page_title: Sherlock Holmes
section_title: Holmesian deduction
hierarchical_section_title: Sherlock Holmes / Knowledge and skills / Holmesian deduction
caption_attribution_description: Sherlock Holmes in "The Adventure of the Abbey Grange."
context_page_description: Sherlock Holmes is a fictional private detective created by British author Sir Arthur Conan Doyle. Referring to himself as a "consulting detective" in the stories, Holmes is known for his proficiency with observation, deduction, forensic science, and logical reasoning that borders on the fantastic, which he employs when investigating cases for a wide variety of clients, including Scotland Yard.
  First appearing in print in 1887's A Study in Scarlet, the character's popularity became widespread with the first series of short stories in The Strand Magazine, beginning with "A Scandal in Bohemia" in 1891; additional tales appeared from then until 1927, eventually totalling four novels and 56 short stories. All but one are set in the Victorian or Edwardian eras, between about 1880 and 1914. Most are narrated by the character of Holmes's friend and biographer Dr. John H. Watson, who usually accompanies Holmes during his investigations and often shares quarters with him at the address of 221B Baker Street, London, where many of the stories begin.
  Though not the first fictional detective, Sherlock Holmes is arguably the best known.
context_section_description: Holmes observes the dress and attitude of his clients and suspects, noting skin marks (such as tattoos), contamination (such as ink stains or clay on boots), emotional state, and physical condition in order to deduce their origins and recent history. The style and state of wear of a person's clothes and personal items are also commonly relied on; in the stories Holmes is seen applying his method to items such as walking sticks, pipes, and hats. For example, in "A Scandal in Bohemia", Holmes infers that Watson had got wet lately and had "a most clumsy and careless servant girl". When Watson asks how Holmes knows this, the detective answers:
  It is simplicity itself ... my eyes tell me that on the inside of your left shoe, just where the firelight strikes it, the leather is scored by six almost parallel cuts. Obviously they have been caused by someone who has very carelessly scraped round the edges of the sole in order to remove crusted mud from it. Hence, you see, my double deduction that you had been out in vile weather, and that you had a particularly malignant boot-slitting specimen of the London slavey.
  In the first Holmes story, A Study in Scarlet, Dr. Watson compares Holmes to C. Auguste Dupin, Edgar Allan Poe's fictional detective, who employed a similar methodology. Alluding to an episode in "The Murders in the Rue Morgue", where Dupin determines what his friend is thinking despite their having walked together in silence for a quarter of an hour, Holmes remarks: "That trick of his breaking in on his friend's thoughts with an apropos remark... is really very showy and superficial". Nevertheless, Holmes later performs the same 'trick' on Watson in "The Cardboard Box" and "The Adventure of the Dancing Men".
  Though the stories always refer to Holmes's intellectual detection method as "deduction", he primarily relies on abduction: inferring an explanation for observed details. "From a drop of water", he writes, "a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other". However, Holmes does employ deductive reasoning as well. The detective's guiding principle, as he says in The Sign of Four, is: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth". This type of problematic reasoning has been referred to as the Holmesian fallacy, or the Sherlock Holmes fallacy.
  Despite Holmes's remarkable reasoning abilities, Conan Doyle still paints him as fallible in this regard (this being a central theme of "The Yellow Face").
caption: Sidney Paget illustration of Holmes for "The Adventure of the Abbey Grange"
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