Donne says that he is diminished by any man's death because we are all one. He makes this clear by using the continent metaphor. If mankind is a continent, then one would think that one death would be like a continent losing a clod of earth: it would not diminish the whole. But, he says, one man's death does diminish the whole just the same as if a large land mass (the promontory) were carried out to sea.
