Speaking at the Queen Elizabeth Hall a few weeks ago, to mark the publication of A Most Wanted Man, John Le Carré, who is 76, spoke touchingly about writing novels in old age.

"I realise there are a whole lot of things I can't do any more and things I can't feel any more. I'm frightened by the example of Graham Greene, who I felt shouldn't have published some of his later stuff. I don't care for it. I don't feel that I must go on delivering novels. I'd like to leave on a decent note." Specifically, he hoped that, if he seemed likely to do otherwise, there'd be a friend behind him wielding a hammer.

Plenty of novelists have persisted into painful diminution. One upsetting example is Evelyn Waugh's last published fiction, Basil Seal Rides Again (1963), which is not only artistically feeble but also a nasty expression of paternal sexual jealousy. Waugh spoke the truth in calling it a "senile attempt to recapture the manner of my youth". What made it specially pitiful was that he had attempted to resurrect the recurrent comic character of his brilliant early novels and failed completely. He was prodding a corpse.

It is a law of aesthetics that only the original novelist himself can re-enter the fictional worlds he has created, to carry the story on and change its outcomes. That's why every supposed sequel or prequel ever written by anybody else - from the innumerable attempts to extend Jane Austen's tediously small oeuvre to the hope of cashing in on the James Bond franchise - is not only illicit but a dud.