An extraordinary claim is one which is not supported by the available, or ordinary, evidence. Support for such a claim must therefore come from newly observed evidence, or a new recognition of existing evidence, which is extraordinary.
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.
Others have put forward very similar ideas with different phrasing. Pierre-Simon Laplace (1749–1827) said "The weight of evidence for an extraordinary claim must be proportioned to its strangeness". 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 "On the Extraordinary: An Attempt at Clarification" (1978), sociologist Marcello Truzzi said "an extraordinary claim requires extraordinary proof."
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 finally confessed to doping in 2013.
Criticism of the aphorism
The aphorism has been criticized both for its apparent support of "orthodoxy" by raising the evidential standard for claims which are outside current social consensus, and for introducing subjectivity and ambiguity in determining what merits an "extraordinary claim". David Deming writes: "[s]cience does not contemplate two types of evidence. The misuse of ECREE ["extraordinary claims require extraordinary evidence"] to suppress innovation and maintain orthodoxy should be avoided as it must inevitably retard the scientific goal of establishing reliable knowledge."
- Burden of proof (philosophy)
- Hitchens's razor
- Razor (philosophy)
- Theory of justification
- Marc Kaufman, First Contact: Scientific Breakthroughs in the Hunt for Life Beyond Earth, Simon and Schuster, p. 124.
- Sagan, Carl (December 14, 1980). "Encyclopaedia Galactica". Cosmos. Episode 12. 01:24 minutes in. PBS.
- "A Stepchild of Science Starts to Win Friends". U.S. News & World Report. 1978-07-31. pp. 41–42. Retrieved 2017-10-14.
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.
- Rao, K.R., 1978, Psi: Its place in nature. Journal of Parapsychology vol 42.
- Flournoy, Théodore (1899). Des Indes à la planète Mars: étude sur un cas de somnambulisme avec glossolalie. Slatkine. pp. 344–345.*Flournoy, Théodore (2007). From India to the Planet Mars: A Study of a Case of Somnambulism. Daniel D. Vermilye, trans. Cosimo, Inc. pp. 369–370. ISBN 9781602063570.
- Berkes, Anna (November 14, 2008). "Who is the liar now?". monticello.org. Thomas Jefferson Foundation. Retrieved October 29, 2016. Letter to Daniel Salmon on 15 February 1808 discusing the nature and origin of meteorites. U.S. Library of Congress image
- Marcello Truzzi, "On the Extraordinary: An Attempt at Clarification", Zetetic Scholar, Vol. 1, No. 1, p. 11, 1978.
- Deming, D. Philosophia (2016) 44: 1319. https://doi.org/10.1007/s11406-016-9779-7