The whole John Donne meditation the title is referencing:

    No man is an island,
    Entire of itself;
    Every man is a piece of the continent,
    A part of the main.
    
    If a clod be washed away by the sea,
    Europe is the less,
    As well as if a promontory were:
    As well as if a manor of thy friend's
    Or of thine own were.
    
    Any man's death diminishes me,
    Because I am involved in mankind.
    And therefore never send to know for whom the bell tolls;
    It tolls for thee

(had to memorize it in High School, and its one of my favorites. It was written over 400 years ago and has aged quite well.)