What is "extraordinary evidence"? ??
A lot of evidence, an extraordinary amount. Or, some otherwise unusually persuasive evidence.
The less plausible the claim, the stronger the evidence will have to be before we can believe the claim.
This adage has never made any sense to me. How do you decide what kind of claim is which?
We have theories that try to make sense of the world. Call that our intellectual scaffolding.
Some claims ("I have thirty-five cents in my pocket") are easy to believe. They fit with our existing theories.
Others can't be believed unless we tear down some of our scaffolding and erect replacements. Consider Einstein's relativity theory. That couldn't be accepted without a terrific amount of tearing down and rebuilding. But the evidence was compelling, so we tore down and rebuilt. The claim was extraordinary, and the evidence was extraordinary too. We now have to look at the world in a different way.
How do those terms make sense at the same time? If "extraordinary" claims is a code word for "supernatural" claims, it seems like "extraordinary" evidence would be supernatural evidence.
It has nothing to do with natural or supernatural. A fossil rabbit in the cretaceous would be extraordinary, but it wouldn't be magical.
So for instance, perhaps the spectral evidence introduced at the Salem Witch Trials to prove the extraordinary accusations of witchcraft might qualify. But most people do not think of the Salem Witch Trials as a crowning moment in the history of rationality. I don't see that a rational person ought to include their personal judgements of how extraordinary the claim is into their consideration at all, and would simply hold everything to the same standard of ordinary evidence. What do the facts care whether you are personally incredulous about them or not?
Personal incredulity is not the test. Making sense of a claim in light of what we already know about the world is the test.
I used to have a scale of plausibility. Maybe I can reproduce it here, or come up with something similar:
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1. Regular claims: "I have 35 cents in my pocket."
Easy to believe. If you tell people this, most reasonable people will believe you just based on your claim."
2. Harder claims: "I have $100,000 in my pocket."
Most reasonable people would take this with a grain of salt. It could, in some sense, be true, but we hesitate to believe without support.
3. Implausible claims: "I have three billion dollars in my pocket, in pennies."
Reasonable people would dismiss this as presumptively false. We might wonder whether this could be made true in some bar-bet form, by redefining "penny," or "pocket," or "three billion dollars," but we assume the claim is false.
This is an extraordinary claim. We aren't closed to the possibility that it is true, but, unless strong evidence is provided, reasonable people will assume that it is false.
4. Wacko claims: "I have been to Mars sixteen times. On one trip, I met the reincarnation of Abraham Lincoln who used the Vulcan mind meld to teach me how to travel faster than light."
This is false, right? It's not impossible, but it couldn't be true unless
so much of what we already thought we knew was wrong. Think how much scaffolding we'd have to tear down and rebuild. How would one even begin to prove such a claim?
And yet, we would still entertain the claim, if the supporting evidence was strong enough.
I believe the claim is false. I can't imagine what evidence would support it. But the falseness of the claim is presumptive.
5. Self contradiction: "A married bachelor gave me a square circle. An omnipotent god couldn't defeat iron chariots."
Such claims aren't just presumptively false; they are false, period. No evidence, no matter how strong, can support a contradiction.
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So there we have a range of types of claims, from the mundane to the impossible. The further down the list you go, the stronger the evidence would have to be before we could believe the claim--until we reach the final category, contradictions, which cannot be supported by any evidence at all.
The more extraordinary the claim is, the more extraordinary the evidence has to be.
I hope this helps.
Suppose you encounter the claim that Colorado doesn't exist, that it appears on maps due to a conspiracy of cartographers, and that people who claim to have been to Colorado do so because they have been bribed or coerced.
You dismiss that claim as false, right? Because it contradicts things that you think you know, about maps, about people, about conspiracies, about Google, about Pikes Peak, about marijuana laws. So
many things thought to be true would have to be reexamined, and discarded, in order for this claim to be true.
It's an extraordinary claim. Category 3 at least, right?
And yet if it were true, you'd want to know about it. You would examine the evidence if the evidence seemed strong enough to be worth examining. You don't know what the evidence would be. You assume that the evidence doesn't exist. But, if someone presented you with evidence that seemed strong enough, you'd consider it with an open mind.
But it would have to be huge. The evidence would have to be extraordinary.
And, in the absence of such evidence, reasonable people get to presume the claim is false.
Because extraordinary claims require extraordinary evidence.