From The New Dictionary of Cultural Literacy,

No man is an island

No one is self-sufficient; everyone relies on others. This saying comes from a sermon by the seventeenth-century English author John Donne.

A full version of the verse is:

“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…”

— John Donne, from Meditation XVII (1624)

But, well, I actually know this verse from About A Boy [IMDb].

Although I sometimes have a feeling that I am, an island. But I know that I’m not. Nobody does.