Full Idea
The credibility of a witness is in part a function of the story being reported. When the story claims to have infinite value, the temptation to lie for personal benefit is asymptotically infinite.
Clarification
'Asymptotic' means gets closer, without limit
Gist of Idea
The reliability of witnesses depends on whether they benefit from their observations
Source
report of Pierre Simon de Laplace (Philosophical Essay on Probability [1820], Ch.XI) by Ian Hacking - The Emergence of Probability Ch.8
Book Reference
Hacking,Ian: 'The Emergence of Probability' [CUP 1975], p.72
A Reaction
Laplace seems to especially have reports of miracles in mind. This observation certainly dashes any dreams one might have of producing a statistical measure of the reliability of testimony.