"The usual touchstone of whether what someone asserts is mere persuasion or at least a subjective conviction, i.e., firm belief, is betting. Often someone pronounces his propositions with such confident and inflexible defiance that he seems to have entirely laid aside all concern for error. A bet disconcerts him. Sometimes he reveals that he is persuaded enough for one ducat but not for ten. For he would happily bet one, but at ten he suddenly becomes aware of what he had not previously noticed, namely that it is quite possible that he has erred."

--Immanuel Kant, Critique of Pure Reason (A824/B852); seen on http://kenfeinstein.blogspot.com/2011/01/kant-on-betting-and-prediction-markets.html as linked by Marginal Revolution