You must look into people, as well as at them. Almost all people are born with all the passions, to a certain degree; but almost every man has one prevailing one, to which the others are subordinate. Search every one for that ruling passion; pry into the recesses of his heart, and observe the different workings of the same passion in different people; and when you have found out the prevailing passion of any man, remember never to trust him where that passion is concerned. Work upon him by it, if you please; but be upon your guard yourself against it, whatever professions he may make you.

Lord Chesterfield (1694-1773) English statesman, wit [Philip Dormer Stanhope]

Letter to his son, #112 (4 Oct 1746)
    (Source)