Extraordinary Claims Require Extraordinary Evidence

Extraordinary claims require extraordinary evidence

American scientist and science popularizer 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 (possibly 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.

A few years later, American writer and professor of biochemistry Isaac Asimov expressed the same basic idea when he said, “I’ll believe anything, no matter how wild and ridiculous, if there is evidence for it. The wilder and more ridiculous something is, however, the firmer and more solid the evidence will have to be.”

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 at least 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…”

For some time I wasn’t able to trace the formulation of this idea any further back. I’ve now run across the basic 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.

Several years ago, a group of skeptics were sitting at a table in a restaurant. One of the group was wearing a tee-shirt with the phrase imprinted. Their waitress commented, “You must work for an insurance company.” Of course, this 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.


Writing in the 9th century might lead you to give some apologies as an introduction.

Here is the basic outline from that time era that some people followed when saying something….

First Apology – Apology for saying whatever the author is planning to say or that he presumes to say it at all. He might also give recognition to whoever is supporting him (or who he would like to support him or gain some favor from).

Second Apology – Apology for saying it the way the author is planning to say it (others can say it better than he can).

Third Apology — The author admits that others know more than he does about whatever he has to say, and he bows to their superior knowledge to correct him where he goes astray.

Author’s Justification: The author thinks that no one else is going to say it (accurately) unless he does, or that no one might say it at all. He expresses a feeling of obligation to the task he has set himself. He feels he has been pressed into his situation by his personal passion on the subject and he is being forced to act. The author feels he is stepping up to the plate when others have not done so.

See Nennius’ apology (from “Historia Brittonum”) for an example of what I’m talking about below:

I, Nennius, disciple of St. Elbotus, have endeavoured to write some extracts which the dulness of the British nation had cast away, because teachers had no knowledge, nor gave any information in their books about this island of Britain. But I have got together all that I could find as well from the annals of the Romans as from the chronicles of the sacred fathers, Hieronymus, Eusebius, Isidorus, Prosper, and from the annals of the Scots and Saxons, and from our ancient traditions. Many teachers and scribes have attempted to write this, but somehow or other have abandoned it from its difficulty, either on account of frequent deaths, or the often recurring calamities of war. I pray that every reader who shall read this book, may pardon me, for having attempted, like a chattering jay, or like some weak witness, to write these things, after they had failed. I yield to him who knows more of these things than I do.

He apologizes for two pages as an introduction and then he has this section above titled “The Apology of Nennius.” I’m not sure that there are any recorded apologies as long or involved as Nennius’ apologies. At least there are none that I am aware of.

Thus results the un-popularized comment, “You are apologizing like Nennius” (something that I made up years ago and which I’ve just decided to try and promote here).

Finally, the author says what he has to say and most of it is wrong.