Unfortunately, “postmodern” is a term bon à tout faire. I have the impression that it is applied today to anything the user of the term happens to like. Further, there seems to be an attempt to make it increasingly retroactive: first it was apparently applied to certain writers or artists active in the last twenty years, then gradually it reached the beginning of the century, then still further back. And this reverse procedure continues; soon the postmodern category will include Homer.
