"Extraordinary claims require extraordinary evidence" (sometimes shortened toECREE),[1] also known as theSagan standard, is anaphorism popularized by science communicatorCarl Sagan. He used the phrase in his 1979 bookBroca's Brain and the 1980 television programCosmos. It has been described as fundamental to thescientific method and is regarded as encapsulating the basic principles ofscientific skepticism.
The concept is similar toOccam's razor in that bothheuristics prefer simpler explanations of a phenomenon to more complicated ones. In application, there is some ambiguity regarding when evidence is deemed sufficiently "extraordinary". It is often invoked to challenge data and scientific findings, or to criticize pseudoscientific claims. Some critics have argued that the standard can suppress innovation and affirmconfirmation biases.
PhilosopherDavid Hume characterized the principle in his 1748 essay "Of Miracles". Similar statements were made by figures such asThomas Jefferson in 1808,Pierre-Simon Laplace in 1814, andThéodore Flournoy in 1899. The formulation "Extraordinary claims require extraordinary proof" was used a year prior to Sagan, by scientific skepticMarcello Truzzi.
An interesting debate has gone on within the [Federal Communications Commission] between those who think that all doctrines that smell ofpseudoscience should be combated and those who believe that each issue should be judged on its own merits, but that theburden of proof should fall squarely on those who make the proposals. I find myself very much in the latter camp. I believe that the extraordinary should certainly be pursued. But extraordinary claims require extraordinary evidence.
The aphorism "Extraordinary claims require extraordinary evidence", according to psychologist Patrizio Tressoldi, "is at the heart of thescientific method, and a model forcritical thinking,rational thought andskepticism everywhere".[3][4][5] It has also been described as a "fundamental principle of scientific skepticism".[6] The phrase is often used in the context ofparanormal and otherpseudoscientific claims.[7][8][9] It is also frequently invoked inscientific literature to challenge research proposals,[10] like a new species ofAmazonian tapir,[6] biparental inheritance ofmitochondrial DNA,[11] or aHolocene "mega-tsunami".[12]
The concept is related toOccam's razor as, according to such aheuristic, simpler explanations are preferred to more complicated ones. Only in situations where extraordinary evidence exists would an extraordinary claim be the simplest explanation.[7] It appears inhypothesis 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 strongerBayesian prior to the acceptance of the null hypothesis as opposed to its rejection.[13]
Science communicatorCarl Sagan popularized the aphorism in his 1979 bookBroca's Brain,[2][14] and in his 1980 television showCosmos in reference to claims aboutextraterrestrials visiting Earth.[15] Sagan had first stated the eponymous standard in a 1977 interview withThe Washington Post.[16] However, scientific skepticMarcello Truzzi used the formulation "Extraordinary claims require extraordinary proof" in an article published byParapsychology Review in 1975,[16] as well as in aZetetic Scholar article in 1978.[17] Two 1978 articles quoted physicistPhilip Abelson—then the editor of the journalScience—using the same phrasing as Truzzi.[18][19]
In his 1748 essay "Of Miracles", philosopherDavid Hume wrote that if "the fact ... partakes of the extraordinary and the marvellous ... the evidence ... received a diminution, greater or less, in proportion as the fact is more or less unusual".[20] Deming concluded that this was the first complete elucidation of the standard. Unlike Sagan, Hume defined the nature of "extraordinary": he wrote that it was a large magnitude of evidence.[20][21]
Others had also put forward very similar ideas.Quote Investigator cites similar statements fromBenjamin Bayly (in 1708),Arthur Ashley Sykes (1740),Beilby Porteus (1800),Elihu Palmer (1804), andWilliam Craig Brownlee (1824).[16] The French scholarPierre-Simon Laplace, in essays (1810 and 1814) on thestability of the Solar System, wrote that "the weight of evidence for an extraordinary claim must be proportioned to its strangeness".[5][16]Thomas Jefferson in an 1808 letter expressedcontemporary skepticism of meteorites thus: "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."[22][23]
Sagan did not describe any concrete or quantitative parameters as to what constitutes "extraordinary evidence", which raises the issue of whether the standard can be applied objectively.[5][14][24] Academic and climate change denialistDavid Deming notes that it would be "impossible to base all rational thought and scientific methodology on an aphorism whose meaning is entirely subjective". He instead argues that "extraordinary evidence" should be regarded as a sufficient amount of evidence rather than evidence deemed of extraordinary quality.[25] Tressoldi noted that the threshold of evidence is typically decided through consensus. This problem is less apparent in clinical medicine and psychology where statistical results can establish thestrength of evidence.[5]
Deming also noted that the standard can "suppress innovation and maintain orthodoxy".[25] Others, likeEtzel Cardeña, have noted that many scientific discoveries that spurredparadigm shifts were initially deemed "extraordinary" and likely would not have been so widely accepted if extraordinary evidence were required.[26][27] Uniform rejection of extraordinary claims could affirmconfirmation biases in subfields.[27] Additionally, there are concerns that, when inconsistently applied, the standard exacerbatesracial and gender biases.[28] PsychologistRichard Shiffrin has argued that the standard should not be used to bar research from publication but to ascertain what is the best explanation for a phenomenon.[29] Conversely, mathematical psychologistEric-Jan Wagenmakers stated that extraordinary claims are often false and their publication "pollutes the literature".[30] To qualify the publication of such claims, psychologist Suyog Chandramouli has suggested the inclusion of peer reviewers' opinions on their plausibility or an attached curation of post-publication peer evaluations.[27]
Cognitive scientist andAI researcherBen Goertzel believes that the phrase is utilized as a "rhetorical meme" without critical thought. PhilosopherTheodore Schick argued that "extraordinary claims do not require extraordinary evidence" if they provide the most adequate explanation.[9] Moreover, theists and Christian apologists likeWilliam Lane Craig have argued that it is unfair to apply the standard toreligious miracles as other improbable claims are often accepted based on limited testimonial evidence, such as an individual claiming that they won the lottery.[31][32]
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