The wisest and most experienced are generally the least credulous. But the man scarce lives who is not more credulous than he ought to be, and who does not, upon many occasions, give credit to tales, which not only turn out to be perfectly false, but which a very moderate degree of reflection and attention might have taught him could not well be true. The natural disposition is always to believe.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“Their credulity, accordingly, is excessive, and it requires long and much experience of the falsehood of mankind to reduce them to a reasonable degree of diffidence and distrust. In grown-up people the degrees of credulity are, no doubt, very different. The wisest and most experienced are generally the least credulous. But the man scarce lives who is not more credulous than he ought to be, and who does not, upon many occasions, give credit to tales, which not only turn out to be perfectly false, but which a very moderate degree of reflection and attention might have taught him could not well be true. The natural disposition is always to believe. It is acquired wisdom and experience only that teach incredulity, and they very seldom teach it enough. The wisest and most cautious of us all frequently gives credit to stories which he himself is afterwards both ashamed and astonished that he could possibly think of believing.” source