"When you have eliminated the impossible, whatever remains, however improbable, must be the truth." It's one of Sherlock Holmes' most famous sayings - and sprung to mind once again in January, when Orion Books announced that there will be a new, fully authorised book featuring the crime-buster from 212b Baker Street. It is, of course, impossible for the long-dead Sir Arthur Conan Doyle to write a new tale. It's improbable that anyone can truly attempt to recreate the style and atmosphere of the original stories, written over 100 years ago. But writer Anthony Horowitz is going to try.
The Conan Doyle Literary Estate has asked the author of the best-selling Alex Rider children's books - and perhaps more pertinently the screenwriter of television detective capers Hercule Poirot, Midsomer Murders and Foyle's War - to come up with a new novel charting the adventures of Sherlock Holmes and his sidekick Watson. And it seems that, unlike many takes on the Holmes series of late, Horowitz is keen to play it pretty straight.
"My Holmes is going to be exactly the Holmes of the novels without any new information on my part, I don't want to take any liberties with this great iconic figure," he told the BBC. "I do have a certain reservation about reinventions of old famous books, which sometimes have a smack of desperation about them. But I simply couldn't resist this opportunity to write a brand-new adventure for this iconic figure and my aim is to produce a first-rate mystery for a modern audience while remaining absolutely true to the spirit of the original."
It's interesting that Horowitz should call Sherlock Holmes iconic. He's right, of course, but essentially Conan Doyle's creation was a very Victorian detective, carrying a cane and wearing an "ear-flapped travelling cap". And yet, somehow, this man very much of his time has lasted. One of last year's most entertaining new TV events was the new BBC adaptation of Sherlock - which updated the story to the 21st century but saw Benedict Cumberbatch and Martin Freeman on fine form as a more modern Holmes and Watson. It was such a success, a new series is planned for later this year. And which film resurrected Guy Ritchie's increasingly dismal directorial career? Sherlock Holmes. True, it worked because Robert Downey Jr was excellent as Holmes - but the script was not enjoyable enough to encourage the filming of a sequel.
So why has Sherlock Holmes endured? It helps that the characterisation is so good - even beyond the famous double act. Conan Doyle created a villain in Professor Moriarty so sinister and dastardly that he actually kills Holmes. It was a decision Conan Doyle would come to regret - and he somewhat unconvincingly resurrected his most famous character some years later. But although Moriarty only appears in two books, he had an impact on literary baddies that went far beyond his actual participation in the stories. And yet, as co-creator of the recent Sherlock television series Steven Moffat admitted in the Guardian newspaper last year, he became the template for everyone trying to write a super-villain.
Without question, then, it's the characters populating Conan Doyle's creations that has made them last. After all, of the 56 short stories and four novels that make up the original Sherlock Holmes canon, most people have only heard of The Hound of the Baskervilles. But everyone knows that Holmes uses his incredible powers of deduction to solve crimes, that he has an arrogance bordering on egotistical, that he may turn up in an odd disguise so the villain can be fooled, and that he'll hand over the culprit to an increasingly disbelieving Inspector.
And such portrayals have enchanted both fans and literary critics alike. Famous 20th-century poet TS Eliot was a huge devotee of Conan Doyle's work. "It is, of course, the dramatic ability, rather than the pure detective ability, that does it ...," he wrote, reviewing The Complete Sherlock Holmes Short Stories. "The content of the story may be poor; but the form is nearly always perfect."
Perhaps that's why Holmes has been so bendable to the will of adaptations (by some registers he may be the most filmed fictitious character ever) - remaining faithful to the original stories isn't as important as getting the atmospheric world of Holmes right. There will be murky deeds, shouts of murder in fogbound streets, and a mystery to solve - that much we know. The mystery, however, can be new.
And Sherlock Holmes has lasted not only because there have there been new mysteries, but because every fictional detective since has had something of Holmes within them - the knowing intelligence of Agatha Christie's Poirot or the flawed personality of Ian Rankin's Inspector Rebus.
Even the double act of, say, Inspector Morse and Lewis has distinct echoes of Sherlock Holmes and stoical sidekick Dr Watson. In that sense, Holmes' very modernity is probably why he has remained so popular.
Still nostalgia has played a part too - as Conan Doyle biographer Andrew Lycett posited when his book was published in 2007. "People are looking for an era when things were more clear-cut and stable," he said to the BBC at the time. "Holmes seems to epitomise a world where crimes can be solved through the use of reason and observation. Actually, if you read the stories carefully that's often not the case - but it is the perception".
So it will indeed be interesting to see how Horowitz copes with the weight of not just history, but expectation, when his book is published in September. "I fell in love with the Sherlock Holmes stories when I was 16 and I've read them many times since," he said last week. One thing's for sure, it's to be hoped Horowitz doesn't follow Conan Doyle's template too closely - as TS Eliot seemed to suggest all those years ago, it might mean the character is recognisable... but the story isn't all that good. Maybe writing a Sherlock Holmes story isn't so elementary after all.
* Ben East