Part 2. How Sherlock Holmes Thinks Backwards.
An example of Holmes thinking backwards can be read in A Study of Scarlet where Holmes describes the history of a watch recently acquired by Watson. After looking at the watch for just seconds, not only did Holmes accurately tell how Watson got the watch, Holmes correctly described character of the watch’s former owner, Watson’s eldest brother. Holmes had inferred his conclusions based the watch’s style, age, initials on the back, and that the watch had been recently cleaned. From this he deduced the watch had belonged originally to Watson’s father and was passed along to the oldest son. He then went on to describe the son as untidy and careless who bounced back and forth several times from success to poverty, but eventually took to drink and died. Watson was stunned that Holmes was so right, and accused Holmes of investigating his poor, unhappy brother. Holmes had not done anything of the sort.
Rather, Holmes was a keen observer and from the wear, dents, and scratches to the watch, and from these clues deduced the history of its previous owner. Holmes solved cases by using his knowledge and reasoning — often working backwards — to solve mysteries. He did this by eliminating factors, a process he called the method of exclusion, “By the methods of exclusion, I had arrived at this result, for no other hypotheses would meet the facts. (The Complete Sherlock Holmes, A Study of Scarlet, p. 69)” This statement tells Watson, and us, that facts should not be twisted to match a hypothesis. Throughout the many stories, Holmes reminds Watson of this method. The last reminder came late in Holmes’ career (The Complete Sherlock Holmes, His Last Bow, The Bruce-Partington Plans, p. 886).
After accepting Holmes’ plea that he did not investigate Watson’s brother, Watson accuses Holmes of guessing. An action that Holmes firmly denies. “No, No: I never guess. It is a shocking habit—destructive to logical facility.” (The Complete Sherlock Holmes, The Sign of Four, p. 79)
Like Holmes, doctors do the same thing when conducting the differential diagnosis (DD). Doctors use a process of exclusion and don’t guess. Before dismissing or excluding a cause the doctor must come up with a good and valid reason. The complexity of the DD depends on the chief complaint. Something that seems simple, say a productive cough (coughing up sputum or mucous), can be quite challenging to determine the cause with more than 70 possible diagnoses ranging from a simple cold to pneumonia, to abscesses, to genetic disorders, to even poisonings.
Alternatives and the Devil’s Advocate
Along the same lines as a differential diagnosis is Holmes’ use of alternative reasons or hypotheses. In Black Peter, about a grisly harpoon murder published in 1904, Holmes tells Watson, “One should always look for a possible alternative, and provide against it. It is the first rule of criminal investigation.” (The Complete Sherlock Holmes, Black Peter, p. 536) Holmes is onto something when he talks of an alternative, and his message is very important today, especially for groups. When groups stop considering alternative hypotheses and seek consensus, groups are prone to making bad decisions. This action is called groupthink, a term coined by Irving Janis in the 1960s who studied several major America foreign policy failures ranging from Pearl Harbor to the Vietnam War. Janis found that cohesive groups (actually committees in his study) prevented contradictory views causing the groups to stop critical testing, analyzing, and evaluating ideas.
Groupthink is pervasive and has bungled battles, wars, national economies, and even results in avalanche accidents. To prevent groupthink, groups should follow Holmes’ advice and always seek out alternative hypotheses. Janis also suggested another tactic, borrowed from the Catholic Church, the advocatus diaboli or devil’s advocate.
Established in the sixteenth century and employed for 400 years, the devil’s advocate was a cannon lawyer who argued against the declaration of a person being a saint. The advocatus diaboli looked for holes and mistakes in reasoning, and argued against a consensus. This is a vital member for any group that seeks to make good decisions. A devil’s advocate should propose alternative hypotheses and make sure relevant data is collected. Too often data is missed, dismissed or ignored, and Holmes knew this. [In 1983 the advocatus diaboli position was ended and the duties transferred to the promotor iustitiae.]
Early in A Study of Scarlet (p. 15) Holmes tells Watson, “It is a capital mistake to theorize before you have all the evidence. It biases the judgment.” Holmes does not tend to dismiss clues. He records and remembers data. He gives his reason to Scotland Yard inspector Lestrade as “We may come across something later which will bear upon it.” (The Complete Sherlock Holmes, The Adventures of the Six Napoleons, p. 555) As cunning and cleaver has Holmes appears he is not infallible. Like the rest of us, he too sometimes jumped to conclusions when he relied on incomplete data. Holmes revealed to Watson in The Speckled Band (p. 253), he [Holmes] had “…come to an entirely erroneous conclusion which shows, my dear Watson, how dangerous it always is to reason from insufficient data.”
For both doctors and detective sleuths answers are not found by simply collecting more data. Nor is it about collecting the right data as the “right data” can used selectively to confirm an intended result. But sometimes those results are wrong or have a bad outcome. This is known as a confirmation bias and with avalanches the results can be deadly. By collecting data that reduces uncertainty helps to reduce the affect of confirmation bias.
When it comes to collecting data, Holmes was an expert observer, and that is what we should strive to be too. In The Blanched Soldier, Holmes found himself alone as Watson had gone off “to find a wife”. Instead of Watson’s usual narration, Holmes tells the story and it starts with his meeting of Mr. Dodd. Much in the same way that Holmes had evaluated Watson’s watch, Holmes did the same in evaluating and describing Mr. Dodd who had just set down before Holmes. Mr. Dodd was stunned as Holmes described his history, based just from Dodd’s appearance. Dodd’s stated, “You see everything.” Holmes offered a simple reply, and one that’s important for all decision makers, “I see no more than you, but I have trained myself to notice what I see. (The Complete Sherlock Holmes, The Blanched Soldier, (p. 959). Wise words.
Usually Sherlock Holmes did not work alone, and whether he was with his sidekick Dr. Watson or someone else, Holmes found a great benefit to summarize a situation or case to another person. As Holmes starts his investigation into a missing racehorse and the murder of its trainer, he tells Watson, “At least I have a grip of the essential facts of the case. I shall enumerate them to you, for nothing clears up a case so much as stating it to another person, and I can hardly expect you to co-operate if I do not show you the position from which we start (The Complete Sherlock Holmes, Silver Blaze, p. 313).” Doctors and detectives are taught and seek to perfect their narratives as to the who, what, when, where, how and why of a case. The narrative communicates clearly the situation and is an action that all decision makers should practice. Not only do narratives communicate about a situation and its facts, the narrative also communicates the interpretation of the situation and facts, and that is what influences decisions. A good narrative is critical to making good decisions.
Summary — Part 2
To wrap up Part 2, Holmes thinks backwards. He starts with the problem and seeks the facts. Holmes collects data; data that reduces uncertainty, so he can make inferences to what had happened. A physician follows an identical path starting with the chief complaint and then gathers the patient’s story, physical findings and nonfindings (pertinent negatives). Armed with facts, Holmes formulates a hypothesis and seeks out alternatives too. The physician does the same when employing the differential diagnosis, a reasoned consideration, in order of likelihood, of several maladies from which a patient with these signs and symptoms might be suffering (Doctors Stories, Hunter, 1991, p. 55).” Holmes with several hypotheses to ponder now starts his method of exclusion. The doctor does the same thing by eliminating maladies based on the facts. Both the Holmes and the doctor must rule out competing hypotheses until the most reasonable explanation remains. The challenge is not to twist the data to fit the hypothesis, but rather to twist or reject the hypotheses to fit the data. Both Holmes and the doctor work from effect to cause.
At this point you might be thinking that Holmes’ methodology and the doctor’s diagnosis work well for explaining what has happened, but how can it be applied to deducing what might happen in the future. In Part 3 (next week) I’ll tell you how use this strategy — thinking from effect to cause — with avalanches.
Note: All references to Holmes can be found in The Complete Sherlock Holmes, Sir Arther Conan Doyle, first published in 1924, 1926, 1927, 1930, republished in 2009 by Barnes & Noble, Inc. New York, NY.
Thanks for reading.