"Extraordinary Claims Require Extraordinary Evidence"

Carl Sagan is credited with the popular skeptical phrase, “Extraordinary claims require extraordinary evidence” or, later, “Extraordinary claims demand extraordinary evidence.” At or about the same time (probably slightly later than Sagan), founding Skeptical Inquirer editor Marcello Truzzi is on record using the phrase, “Extraordinary claims require extraordinary proof.” All three of these variations of this concept are still currently in circulation.

Tracing this thought back in time, we find American magician and skeptic Joseph F. Rinn expressing this idea in a 1911 Washington Post article debunking psychics. His version was “Wonderful phenomena demand wonderful evidence in their support.” Skipping back a little further we find French scholar Pierre-Simon Laplace considering this concept in some detail. In 1814, he argued that “the more extraordinary the event, the greater the need of its being supported by strong proofs,” and “the probability of the falsehood increases in the measure that the deed becomes more extraordinary,” and “the probability of the error or of the falsehood of the witness becomes as much greater as the fact attested is more extraordinary.” Taking it a little further, he said, “There are things so extraordinary that nothing can balance their improbability.”

Thomas Jefferson’s formulation of this concept predates Laplace by six years where he suggests that extraordinary claims’ “verity needs proofs proportioned to their difficulty” in 1808….

“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. A cautious mind will weigh well the opposition of the phenomenon to everything hitherto observed, the strength of the testimony by which it is supported, and the errors and misconceptions to which even our senses are liable.”

We can trace this idea back one step further to Scottish philosopher David Hume. His formulation of the idea in 1739 was “A wise man, therefore, proportions his belief to the evidence.” Expanding a bit, he said, “Suppose, for instance, that the fact, which the testimony endeavours to establish, partakes of the extraordinary and the marvellous; in that case, the evidence, resulting from the testimony, admits of a diminution, greater or less, in proportion as the fact is more or less unusual,” and “Such an event, therefore, may be denominated extraordinary, and requires a pretty strong testimony, to render it credible…”

He also said, "When any one tells me, that he saw a dead man restored to life, I immediately consider with myself, whether it be more probable, that this person should either deceive or be deceived, or that the fact, which he relates, should really have happened. I weigh the one miracle against the other; and according to the superiority, which I discover, I pronounce my decision, and always reject the greater miracle."

For some time I wasn’t able to trace the formulation of this idea any further back. I’ve now run across the concept expressed by Michel Eyquem de Montaigne (1533-1592) as “[I]t is far more probable that our senses should deceive us, than that an old woman should be carried up a chimney on a broom stick; and that it is far less astonishing that witnesses should lie, than that witches should perform the acts that were alleged.”

The phrase, “Extraordinary claims require extraordinary evidence” has become very popular in the freethought and skeptic communities where it can be found on everything from bumper-stickers to tee-shirts. A waitress in a restaurant once approached a freethinker wearing a tee-shirt with the phrase imprinted. She commented, “You must work for an insurance company,” which brought much laughter to everyone at the table.

“Extraordinary claims require extraordinary evidence” is more than just a good policy for insurance companies. It is a most useful idea to employ when assessing any extraordinary claims, and it is an essential aspect of critical thinking.