In these particular lines from "In Memory of W. B. Yeats," Auden reminds us that the meaning of a poem is never fixed, that it is subject to myriad interpretations, long after the poet who wrote it has passed away. Yeats may have passed away, but the words of this "dead man" live on, "modified in the guts of the living." In other words, the meaning of Yeats's poetry, like that of all poetry, will be changed by us and subsequent generations.

Time changes, and we change with them. And it's the same with poems. How we interpret poems today is different from how literary critics and scholars of former times interpreted them. What were once dominant schools of thought in English Literature are now completely out of date. By the same token, what is now the prevailing discourse will itself in due course become decidedly passé.

Auden's reference to "the guts of the living" indicates that attributing meaning to poems isn't simply an intellectual exercise. In order for a poem to live on, to continue enjoying cultural relevance, it has to be felt at an emotional level. As emotions are notoriously subjective, it's therefore inevitable that they will give rise to often radically different interpretations of a particular poem.

In that sense, the words of a poem will be "modified in the guts of the living" to such an extent that the dead poet probably wouldn't recognize the new interpretations of his work were he alive to see them.