Lourdes effect
The term Lourdes effect has been coined by the Belgian philosopher and skeptic Etienne Vermeersch for the tendency of supposed supernatural powers to resist clear demonstration. Vermeersch suggests that, if the miraculous healing powers of Lourdes are real, one would expect the Virgin Mary or God to reattach a severed arm as often as more ambiguous cures of paralysis or blindness; and that the lack of such undeniable healings is strong evidence against the subtle ones as well.