Men of no more than ordinary discernment never rate any person higher than he appears to rate himself.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“If those companions, however, have not both more discernment and more generosity than ordinary, though they may have some kindness for him, they have seldom much respect; and the warmth of their kindness is very seldom sufficient to compensate the coldness of their respect. Men of no more than ordinary discernment never rate any person higher than he appears to rate himself. He seems doubtful himself, they say, whether he is perfectly fit for such a situation or such an office; and immediately give the preference to some impudent blockhead who entertains no doubt about his own qualifications.” source