One of the most popular candidates was Dr. Joseph Bell (1837–1911) a forensic surgeon at Edinburgh Royal Infirmary. Bell emphasized the importance of close scientific observation in making a diagnosis. Its said he would often pick a stranger and, by observing him, deduce his occupation and recent activities to demonstrate this. He is also said to have been involved in several police investigations, mostly in Scotland.
His claim is further strengthened by the fact that Doyle served as his clerk for a time before writing the adventures of Sherlock Homes and is said himself to have credited Bell as a source of inspiration. Although there is scant evidence of the BBC's assertion in the drama documentary 'Murder Rooms: The Dark Beginnings of Sherlock Holmes' that Doyle cast himself as Watson to Bell's Homes.
Today Bell's home is the Japanese Consulate in Edinburgh. Where a bronze plaque erected by the Japan Sherlock Holmes Club celebrates his connection to Conan Doyle and Sherlock Holmes.
Another candidate, also credited by Doyle as a source of inspiration, was Sir Henry Littlejohn (1826–1914). A famous Scottish surgeon, forensic scientist and public health pioneer, who served as as Surgeon of Police and Medical Officer of Health for Edinburgh for almost 50 years. It's said he taught Doyle forensic medicine during his studies at the University of Edinburgh's medical school. He's reputed to have worked closely with Bell on a number of cases and it has been suggested both Bell and Littlejohn were called in to assist during the investigation of the “Jack the Ripper” murders.
In 2013 a fresh candidate for the inspiration behind Sherlock Homes was put forward, John Norman Collie (1859–1942). Collie was a skilled mountaineer, incessant pipe smoker, and confirmed bachelor. A scientist by profession, with a PhD in chemistry and an analytical mind honed by research. His wide ranging interests included everything from Chinese porcelain and fishing to claret and horse racing. He was also fond of Homes trademark tweeds and said to possess a sardonic wit.
On a visit to the Isle of Skye, he formed a life long friendship with local man John MacKenzie and the two men would spend the next 50 years exploring and mapping Skye's famous Cuillin, opening up the prospect of a full unbroken traverse of the Cuillin Ridge, part of which is named after him (Collies Ridge). Today the Cuillin Ridge traverse is considered one of the UK's greatest mountain challenges. Along the way he invented a portable barometer to measure the height of the peaks and had a mountain named after him, Sgurr Thormaid (Norman's Peak).
Having conquered Skye's Cuillin, he narrowly avoided death during the world's first attempt at a Himalayan 8000-metre peak, Nanga Parbat. As well as climbing in the Lake District, Alps and Caucasus. During his later exploration of the Canadian Rockies he notched up no less than twenty-one first ascents and named more than thirty peaks.
In between climbing he also manage to take the worlds first x-ray for diagnosing medical conditions and invent the neon lamp.
He also credited with giving credence to the legend of the Cairngorm's Big Grey Man or Am Fear Liath Mor. When upon hearing hearing footsteps behind him he was overtaken by an unexplained fear and ran five-miles back to civilisation from the summit of Ben Macdui (Britain's second highest mountain), never to return. Although he did return to Skye every year, until his death in 1933, where he is buried alongside John MacKenzie.
While not having the strongest case to being the inspiration for Homes, he is probably the only one of the three who could have taken on Moriarty at the top of the Reichenbach Falls and survived.