Do extraordinary claims require extraordinary evidence?

Pseudo-skepticism and the issue of burden of proof

A UFO over Manitoba, Canada? It certainly is an extraordinary claim… Image from cbc.ca.

You might have heard the catch phrase “Extraordinary claims require extraordinary evidence,” which is often uttered by skeptics against anyone who purports to believe in extra sensorial perception, UFOs, unlikely conspiracy theories, god, and a large variety of other dubious notions.

It is often attributed to one of the founding fathers of the American skeptical movement in the 1970s, the astronomer Carl Sagan, who did in fact use it in a Cosmos/PBS segment aired on 14 December 1980. Sagan, however, borrowed the phrase from Marcello Truzzi, an American (and Danish born) sociologist who co-founded (together with the legendary Paul Kurtz) what was known as the Committee for the Scientific Investigation of Claims of the Paranormal (CSICOP) in 1976. (Nowadays CSICOP is known as CSI, the Committee for Skeptical Inquiry.)

Truzzi’s original phrase was “An extraordinary claim requires extraordinary proof,” as he put it in the first issue of his journal, Zetetic Scholar, published in 1978. In turn, Truzzi himself borrowed the sentiment from the 18th French astronomer Pierre-Simone de Laplace, who said that “the more extraordinary a fact is, the more it needs to be supported by strong evidence.” The Scottish Enlightenment skeptic David Hume said something similar when he wrote, in his essay On Miracles, that “A wise man proportions his belief to the evidence” and should “always reject the greater miracle.”

Regardless of whether we refer to the notion as Sagan’s standard or Hume’s dictum (I preferred the latter), the idea was interpreted very differently by Truzzi on the one hand and by Paul Kurtz and Carl Sagan on the other, leading to a major schism early on in the history of modern skepticism and—most importantly—is at the root of what it means to be a skeptic, or a reasonable person. So it’s worth digging in a little deeper into the whole affair.

Truzzi was once characterized by Kurtz as “the skeptic’s skeptic,” which, knowing Paul’s sarcastic sense of humor, was probably intended as a very backhanded compliment. Here is the story, in brief, before we get to the epistemological meat of the matter.

Truzzi had founded a skeptically-inclined journal known as Explorations, but agreed to make it the official magazine of CSICOP under the title The Zetetic. “Zetetic” is a Greek word that is essentially synonymous with skeptic, both of them meaning inquirer, though zetesis more specifically means research while skepsis indicates thorough investigation. Six of one, half a dozen of the other, as we would say today.

Truzzi served as editor of the magazine during its first year of publication, until August 1977, but soon found himself at odds with the rest of the board of CSICOP. Why? Because Truzzi thought that true skepticism implied a stance of complete neutrality, of agnosticism, toward extraordinary claims, and that therefore it would be intellectually honest to invite authors who supported notions like the paranormal, UFOs, etc. to contribute to CSICOP’s effort. One can sympathize with this “let a thousand flowers bloom” attitude, though I would argue below that Truzzi was wrong about this, in a very interesting and revealing way.