There are no beginnings and no ends, no sequences which cannot be reversed, no hierarchy of textual 'levels' to tell you what is more or less significant. All literary texts are woven out of other literary texts, not in the conventional sense that they bear the traces of 'influence' but in the more radical sense that every word, phrase or segment is a reworking of other writings which precede or surround the individual work. There is no such thing as literary 'originality', no such thing as the 'first' literary work: all literature is 'intertextual.' A specific piece of writing thus has no clearly defined boundaries: it spills over constantly into the works clustered around it, generating a hundred different perspectives which dwindle to vanishing point.
Terry Eagleton - Introduction to Literary Theory