I recently heard a debate where one of the debaters rejected the claim that extraordinary claims require extraordinary evidence. I was shocked to hear this; not only because I have never heard anyone reject this before, but because I regard this as a fundamental principle of scientific skepticism, and from my experience, so does the scientific community. Should this aphorism be rejected by critical thinkers? As you might have guessed, the answer is an emphatic "No." In critical thinking, it is imperative that one proportions his or her belief to the evidence. But we're going to need to break this down.
Imagine you are trying to convince someone that people can be possessed by demons. Your evidence you provide is a) the Vatican performs exorcisms, b) you've read stories on the Internet and in books about hundreds of demonic possessions, and c) you know this guy whose friend's brother-in-law was possessed by a demon. The person whom you are trying to convince tells you that your claim is extraordinary and the evidence you are providing to support that claim is far from extraordinary. At this point, you have several options to convince them that your claim is true:
Is this claim, in fact, extraordinary? Roughly 2500 years ago, Socrates established the rule that basically states if any intelligible conversation is to be had, terms need to be clearly defined and understood by all those involved. Webster's defines extraordinary as "very unusual or remarkable." These are subjective words that can mean different things to different people. One could argue that since this "happens all the time," that it is far from unusual or remarkable. But Webster didn't suggest that extraordinary claims require extraordinary evidence—David Hume did more than 200 years before Carl Sagan popularized the aphorism. In David Hume's essay Of Miracles, Hume discusses miracles defined as "a transgression of a law of nature by a particular volition of the Deity, or by the interposition of some invisible agent," which would be a specific class of extraordinary event. While this doesn't cover the entirety of extraordinary events, it does clearly define perhaps the most common—and one that pertains to our example of demonic possession. So if we are borrowing Hume's rule of scientific skepticism, to be consistent, we must also borrow his use of the word "extraordinary," otherwise, we would be committing the fallacy of equivocation, which is using an ambiguous term in more than one sense, thus making an argument misleading. Is this claim, in fact, extraordinary? Yes. It certainly is.
Perhaps you feel that your evidence is "extraordinary?" It's a good idea to pause for a moment and consider the problem of cognitive overreaching. We can say that there is plenty of evidence for people claiming to be possessed by demons; what we cannot say is that there's plenty of evidence for people actually being possessed by demons. Your claim is that people can be possessed by demons, not that people can claim to be possessed by demons. So while you might have evidence, it is evidence for the wrong claim. As for the fact that the Vatican performs exorcisms, this is the only evidence that the Vatican believes in, or claims to believe in, demonic possession. One can accept this on faith but not offer it up as evidence. Is the evidence here "extraordinary?" No. Not in any sense of the word.
Now let's look at the last option: reject this aphorism, "extraordinary claims require extraordinary evidence." I suggest that those who do reject this, only do so for claims in which they have a vested interest in the conclusion. Consider the following example: If someone claimed to have seen pigs fly, the fact that pigs did fly would certainly be extraordinary. If they showed you a photo of pigs flying, would you consider that sufficient evidence to warrant the belief that they saw pigs that fly? I would suspect that no reasonable person would accept such a claim purely based on a photo that can be easily doctored by an eleven-year-old with a smartphone. We can also look at the miraculous claims of tens of thousands of religions and cults both past and present, and we would find that people who accept extraordinary claims of their own religion or cult based on ordinary evidence reject the extraordinary claims of other religions and cults based on the lack of extraordinary evidence. This is referred to as compartmentalization, which is a psychological phenomenon where people suspend their use of critical thinking for one or more categories or beliefs. This explains how some very intelligent people can continue to hold some very irrational beliefs.
In the debate which I referred to earlier, the debater who rejected the aphorism, "extraordinary claims require extraordinary evidence," did so by providing an example. Here is the example, paraphrased:
If someone were to claim to win millions in the lottery, the fact that they did win would certainly be extraordinary. However, to believe them, you might simply require them to show you the winning ticket. This is not extraordinary evidence. Therefore, extraordinary claims do not require extraordinary evidence.
The usage of "extraordinary" in this context is a departure from what Hume had in mind. Winning the lottery is certainly rare for any given individual, but it happens several times a day. Clearly, winning the lottery is a categorically different claim than say being possessed by a demon. The word we use to represent the claim (i.e., extraordinary, miraculous, amazing, novel, improbable, etc.) is secondary to what we mean by "extraordinary claims require extraordinary evidence." While articulation of precisely what we mean may be challenging, it is well demonstrated and practically universally agreed upon through examples, such as the flying pigs or miraculous claims of cult members.
Extraordinary claims do require extraordinary evidence. What qualifies as "extraordinary" is debatable, but this continues to be a foundational idea in scientific skepticism. If you wish to make exceptions to this rule for claims of faith or other emotionally-based claims (e.g., your kids are perfect) then go ahead. Just realize that you are leaving the realm of critical thinking and your claims are no longer welcome in rational debate. However, if you want your claims to be taken seriously among those who value reason and critical thinking, evaluate your own extraordinary claims using the same skepticism you would when evaluating someone else's extraordinary claims—especially those claims that contradict yours.
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