Book Excerpt: Reasoning Backwards
Written by Dr. Thomas W. Young   

EVEN IF HE WASN’T THE FIRST FICTION DETECTIVE ever portrayed, Sherlock Holmes is without a doubt the most enduring and influential fictional detective of all time.

In 2012, Guinness World Records announced that Holmes has the world record as the most portrayed literary human character in film and television. He has been on the screen 254 times (Dracula beat Holmes with 272 films, but Dracula is a non-human character). Since 1887, more than 75 actors have depicted Sherlock Holmes. The Guinness organization recognizes Sherlock Holmes as a “literary institution”: someone who is just as “compelling” today as he was 125 years before today.

Holmes’ fame extends throughout the entire world. He is very popular in China. The people there viewed the first episode of the third season of the British Broadcasting Corporation’s production of Sherlock over seven million times—somewhat comparable to the ten million times it was viewed in the United Kingdom on prime time. When British Prime Minister David Cameron visited China, Chinese subscribers on a social media site peppered him with questions about the television program.

Sherlock Holmes is as popular as ever everywhere you may go.

Some even look at him wistfully in this modern age, wishing that modern forms of forensic evidence like DNA had not made irrelevant the kind of Sherlock-Holmes-style deduction described in short stories and novels. According to one reporter from the United Kingdom, it is the “size of the DNA database” and not Holmes-like reasoning that puts criminals behind bars these days. This reporter does not understand the power that Sherlock Holmes and his analytical methods have on modern-day forensic doctors and scientists.

Sherlock Holmes is the invention of author Sir Arthur Conan Doyle (1859–1930).

Many authors of detective fiction often use Conan Doyle’s general ideas. These authors depict brilliant detectives—people who are smarter than rank-and-file police officers—who solve mysteries through intuition and deduction. The fictional heroes are peculiar people with both serious personal problems and endearing quirks, who wander about looking for “clues.” At the conclusion of each tale, the brilliance of each detective catches everyone off guard when he or she weaves the clues into an amazing account. The “bad guy” typically acknowledges the truthfulness of that account by confessing at the end of the story.

Sherlock Holmes had as his sidekick a medical doctor, the famous Dr. Watson. At the conclusion of one Sherlock Holmes novel, A Study in Scarlet (1888), Holmes explains how he is able to solve his cases (Holmes talks and Watson responds):

In solving a problem of this sort, the grand thing is to be able to reason backwards. That is a very useful accomplishment, and a very easy one, but people do not practise it much. In the every-day affairs of life it is more useful to reason forwards, and so the other comes to be neglected. There are fifty who can reason synthetically for one who can reason analytically.

“I confess,” said I, “that I do not quite follow you.”

I hardly expected that you would. Let me see if I can make it clearer. Most people, if you describe a train of events to them, will tell you what the result would be. They can put those events together in their minds, and argue from them that something will come to pass. There are few people, however, who, if you told them a result, would be able to evolve from their own inner consciousness what the steps were which led up to that result. This power is what I mean when I talk of reasoning backwards, or analytically.

“I understand,” said I.

Holmes describes what a person does when he reasons forward: by learning the train of events and knowing (or predicting) what the result of those events would be. He describes this as something that most people do.

Rare and brilliant individuals like Sherlock Holmes, however, solve cases by reasoning backward: by learning the result and, through intuition, describing the multiple steps in the past that led to the result. When a person learns the events of the past first and then describes the result, the reasoning moves forward. When a person learns the result first and then says what led to the result, the reasoning moves backward.

It seems plausible to the casual reader that if there is someone brilliant enough like Sherlock Holmes, that person could solve crimes with the same method of reasoning backwards. The reader is tricked into thinking that backward reasoning is brilliant, but it is not. Backward reasoning does not work. Those described by Sherlock Holmes as “most people” do not do it because it does not work.

If I had often used the Sherlock Holmes’ method when I was employed as a medical examiner, I would not have been able to keep my job for as long as I did. Citizens and civic leaders do not appreciate death investigators and police officers who use methods that do not work. The failure of backward reasoning in a case becomes painfully apparent as more is learned about the case.

There is a simple explanation why backward reasoning does not work. For any result, any set of clues, there may be numerous possible “trains of events” that could explain the result. Sherlock Holmes and Conan Doyle may imagine only one “train of events” and may persuade the reader that there is only one possible “train of events,” so the reader never considers that there are other possibilities or even imagines them.

Consider an example of backward reasoning that took place earlier in A Study in Scarlet. Early in the story, Dr. Watson is introduced to Sherlock Holmes. Holmes looks at Watson and draws the following conclusions about him:

Here is a gentleman of a medical type, but with the air of a military man. Clearly an army doctor, then. He has just come from the tropics, for his face is dark, and that is not the natural tint of his skin, for his wrists are fair. He has undergone hardship and sickness, as his haggard face says clearly. His left arm has been injured. He holds it in a stiff and unnatural manner. Where in the tropics could an English army doctor have seen much hardship and got his arm wounded? Clearly in Afghanistan.

Of course, Dr. Watson is amazed by how accurately Holmes determined Watson’s past from simple but subtle observations. Readers of Sherlock Holmes’ novels are also amazed at the brilliance of the super sleuth, but neither Watson nor the reader realize that there are numerous other possible explanations for these observations.

Is the military the only place where one can develop what seems to be “the air of a military man?” Why does Watson have to be an army doctor? Why not a navy doctor?

Are the tropics the only place where one can develop a tan? Is Afghanistan the only country in the “tropics?” Who says that hardship and sickness from military service is the only explanation for a haggard face? Aren’t there other places and other ways besides a military conflict where one can wound his left arm?

To the reader, the Afghanistan war is made to seem like the best explanation for all of the findings, but how would we know that it truly is the best explanation?

There is an answer to that question. What if we as detectives were to ask Dr. Watson the following: “Dr. Watson, tell us about yourself. What have you been doing all of these years? Where have you gone? What have you gone through?”

Perhaps Dr. Watson himself could independently and spontaneously provide an explanation for the military bearing, the tan, the haggard face, and the injured arm through his answers. If he freely told his story and if that story happened to explain all of the observations made by Sherlock Holmes, wouldn’t that then be the best explanation?

Of course, to do that would be reasoning forward—learning the “train of events” and seeing if those events could explain the result. Asking Dr. Watson would be straightforward, but Sherlock Holmes wants to work in a less straightforward, less certain way. Why is that considered brilliant?

Here is another way to understand this concept, all using the fact that

2 + 2 = 4

Consider the numbers to the left of the equal sign to be a “train of events”—where one event is added to another event in a series of events. I use only two “events” in this illustration—two added to two—to keep things simple. Now consider the number to the right of the equal sign to be the result of what happened on the left side of the equal sign. Using forward reasoning, we would learn what came before (two being added to two) and see if the result (four) makes sense with what came before. In other words, using Dr. Watson’s freely and spontaneously offered story, we would see if his “train of events” explains the clues in Dr. Watson’s body. What he said could explain the result…

2 + 2 = 4

…or it might not explain the result:

2 + 3 ≠ 4

The facts from Dr. Watson might “add up” or they might not “add up.” With this forward approach, the detective learns all available facts. These facts are gathered both from eyewitnesses and observable results of the past events. The detective will then see if everything “adds up.” If everything does not add up, there is a red flag; more investigation is called for and more non-leading questions need to be asked.

On the other hand, the backward reasoning approach attempts to tell the past events from the result. Witness accounts are ignored with backward reasoning. Instead, an arrogant and brilliant detective tells the witnesses what they should have supposedly seen. The detective is the one who “fills in the blanks”:

__ + __ = 4

The arrogant and brilliant detective may fill in the blanks with 2 and 2, figuring that to be the best explanation, but in truth, combinations of numbers too numerous to count could be placed into the blanks: 1 and 3, 4 and 0, negative 1 and positive 5, 1.999 and 2.001, etc.

Two and two cannot be considered the best possibility among millions of possibilities. Selecting the best explanation that way would be like correctly guessing the right numbers in a combination lock or the correct order of letters and numbers in a computer password!

Reasoning backward from a result to a complex train of events that is beyond what a person could imagine is not brilliant. It is just the opposite. It is simply guessing with no hope of learning the correct answer.

Perhaps you are a person who does not like mathematics. Maybe you would prefer to think about white beans.

Charles Sanders Peirce (1839–1914) was a logician, mathematician, philosopher, and scientist who lived at much the same time as Arthur Conan Doyle. He offered an illustration of a bag that contained only white beans [“Illustrations of the logic of science. Sixth paper—Deduction, induction, and hypothesis”, Popular Science Monthly 1878;13:470–482]. If you reached into that bag and grabbed several of those beans, you could reasonably and certainly conclude that when you opened your hand, you would see white beans. He called this a deduction. A deduction is a conclusion that is guaranteed to be true as long as the other items are true: the bag contains only white beans and you grabbed the beans from that bag. On the other hand, a hypothesis is seeing someone holding white beans in his hand and concluding that he got the beans from the bag of white beans. A hypothesis is a guess requiring further proof. The beans may have come from that bag, but they may have also come from somewhere else: from another bag in another room or from the grocer down the street or from who knows where.

You may have noticed that with the deduction above, the train of events leading to the result is known and witnessed. On the other hand, a hypothesis involves trying to guess or surmise the past events that led to the result without knowing or considering what was witnessed. If the hypothesis is a guess requiring further proof, what would be that proof? Asking the person with the beans in his hand what he did, of course. If he said he got the beans from the special bag with only white beans, you could then conclude that what he said could have happened. His account is consistent with the result. That would be a deduction.

But if his hand contained only black beans and he said that he grabbed them from the special bag with only white beans, then what he said could not have happened. His account is not consistent with the result. That would also be a deduction.

Back in 2009, I wrote a statement that I believe sums up what I have written above. I call it the Inferential Test:

One can be reasonably certain if witness accounts of the past are consistent or not consistent with physical evidence in the present, but one cannot reliably surmise past events from physical evidence unless there is only one plausible explanation for that evidence.

I know that sounds wordy and it seems hard to understand at first. What would be a simpler way to say it? How about this: You can listen to an eyewitness with an open mind and see if what he says fits the clues, but you cannot make up a story from the clues and expect it to be true.

“Clues” are what police officers search for at crime scenes or items doctors look for when they examine patients or dead bodies. It is a word I borrowed from detective fiction to illustrate. When a medical examiner looks for clues in dead bodies, it is important for this forensic doctor not to invent a scenario to explain those clues. The doctor should listen to eyewitness accounts of what happened in the past and compare those accounts to the clues he found. Doctors who build scenarios before carefully listening with an open mind to eyewitnesses cause innocent people to be sent to prison.

I know what I have written so far is simple and it seems obvious. It seems obvious that weaving a story around clues while failing to listen to eyewitnesses, the people who were actually there to see what happened, would be asking for trouble.

But what would happen if someone who was seemingly brilliant and had impressive credentials—someone like Sherlock Holmes—were to get up on a witness stand in a courtroom and engage in Sherlock Holmes-like behavior, weaving stories from clues? What if everybody on the jury believed that impressive expert? What if people were sent to prison because of that kind of testimony? Who could stop him? What if nobody was willing to stop him? What if he were allowed to go on, year after year, doing this? What if people actually confessed to doing what he stated in his invented theory? Does this actually happen?

Sadly enough, this actually happens. Many forensic doctors behave this way all the time, everyday; and attorneys, jurors, judges, and the public buy into it. It is a great tragedy with no end in sight.

About the Author

Dr. Thomas W. Young, a forensic pathologist and full-time forensic doctor for nearly 30 years, has testified in court over 460 times both as a prosecution and defense expert. He is a fellow of the American Academy of Forensic Sciences and the National Association of Medical Examiners. He has written extensively and been published in peer-reviewed journals. He served as a chief medical examiner successfully for nearly 12 years and is a former director of a training program in forensic pathology. As a forensic practitioner, what makes him uniquely qualified to write this book are not only his credentials, but also his research on real-world trials and case studies, his examination of current practices, and his close study of deductive and inductive logic. Thanks to his years as a chief medical examiner and as a current independent forensic-pathology consultant, Young has a deep understanding of the problem about which he writes.

This article appeared in the Summer 2018 issue of Evidence Technology Magazine.

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