It is one of those questions that I cannot answer with a proper statement,
it is more a case of "Name a book, and I'll tell you if it's a classic!"
(and that answers nothing!)
I define a classic by the ongoing devotion to it from readers, by the influence it had
on the thinking of the time, either through content or style.
Can a classic be defined by it's publication date? I'm not sure, Hemingway was
Echo's example, another could be George Orwell, who published Animal Farm
& Nineteen Eighty Four after World War Two.
I'm going to be pondering this one now!