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.
