The philosophy and practicality of deduction and induction

Often Sherlock Holmes would say to Watson, “You know my methods. Apply them.” But what are Sherlock Holmes’ methods?

It is obvious from any casual reading of the stories that he was well read in a variety of topics despite what Watson may have said from time to time, and he used that knowledge to great advantage. He kept meticulous records and references in several forms. Watson mentions his index of biographies in ‘The Adventure of the Empty House,’ the book (later several volumes) in which he daily filed the agony columns (the modern reader might know them as personal classified advertisements) of the daily London newspapers, a set of American Encyclopedias in ‘The Adventure of the Five Orange Pips,’ the Continental Gazetteer in ‘A Scandal in Bohemia.’

In one memorable exposition from ‘A Study in Scarlet,’ he says the following regarding knowledge: “I consider that a man’s brain originally is like a little empty attic, and you have to stock it with such furniture as you choose. A fool takes in all the lumber of every sort that he comes across, so that the knowledge which might be useful to him gets crowded out, or at best is jumbled up with a lot of other things, so that he has a difficulty in laying his hands upon it. Now the skilled workman is very careful indeed as to what he takes into his brain-attic. He will have nothing but the tools which may help him in doing his work, but of these he has a large assortment, and all in the most perfect order. It is a mistake to think that that little room has elastic walls and can distend to any extent. Depend upon it there comes a time when for every addition of knowledge you forget something that you knew before. It is of the highest importance, therefore, not to have useless facts elbowing out the useful ones.”

Holmes’ philosophy came out in several instances. The most common axiom he used to explain his methods is: “When you have eliminated the impossible, whatever remains, however improbable, must be the truth.” It is used in five separate places: twice in ‘The Sign of Four,’ ‘The Adventure of the Beryl Coronet,’ ‘The Adventure of the Bruce-Partington Plans,’ and ‘The Adventure of the Blanched Soldier.’ Another precept that Holmes employed often is this: “It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts.” Or the similar quote, “Data! data! data! I can’t make bricks without clay.” Often, when describing the facts of a case, he would mention one or more cases from his vast memory of crime that were similar to the current one. On one such occasion in ‘A Study in Scarlet,’ Holmes remarks, “There is nothing new under the sun. It has all been done before.”

His ultimate goal is to discover the proper starting point, to grasp the important facts, ignore the unimportant ones, and work out the most likely chain of events. This topic could become a textbook, but I could hardly expect to encompass it in this space. Until that epic tome is published, this brief glimpse into the mind of Sherlock Holmes will have to suffice.

NEXT WEEK: Codes and ciphers

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