Not only every great poet, but every genuine, but lesser poet, fulfils once for all some possibility of language, and so leaves one possibility less for his successors.
Good poetry could not have been otherwise written than it is. The first time you hear it, it sounds rather as if copied out of some invisible tablet in the Eternal mind than as if arbitrarily composed by the poet.
What a poem means is as much what it means to others as what it means to the author; and indeed, in the course of time a poet may become merely reader in respect to his own works, forgetting his original meaning.