The phrase "No man is an island" is a metaphor that conveys the idea that individuals are interconnected and interdependent on each other, and no one can exist in isolation without being influenced by or having an impact on others. Lines 8-9 contribute to the overall meaning of this metaphor by emphasizing the interconnectedness of humanity:
"...Therefore never send to know for whom the bell tolls;
It tolls for thee."
These lines, written by John Donne in his meditation "Meditation XVII," express the notion that when the tolling of a funeral bell is heard, it is not just a signal of the death of an individual; it is a reminder that each person's life is intertwined with the lives of others. The tolling bell serves as a symbol of the interconnectedness of humanity, suggesting that the death of one person affects the whole community or society. In this context, the lines emphasize that no one exists in isolation, and the experiences and fates of individuals are interconnected and shared.
In essence, lines 8-9 underscore the core message of "No man is an island" by highlighting the idea that we are all part of a larger whole, and our actions, experiences, and existence are interconnected with those of others.