When  a  twelfth-century  youth  fell in  love  he  did  not take  three  paces
 backward, gaze into her eyes, and tell her she was too beautiful to live.  He
 said he would step outside and see about it. And if, when he got out, he met
 a man and  broke his head--the  other man's head, I mean--then that proved
 that  his--the  first  fellow's--girl  was a  pretty  girl.  But if the other fellow
 broke  _his_  head--not his own,  you know,  but the other fellow's--the other
 fellow  to  the  second  fellow,  that  is,  because  of  course the other fellow
 would  only be  the other fellow to him,  not the first  fellow who--well, if he
 broke  his  head,  then _his_ girl--not the other  fellow's,  but the fellow who
 _was_ the-- Look here,  if A broke B's head, then A's  girl  was a  pretty  girl;
 but if B broke A's head, then A's girl wasn't a pretty girl, but B's girl was.

Jerome K. Jerome, "The idle thoughts of an idle fellow",
  cited in M. Kline, "Mathematics in Western culture"