Thread: Can someone explain the usage of the phrase "Extraordinary claims require extraordinary evidence"?

That is a preposterous claim! You could as well have quoted a figure of 10% or 0.1%. There is no objective level for "extraordinary".

I have to admit that in retrospect, that was a bit of a blanket statement.
However, due to the digital nature of how our brains parse beliefs (I believe/I don't believe), I feel that there must be a point somewhere where a belief falls below the threshold that warrants additional effort to investigating it. 1% is admittedly a rather arbitrary percentage though.

There certainly is a danger of suppressing research based on this phrase, particularly when used by those with a poor grasp of probability and an ideological axe to grind.

I have to disagree with some of your later sentiments though. Correct me if I'm wrong but it sounds like you are advocating the idea that absence of evidence is not evidence of absence. I disagree with this often abused notion. Absence of evidence can range from anywhere between very weak to strong evidence of absence. It is contingent on how likely the observation in question would result from a cause.

For example: the absence of evidence for extraterrestrial life qualifies as very weak evidence of absence since we do not possess instrumentation that is powerful enough to test a significant sample of the universe for life. What's more, there is indirect evidence such as clouds of amino acids and water that at the very least make the formation of life outside our planet possible, if not very probable given the wide field of space and time for it to occur.

However, in the case of ghosts, absence of evidence is very strong evidence for absence because we have had ample opportunity to test causes that should result in such an observation. They have all turned up dry. We also have indirect evidence to suggest that purported ghost sightings are likely the result of infrasonic frequencies that occurs due to old fashioned plumbing and other causes. Frequencies around 18Hz (basically the range of tiger growls) can evoke feelings of dread, an evil presence and terror in subjects. 19Hz can cause outright hallucinations.

For a more in depth critique by someone better versed than me:
http://lesswrong.com/lw/ih/absence_o...ce_of_absence/