Sherlock Holmes was based on a real man, Scottish surgeon Joseph Bell, whom Arthur Conan Doyle had served as a clerk in the Edinburgh Royal Infirmary.
Bell was famous for making deductions about his patients. He greeted one by saying, “Ah, I perceive that you are a soldier, a noncommissioned officer, and that you have served in Bermuda.”
When the man acknowledged this, Bell addressed his students. “How did I know that, gentlemen? The matter is simplicity itself. He came into the room without taking his hat off, as he would go into an orderly’s room. He was a soldier. A slight authoritative air, combined with his age, shows that he was a noncommissioned officer. A slight rash on the forehead tells me that he was in Bermuda, and subject to a certain rash known only there.”
On another occasion Bell challenged his students to identify a bitter drug by taste alone. They watched him dip a finger into the tumbler and taste it, and reluctantly followed suit. “Gentlemen,” he said with a laugh, “I am deeply grieved to find that not one of you has developed this power of perception which I so often speak about; for if you had watched me closely, you would have found that while I placed my forefinger in the medicine, it was the middle finger which found its way into my mouth.”