Thursday, November 08, 2012

McGrew on ECREE

From his essay on evidence.

Extraordinary Claims and Extraordinary Evidence


Another common slogan, also popularized by Sagan, is that Extraordinary claims require extraordinary evidence. Much depends, of course, on what counts as extraordinary, both in a claim and in evidence. It cannot be simply that a claim is unprecedented. At a certain level of detail, almost any claim is unprecedented; but this does not necessarily mean that it requires evidence out of the ordinary to establish it. Consider this claim: “Aunt Matilda won a game of Scrabble Thursday night with a score of 438 while sipping a cup of mint tea.” Each successive modifying phrase renders the claim less likely to have occurred before; yet there is nothing particularly unbelievable about the claim, and the evidence of a single credible eyewitness might well persuade us that it is true.
The case is more difficult with respect to types of events that are deemed to be improbable or rare in principle, such as miracles. It is generally agreed in such discussions that such events cannot be common and that it requires more evidence to render them credible than is required in ordinary cases. (Sherlock 1769) David Hume famously advanced the maxim that No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous, than the fact, which it endeavours to establish (Beauchamp 2000, p. 87), which may have been the original inspiration for the slogan about extraordinary evidence. The proper interpretation of Hume’s maxim has been a source of some debate among Hume scholars, but one plausible formulation in probabilistic terms is that
P(MT) > P(~MT) only if P(M) > P(T~M),
where M is the proposition that a miracle has occurred and T is the proposition describing testimonial evidence that it has occurred. This conditional statement is not a consequence of Bayes’s Theorem, but the terms of the latter inequality are good approximations for the terms of the exact inequality.
P(M) P(TM) > P(~M) P(T~M) when both P(~M) and P(TM) are close to 1. There is, then, a plausible Bayesian rationale for Hume’s maxim so long as we understand it to be an approximation.

It does not follow that the maxim will do the work that Hume (arguably) and many of his followers (unquestionably) have hoped it would. Hume appears to have thought that his maxim would place certain antecedently very improbable events beyond the reach of evidence. But as John Earman has argued (Earman 2000), an event that is antecedently extremely improbable, and in this sense extraordinary, may be rendered probable under the right evidential circumstances, since it is possible in principle that

P(TM)/P(T~M) > P(~M)/P(M),

a condition sufficient to satisfy the rigorous condition underlying Hume’s maxim and the slogan about extraordinary events. The maxim is therefore less useful as a dialectical weapon than is often supposed. It may help to focus disagreements over extraordinary events, but it cannot resolve them.







13 comments:

Chris W said...

This is a very clear, helpful statement of the problem. If there's a knock-out "in principle" arument against miracles, ECREE ain't it.

cautiouslycurious said...

Chris,

"If there's a knock-out "in principle" arument against miracles, ECREE ain't it."

I'll bite. Who ever suggested that ECREE is in principle against miracles? It is specifically asking for evidence for them so to suggest that it is in principle against them takes a comedic level of misunderstanding.

Dan Gillson said...

McGrew appears to be intentionally misunderstanding what Sagan means by "extraordinary." It isn't merely that an event is statistically impropbable--or even impossible. It's that a certain kind of events--say, God's supervenience in history--requires a certain kind of evidence, which is, outside of mere testimony, currently lacking.

B. Prokop said...

"outside of mere testimony"

But by its very definition, a supernatural intervention in nature will have no other sort of evidence. If I can come up with natural evidence for a supposedly supernatural event, then it ain't supernatural - its explainable by nature alone.

Now this does not, of course, rule out physical recognition of said event. Example in point: Thomas examining the hands and feet of the risen Christ. But even then, even if you yourself are the witness to the event, all you would have is testimony (in such a case, your own).

So Dan, what you are asking for would by its very existence rule out what one is trying to prove/disprove. You are essentially talking in circles.

Dan Gillson said...

Bob,

I'm not asking for anything more or less. (Having been a good Lutheran, I know better.) Personally, I don't think that relying on testimony alone to explain certain sorts of events is a problem. There is, to borrow from a favorite example of analytic philosophy, no evidence for the fact that I'm in pain, save my mere testimony that I am, in fact, in pain. (Sure, it helps that my behavior indicates as such, but one can't rely on behavior alone. I could be acting, playing, faking, etc.) Likewise for miracles. However, it is a problem for Sagan, and it appears to be a problem for McGrew. My problem with this excerpt from McGrew's article is that he is trading on a confusion of what Sagan means by 'extraordinary', that's all.

Chris W said...

cautiouslycurious,

I think many skeptics treat it as an in-prinicple KO argument, and thus they are the ones contributing to the misunderstanding (whether or not this misunderstanding is "comedic" is your call). Hume started it when he called his maxim "an everlasting check." And some sloppy Christian apologists make it worse when they reject it outright, instead accpeting the rather obvious point it makes, as McGrew helpfully clarifies.

You're right that it *should* be a shorthand way of asking for good evidence for claims with a low prior probability. But to many skeptics it's a conversation stopper, esp. in many combox discussions. The tone is often: "well, ECREE, so there!" I often see it used as if it's a argument in itself instead the beginning of an argument which goes through the specific evidence for whatever specific extraordinary claim is being made.

Chris W said...

Dan,

What's wrong with "mere" testimony? McGrew is right that *in principle* the inequality
P(T|M)/P(T|~M) > P(~M)/P(M) for a certain miracle could be true if we had enough testimony. That's not to say any miracle has yet met this standard. In fact, I'm fairly sure none have.

Martin said...

I agree with Dan. I think the essence of what Sagan means by "extraordinary" is not "improbable", but rather "contrary to our general background knowledge."

If someone claims to have a tennis ball that floats up, then this seems to conflict with our background knowledge of gravity, what is required for thrust, etc.

And such a claim would require a bit more umph behind the evidence in order to believe it. Testimony could work, if it were at least a handful of people who are generally trustworthy, don't have a personal stake in the claim, etc. I might accept the testimony of Carl Sagan alone, for example. Or James Randi. But not Uri Geller.

So I while I think the need for extraordinary evidence might be overstated, I do think that there is equivocation over the word "extraordinary". No thanks to Sagan, of course.

Crude said...

I'd actually take a view different from Bob's here: no witnessed event is known to be supernatural or natural in any relevant sense. 'Supernatural' and 'natural' are red herrings. Some other standard will need to be used to make talk of the extraordinary work.

B. Prokop said...

Crude,

I'm not sure if this helps, but I wasn't referring to supernatural events, but rather to supernatural causes of events. And by supernatural, I mean here an agent exterior to the physical universe.

So the event itself can be quite physical (e.g., water turning into wine), whilst the reason for its occurrence is not.

cautiouslycurious said...

Chris.
“I think many skeptics treat it as an in-prinicple KO argument, and thus they are the ones contributing to the misunderstanding (whether or not this misunderstanding is "comedic" is your call). Hume started it when he called his maxim "an everlasting check."”

Are you speaking of notable skeptics or just some skeptics that you have talked to somewhere on the interwebs? If it’s the former, then I would want examples. If it’s the latter, then I hardly see the point in bringing it up as if it’s a common position. If I responded to arguments with a misconception based on what Christians say on forums, I would be rightly lambasted. Plus, it’s rather pointless to bring up in the context of discussing the argument, you’ll just get is a “yeah, I agree, they are misunderstanding the argument/ignorant.”

Also, calling it an everlasting check doesn’t mean that you have an objection to it in principle. It could simply mean that he believed the conditions for belief would never occur. According to his maxim, a miracle could be believed if “the testimony be of such a kind that its falsehood would be more miraculous than the fact which it endeavors to establish.” So, if we find someone who we can’t fathom to be wrong and if they report to have experienced a miracle, then we would be able to believe them. However, since humans aren’t like that and we make mistakes all the time, we have an everlasting check on miracle claims. As I interpret it, this isn’t about what is permissible in principle and more about what we find in actuality. I could easily imagine myself making such a claim and this is what I would mean by it.

Papalinton said...

"Supernatural" is not a philosophical construct. It is a literary device.

Crude said...

Bob,

I think the problem still remains, but this would be a derail into something I am perpetually going on about. I just can't resist mentioning it whenever it comes up.