Extraordinary claims require extraordinary evidence
From RationalWiki
Extraordinary claims require extraordinary evidence was a phrase made popular by Carl Sagan. It is the heart of the scientific method, and a model for critical thinking, rational thought and skepticism everywhere.
The evidence put forth by proponents of such things as gods, ghosts, the paranormal, and UFOs is highly questionable at best and offers little in the way of proof. Even if we accepted what evidence there is as valid (and it is highly debatable if we should), limited and weak evidence is not enough to overcome the extraordinary nature of these claims.
[edit] Analogy
Alice and Bob are two friends talking after school. Alice tells Bob that she watched a movie the previous evening. Bob believes her easily, because he knows that movies exist, that Alice exists, and that Alice is capable and fond of watching movies. If he doubts her, he might ask for a ticket stub or a confirmation from one of her friends. If, however, Alice tells Bob that she flew on a unicorn to a fairy kingdom where she participated in an ambrosia-eating contest, and she produces a professionally-printed contest certificate and a friend who would testify to the events described, Bob would not be inclined to believe her without strong evidence for the existence of flying unicorns, fairies and ambrosia-eating contests.
