The man of the most exquisite humanity, is naturally the most capable of acquiring the highest degree of self-command.
 Adam Smith, The Theory of Moral Sentiments (1759). copy citation

Context

“The person best fitted by nature for acquiring the former of those two sets of virtues, is likewise necessarily best fitted for acquiring the latter. The man who feels the most for the joys and sorrows of others, is best fitted for acquiring the most complete control of his own joys and sorrows. The man of the most exquisite humanity, is naturally the most capable of acquiring the highest degree of self-command. He may not, however, always have acquired it; and it very frequently happens that he has not. He may have lived too much in ease and tranquillity. He may have never been exposed to the violence of faction, or to the hardships and hazards of war.” source