an extraordinary event.
To the contrary, sceptics routinely reject extraordinary claims of this sort (e.g. miraculous, supernatural, paranormal) because they have a preconception of what kinds of events are ordinary, and what kinds of events are extraordinary. They accept or reject the credibility of a reported event based on their preexisting classification scheme of what is actual, possible, impossible, probable, and improbable.
For them, it goes like this:
i-b) Miracles are inherently implausible.
ii-b) The reported event falls within the stereotypical domain of a miraculous event.
iii-b) Hence, the reported event is inherently implausible.
iv-b) Hence, it requires extraordinary evidence to overcome the presumption of its nonoccurrence.
But, of course, the major premise (i-b) simply begs the question.