From William Shakespeare's Julius Caesar, Act I, Scene 2:

  CAESAR
  Let me have men about me that are fat;
  Sleek-headed men and such as sleep o' nights:
  Yond Cassius has a lean and hungry look;
  He thinks too much: such men are dangerous.
  
  ANTONY
  Fear him not, Caesar; he's not dangerous;
  He is a noble Roman and well given.
  
  CAESAR
  Would he were fatter! But I fear him not:
  Yet if my name were liable to fear,
  I do not know the man I should avoid
  So soon as that spare Cassius. He reads much;
  He is a great observer and he looks
  Quite through the deeds of men: he loves no plays,
  As thou dost, Antony; he hears no music;
  Seldom he smiles, and smiles in such a sort
  As if he mock'd himself and scorn'd his spirit
  That could be moved to smile at any thing.
  Such men as he be never at heart's ease
  Whiles they behold a greater than themselves,
  And therefore are they very dangerous.