Extraordinary claims require extraordinary evidence

An interesting debate has gone on within the [Federal Communications Commission] between those who think that all doctrines that smell of pseudoscience should be combated and those who believe that each issue should be judged on its own merits, but that the burden of proof should fall squarely on those who make the proposals.

[7][8][9] It is also frequently invoked in scientific literature to challenge research proposals,[10] like a new species of Amazonian tapir,[6] biparental inheritance of mitochondrial DNA,[11] or a Holocene "mega-tsunami".

[13] Sagan popularized the aphorism in his 1979 book Broca's Brain,[2][14] and in his 1980 television show Cosmos in reference to claims about extraterrestrials visiting Earth.

Quote Investigator cites similar statements from Benjamin Bayly (in 1708), Arthur Ashley Sykes (1740), Beilby Porteus (1800), Elihu Palmer (1804), and William Craig Brownlee (1824).

[16] The French scholar Pierre-Simon Laplace, in essays (1810 and 1814) on the stability 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 expressed contemporary 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] Science communicator Carl 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 David Deming notes that it would be "impossible to base all rational thought and scientific methodology on an aphorism whose meaning is entirely subjective".

[27] Cognitive scientist and AI researcher Ben Goertzel believes that the phrase is utilized as a "rhetorical meme" without critical thought.