5X5 Episode 40

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5X5 Episode 40
Extraordinary Claims Require Extraordinary Evidence
8th October 2008

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5X5 39 5X5 41
Skeptical Rogues
S: Steven Novella
R: Rebecca Watson
B: Bob Novella
J: Jay Novella
E: Evan Bernstein
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Skepticism 101 - Extraordinary Claims Require Extraordinary Evidence[edit]

You're listening to the Skeptics' Guide 5x5, five minutes with five skeptics, with Steve, Jay, Rebecca, Bob and Evan.


S: This is the SGU 5X5 in our continuing series of Skepticism 101, this week's topic is the phrase "extraordinary claims require extraordinary evidence". This is pretty much the mantra of the skeptical movement, or of skeptical philosophy, and is attributed to Carl Sagan.

B: Actually, Sagan's version is a succinct version of what Hume wrote in his controversial essay on miracles in 1748. Hume said, "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 endeavors to establish."

E: Doesn't exactly roll off the tongue.

B: No; replace "miracle" with "extraordinary claim" and you pretty much have it, but Hume kind of had that sentiment first, but of course Sagan kind of made it real nice and pithy.

J: Let's put it into real world terms. As an example, let's say that a co-worker comes in and says, "I saw a block of ice on the side of the road" and you ask them, "well, how big was it?", and say say "oh, it was, you know, 1ft X 1ft", and then you'd be like, "Oh, well that's strange, but OK", and you probably wouldn't ask for any more qualifying evidence; you'd just assume it fell off some type of truck or whatever. But, let's say that they came in and said, "I saw a block of ice on the side of the road, and it was 20ft X 20ft". You know, at some point the size of that block of ice is going to make you question whether or not they're telling the truth, and you know most people would at some point along that scale ask for more evidence. "Well, where was it?" "What were the circumstances?" "Was it on some—" you know, and you'd start to question it, and I think as a skeptic, I question things much sooner.

R: Well—and the things that we usually address would be like the alien is frozen in the block of ice on the side of the road.

B: (laughs)

S: But yeah, the bigger the block of ice, the more evidence you would need to accept it, or the more unusual the details that are added in. From a scientific point of view, there's really one premise to this principle of extraordinary claims requiring extraordinary evidence, and that is: that we know stuff. You know, each hypothesis isn't starting absolutely de novo from nothing, as if we have no prior knowledge. What it's really saying is an extraordinary claim is something that contradicts stuff that we already know; well-established facts.

B: Or something that's not established at all.

S: Or something—yeah, that's not established; that's a new claim. But there—you know, you could still make assessments based upon first principle of how plausible or likely something is even if it's entirely new. And the more it runs contrary to existing evidence, the more evidence you need—the more extraordinary evidence you need—before it's reasonable to accept it. It really comes down to just math; just piling up the evidence for and against, on two sides of the scale and seeing which one is more.

B: So Steve, what you're saying is that all claims pretty much require about the same amount of evidence; it's just that the ordinary claims already have this huge amount of evidence backing them up that you don't think about.

S: That's partly true, or at least there is nothing against them.

B: Right.

S: Yeah, what makes a claim extraordinary? That's really what it comes down to. If you're making a legitimate scientific assessment, it's that it runs contrary to established data, established observations, experimental facts, scientific theories and models that have been well-established. Why do we think ESP is extraordinary? Why don't we accept someone's say-so; that they had an extra sensory experience? Well, because, you know, all of physics and biology says that it shouldn't be possible. That makes it extraordinary. It doesn't mean that we would never ever accept it, just that the bar for evidence is equally extraordinary to the implausibility of the claim itself. I mean, it makes so much common sense it always is astounding to me when true believers actually try to argue against the principle of extraordinary claims requiring extraordinary evidence.

E: Have you guys ever heard the twist on this, "extraordinary claims require extraordinary investigations"? Budd Hopkins, who made a response to Carl Sagan—and actually Carl Sagan responded to Budd Hopkins in regards to that and said, "first of all, if there's a brontosaurus trampling through the jungles in the Republic of the Congo, should a massive expedition be mounted, with Government funds to find it, or is it so implausible it's not worth serious systematic attention?" And then the second point he made about that: investigations must be true to the spirit of science, and that means skeptical, demanding, rigorous standards of evidence, and you just don't get that with the people who are making the paranormal claims.

S: Yeah, well those like Hopkins, like Budd Hopkins, are essentially trying to say that, "well because we're investigating it it must be legitimate or real". No, you actually need—extraordinary evidence has to emerge from those investigations, and then we'll look at that evidence.

S: SGU 5x5 is a companion podcast to the Skeptics' Guide to the Universe, a weekly science podcast brought to you by the New England Skeptical Society in association with skepchick.org. For more information on this and other episodes, visit our website at www.theskepticsguide.org. Music is provided by Jake Wilson.


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