Sarah Sands: Terry Pratchett's long voyage into the darkness

The announcement last week by Terry Pratchett that he has been diagnosed with Alzheimer's was reluctant and studiedly light. He said he expected to finish his next novel and to lay down the notes for the following one. Inventing a word to describe his situation "an embuggerance" he anticipated the literary obituaries with a sharp reminder: "I am not dead. I will of course be dead at some future point, as will everybody else." And he joked that his GP had asked him whether he suffered memory loss, to which he had replied: "Not that I can recall."

Pratchett is an extraordinarily prolific writer, averaging two short novels a year. He clearly fears his 55 million readers giving up on him. His talent is to create imaginative journeys in other worlds, and he has shown little interest in this one.

It is an indignity for a writer when others decide the outcome of a plot. Alan Bennett, who revealed in his autobiography that he had cancer, kept it quiet for years because he said he did "not want to die in the pages of a newspaper". Also, he did not see cancer as "a way of dramatising my life": it was a bore, it was not his identity. John Mortimer was similarly irritated when I recently asked him about his prolonged ill-health. "My health is the least interesting thing about me."

What is so devastating for Pratchett is not only that Alzheimer's controls identity, affecting the part of the brain in which memory and personality lie, but, most cruelly, it plays tricks with words. Pratchett's playful command of language will soon become a game of hide and seek, and then incomprehension. The author who conjured fantastic other worlds will enter one too terrible to invent.

Iris Murdoch described it as "sailing into the darkness". Her husband, John Bayley, left her brain, a shrunken sponge of a hippocampus, to medical science. The pathologist said there is one unerring fact about Alzheimer's: "It will win."

For the moment, the blogosphere, a natural environment for Pratchett, is full of good wishes and regret. But his readers have a geeky tendency to follow the characters and plot minutely and know much of his work off by heart. The author may bravely proclaim that it is business as usual, but his next novel, Nation, will be read differently.

Among his audience of children and science-fiction addicts, there will also be medical students. Iris Murdoch's last published book in 1995, Jackson's Dilemma, became a kind of medical textbook, as doctors observed how far the language and thoughts of the author had changed. The words were simpler, the reasoning timid. One reviewer said the formidable English philosopher had produced "the work of a 13-year-old schoolgirl who doesn't get out enough". John Bayley described in his biography Elegy to Iris finding his wife staring at the word "puzzled". Words stopped making sense to her. By the end, she could no longer write or remember any of her 27 novels. The destruction of the beautiful mind was complete. I remember meeting Iris Murdoch with John at a literary party in a London garden square and realising her serenity was actually absence. An author whispered: "It is not her any more."

Everyone knows Terry Pratchett, with his beard and his hats and his chirpy curiosity. He has no illusions about himself. When he was created an OBE for services to literature, he said wittily: "I suspect the services to literature consisted of refraining from trying to write any." He is an atheist, rejecting the comfort of other worlds, much as he loved to create them. He is busy stockpiling for his voyage into the darkness, and one watches his departure with horror and awe.