Sherlock Holmes

Sherlock Holmes (1854–1957, according to William S. Baring-Gould) is a fictional detective of the late 19th and early 20th centuries, created by British author and physician Sir Arthur Conan Doyle. Holmes is famous for his prowess at using logic and astute observation to solve cases.

Holmesian (or Sherlockian) deduction

"From a drop of water"—Holmes wrote in an essay described in A Study in Scarlet—"a logician could infer the possibility of an Atlantic or a Niagara without having seen or heard of one or the other." Holmes stories often begin with a bravura display of Holmes' talent for "deduction". It is of some interest to logicians and those interested in logic to try to analyse just what Holmes is doing when he performs his deduction. Holmesian (the British adjective; Americans say "Sherlockian") deduction appears to consist primarily of drawing inferences based on either straightforward practical principles—which are the result of careful inductive study, such as Holmes's study of different kinds of cigar ashes—or inference to the best explanation. In many cases, the inference can be modelled either way. In 2002, Holmes was inducted as an honorary fellow of the Royal Society of Chemistry—the only fictional character so honored—in appreciation of the contributions to forensic investigation.

Related Topics:
Atlantic - Niagara - Logic - British - Americans - 2002 - Royal Society of Chemistry - Fictional character

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Holmes's straightforward practical principles are generally of the form, "If p, then q," where "p" is observed evidence and "q" is what the evidence indicates. But there are also, as one may observe in the following example, often some intermediate principles. In "A Scandal in Bohemia", Holmes deduces that Watson had gotten very wet lately and that he had "a most clumsy and careless servant girl". When Watson, in amazement, asks how Holmes knows this, Holmes answers:

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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.

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In this case, we might say Holmes employed several connected principles such as these:

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• If leather on the side of a shoe is scored by several parallel cuts, they were caused by someone who scraped around the edges of the sole in order to remove crusted mud from it.

(Of course they could be used to remove anything from the shoe, or by someone wishing to damage the shoe.)

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• If a nineteenth-century London doctor's shoes are scraped to remove crusted mud, the person who so scrapes them is the doctor's servant girl.
• If someone cuts a shoe while scraping it to remove encrusted mud, that person is clumsy and careless.

(But of course we only gather that mud was scraped off by the information above.)

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• If someone's shoes had encrusted mud on them, that person has been very wet lately and has been out in vile weather.

(Which seems a very bad turn of speech. People would be likely to get wet often! Also the mud need not be placed there recently anyway.)

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By applying such principles in an obvious way (using repeated applications of modus ponens), Holmes is able to infer from

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:p: The sides of Watson's shoes are scored by several parallel cuts.

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to

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:q1: Watson's servant girl is clumsy and careless.

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and

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:q2: Watson has been very wet lately and has been out in vile weather.

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But perhaps Holmes is not giving a proper explanation—after all Holmes may be well aware of Watson's servant girl. As Watson is a doctor and it has been raining, it is likely he has been out in the rain.

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In other instances of Holmesian deduction, it is more difficult to model his inference as deduction using general principles, and logicians and scientists will readily recognize the method used, instead, as an inductive one—in particular, argument to the best explanation, or, in Charles S. Peirce's terminology, abduction. That Holmes should have called this deduction is entirely plausible, however; in several stories, Holmes is said not to have known anything at all of philosophy, although he quotes Thomas Carlyle.

Related Topics:
Inductive - Argument to the best explanation - Charles S. Peirce - Abduction - Philosophy - Thomas Carlyle

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The instances in which Holmes uses deduction tend to be those where he has amassed a large body of evidence, produced a number of possible explanations of that evidence, and then proceeds to find one explanation that is clearly the best at explaining the evidence. For example, in The Sign of Four, a man is found dead in his room, with a ghastly smile on his face, and with no immediately visible cause of death. From a whole body of background information as well as evidence gathered at and around the scene of the crime, Holmes is able to infer that the murderer is—not the various people that Scotland Yard has in custody (each of them being an alternative explanation)—but rather another person entirely. As Holmes says in the story, "How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?" This phrase has entered Western popular culture as a catchphrase. It also turned up in the Dirk Gently stories by Douglas Adams where the detective uses the opposite phrase "because we know very much about what is improbable, but very little about what is possible".

Related Topics:
The Sign of Four - Scotland Yard - Catchphrase - Dirk Gently - Douglas Adams

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In the latter example, in fact, Holmes's solution of the crime depends both on a series of applications of general principles and argument to the best explanation.

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Holmes's success at his brand of deduction, therefore, is due to his mastery of both a huge body of particular knowledge of things like footprints, cigar ashes, and poisons, which he uses to make relatively simple deductive inferences, and the fine art of ordering and weighing different competing explanations of a body of evidence. Holmes is also particularly good at gathering evidence by observation, as well locating and tracking the movements of criminals through the streets of London and environs (in order to produce more evidence)—skills that have little to do with deduction per se, but everything to do with providing the premises for particular Holmesian deductions.

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In the stories by Conan Doyle, Holmes often remarked that his logical conclusions were "elementary", in that he considered them to be simple and obvious. However, the complete phrase "Elementary, my dear Watson" does not appear in any of the 60 Holmes stories written by Doyle. It does appear at the very end of the 1929 film, The Return of Sherlock Holmes, the first Sherlock Holmes sound film, and may owe its familiarity to its use in Edith Meiser's scripts for The New Adventures of Sherlock Holmes radio series.

Related Topics:
Edith Meiser - The New Adventures of Sherlock Holmes

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It should be noted too, that our modern stereotype of police procedure—someone who looks for physical clues, rather than someone who examines opportunity and motive—comes from Holmes.

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Readers of the Sherlock Holmes stories have often been surprised to discover that their author, Conan Doyle, was a fervent believer in paranormal phenomena, and that the logical, sceptical character of Holmes was in opposition to his own in many ways.

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The word "Sherlock" has entered the language to mean a detective or nosy person; it is also commonly used in American slang to mean a knowledgeable person, as in the sarcastic phrase "No shit, Sherlock", uttered when someone says something obvious.

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Man or machine

"So many regard him as a machine rather than a man." Watson describes Holmes a "desiccated calculating machine", "as deficient in human sympathy as he was pre-eminent in intelligence", and states that "all emotions... were abhorrent to his cold, precise, yet admirably balanced mind."

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In the era of Charles Babbage, Holmes may have been written as a human computer. He treats all he finds as data, information to be interpreted, and does not proceed without all the facts. Like a machine, he does not have a social life and he does not seem to eat or even sleep (even when he is ill).

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However, there are complications for this theory. Although a computer could possibly come up with the idea of getting engaged to a woman to gain information from her, it could not come up with a way of doing this (i.e. convince the woman). A computer would not stoop to disguise or acting as Holmes did. In fact if you consider Holmes's deduction principles above, it seems a very skewed logic. His bipolar nature, skill as a musician and composer, and occasional fondness for showmanship also count against this. While "his cold and proud nature was always adverse... public applause" and "turned away with disdain from popular notoriety" but "for an instant he ceased to be a reasoning machine, and betrayed his human love for admiration and applause... from a friend."

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Consider also Prof. John Sutherland's insights into the moral judgements Holmes (possibly) makes in the conclusion to the story 'The Speckled Band'.

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