Applying the standard
ECREE is related to Occam's razor in the sense that according to such a heuristic, simpler explanations are preferred to more complicated ones. Only in situations where extraordinary evidence exists would an extraordinary claim be the simplest explanation. A routinized form of this appears in hypothesis testing where the hypothesis that there is no evidence for the proposed phenomenon, what is known as the "null hypothesis", is preferred. The formal argument involves assigning a stronger Bayesian prior to the acceptance of the null hypothesis as opposed to its rejection. How to weigh such priors requires careful consideration, and it is usually at this point that arguments between those who make extraordinary claims and those who debunk them occur.
The aphorism was made popular by astronomer Carl Sagan through the 1980 TV show Cosmos. Two 1978 articles, one in U.S. News & World Report and another by Koneru Ramakrishna Rao in the Journal of Parapsychology both quote physicist Philip Abelson, then the editor of Science, using the same phrase. In "On the Extraordinary: An Attempt at Clarification" (1978), sociologist Marcello Truzzi said "an extraordinary claim requires extraordinary proof."
Others have put forward very similar ideas with different phrasing. Théodore Flournoy, in 1899, put forward the principle that "the weight of evidence for an extraordinary claim must be proportioned to its strangeness", attributing the idea to Laplace, whom he quotes saying, in 1814, that "we ought to examine [seemingly inexplicable phenomena] with an attention all the more scrupulous as it appears more difficult to admit them". In 1808, Thomas Jefferson also said "A thousand phenomena present themselves daily which we cannot explain, but where facts are suggested, bearing no analogy with the laws of nature as yet known to us, their verity needs proofs proportioned to their difficulty."
In 2004 the cyclist Lance Armstrong used the phrase "Extraordinary allegations require extraordinary evidence" to discredit allegations of doping put to him by journalist David Walsh. Armstrong was later asked "What is it about you that makes ordinary proof insufficient to bring you down? For murderers, we're not looking for extraordinary proof, we're looking for proof. But you're saying it must be extraordinary. Why?". Armstrong later confessed to doping in 2013.
- Burden of proof (philosophy)
- Hitchens's razor
- Razor (philosophy)
- Theory of justification
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Philip H. Abelson, editor of the authoritative journal Science, agrees that parapsychological research has improved markedly, but he is dubious about the results. "These extraordinary claims require extraordinary evidence," he contends.
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