Do extraordinary claims require extraordinary evidence? This aphorism, popularized by the late Carl Sagan, is intuitively appealing to many people. In particular, New Atheists cite it to reject the occurrence of supernatural events.
However, the aphorism is poorly stated. Extraordinary claims should be regarded as requiring, not "extraordinary evidence," but adequate evidence. To see this, consider someone who claims to have won the lottery. That’s quite the extraordinary claim – the probability of this happening is extremely small. And yet, all it would take to justify this claim is a ticket with the winning numbers - something that isn’t necessarily "extraordinary," but it is adequate.
By using the word "adequate" a further issue is highlighted. That is, what someone considers to be “adequate” will depend on their background beliefs. For example, consider the possibility of life on other planets. If someone thinks that the existence of extraterrestrials is impossible, then no amount of evidence could ever convince them otherwise. But if someone thinks that it’s possible for extraterrestrials to exist then the standards for “adequate evidence” will be quite different.
In like manner, if someone thinks that supernatural events are impossible because they are convinced that God does not exist (or that nothing exists beyond the physical world), then accepting the occurrence of a supernatural event will be practically impossible. On the other hand, if someone believes God exists then they will apply quite a different criterion to determine whether there is adequate evidence to justify a supernatural claim. In sum, extraordinary claims require adequate evidence. But what counts as adequate will depend on a person's background beliefs.
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