As the poster for the Centre for Inquiry notes, extraordinary claims require extraordinary evidence. It’s a popular catchphrase for the skeptical movement, but should be an intellectual policy for everyone.
Regardless of what is being claimed, it requires evidence at the same level of the claim.
Anecdote is not evidence, please note, especially personal anecdote even with the corroboration of other witnesses. People often “see” what they choose to see, and interpret events and objects according to preconceived ideas. Seeing UFOs instead of ordinary aircraft, or chemtrails instead of mundane contrails are examples of this. Evidence is something concrete; a body of facts, not simply interpretation or disingenuous claim.
The Centre lists many claims as the poster indicates – a list that continues to grow – along with a brief introduction to each: claims, evidence and conclusion. Some like leprechauns, the Easter Bunny, tooth fairy, dragons and Xenu seem pretty obviously mythological or (like Xenu) totally fabricated. Others will certainly raise an argument among some religious believers or the superstitious – angels, magic, Heaven, hell, the afterlife and similar religious topics included (it does not yet list Santa Claus, but I expect it will come)..
As RationalWiki puts it,
Extraordinary claims require extraordinary evidence was a phrase made popular by Carl Sagan. It is central to scientific method, and a key issue for critical thinking, rational thought and skepticism everywhere.
The actual phrase was coined by sociologist Marcello Truzzi, but it has been around in other forms for several centuries. In his 1748 work, An Enquiry concerning Human Understanding (chap. 10.4.), David Hume wrote:
In our reasonings concerning matter of fact, there are all imaginable degrees of assurance, from the highest certainty to the lowest species of moral evidence. A wise man, therefore, proportions his belief to the evidence… No testimony is sufficient to establish a miracle, unless the testimony be of such a kind, that its falsehood would be more miraculous than the fact which it endeavors to establish.
Simply because a lot of people believe in something, or accept it as factual doesn’t mean it is true. In a letter to Adam Smith, Hume wrote:
Nothing indeed can be a stronger presumption of falsehood than the approbation of the multitude…
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