The Technique of the Mystery Story

[In one of Doyle's stories, Sherlock Holmes himself states definitely his principles of deduction in what purports to be a magazine article written by himself. The opening paragraph, however, is in the words of the faithful Dr. Watson.]

Its somewhat ambitious title was "The Book of Life," and it attempted to show how much an observant man might learn by an accurate and systematic examination of all that came in his way. It struck me as being a remarkable mixture of shrewdness and absurdity. The reasoning was close and intense, but the deductions appeared to me to be far-fetched and exaggerated. The writer claimed by a momentary expression, a twitch of a muscle, or a glance of an eye, to fathom a man's inmost thoughts. Deceit, according to him, was an impossibility in the case of one trained to observation and analysis. His conclusions were as infallible as so many propositions of Euclid. So startling would his results appear to the uninitiated that, until they learned the processes by which be had arrived at them, they might well consider him a necromancer.