quoted text The art of concluding from experience and observation consists in evaluating probabilities, in estimating if they are high or numerous enough to constitute proof. This type of calculation is more complicated and more difficult than one might think. It demands a great sagacity generally above the power of common people. The success of charlatans, sorcerors, and alchemists — and all those who abuse public credulity — is founded on errors in this type of calculation.

Benjamin Franklin and Antoine Lavoisier, Rapport des commissaires chargés par le roi de l'examen du magnétisme animal (1784), as translated in "The Chain of Reason versus the Chain of Thumbs", Bully for Brontosaurus (1991) by Stephen Jay Gould, p. 195, http://en.wikiquote.org/wiki/Benjamin_Franklin