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Lessons from Sherlock Holmes: Don't Tangle Two Lines of Thought


Holmes often faults the hapless Watson--and many others who come under his exacting gaze--for a failure to use proper logic. But his admonishments often remain general, noting an overall failure to demonstrate the requisite logical finesse without necessarily taking the time to point out where exactly the reasoner went wrong. After all, Holmes has more pressing concerns than correcting, point by point, the fallacies of others' minds.

The "Adventure of Shoscombe Old Place" marks one of those occasions where the detective moves beyond chiding and begins to instruct Watson on how the thing--that is, reasoning--is to be done. The case revolves around a mysterious change in the relationship between Sir Robert Norberton and his widowed sister, Lady Beatrice Falder, on the eve of an important horse race that could make or break Sir Robert's financial fortunes.

As Holmes and Watson are going over the facts of the case to date, Holmes addresses his companion:

"Let us consider our data. The brother no longer visits the beloved invalid sister. He gives away her favourite dog. Her dog, Watson! Does that suggest nothing to you?"

"Nothing but the brother's spite."

"Well, it might be so. Or--well, there is an alternative. Now to continue our review of the situation from the time that the quarrel, if there is a quarrel, began. The lady keeps her room, alters her habits, is not seen save when she drives out with her maid, refuses to stop at the stables to greet her favourite horse, and apparently takes to drink. That covers the case, does it not?"

"Save for the business in the crypt."

"That is another line of thought. There are two, and I beg you will not tangle them. Line A, which concerns Lady Beatrice, has a vaguely sinister flavor, has it not?"

In his reprimand, Holmes singles out a particular fault of Watson's mind: the desire to bring in elements that, while technically related, are not part of the specific logical chain that is being examined. And this fault, if not corrected, could cause the entire chain to break down.

When we reason, we retrieve the irrelevant alongside the relevant.

Normally, when we reason, our minds have a tendency to grab any information that seems to be related to the topic, in the process retrieving both relevant cues and those that seem somehow to be connected but may not actually matter in this particular case. We may do this because of familiarity, or a sense that we've seen this before or should know something even when we can't quite put our finger on it; spreading activation, or the idea that the activation of one little memory node triggers others, and over time, the triggered memories spread further away from the original, creating linked thoughts that now fill our mind; or simple accident or coincidence--we just happen to think of something while thinking about something else. If, for example, Holmes were to magically emerge from the book and ask us, not Watson, to enumerate the particulars of the case at hand, we'd rummage through our memory (What did I just read? Or was that the other case?), take certain facts out of storage (ok: dog; horse; fight; alcohol; am I missing anything?), and in the process, likely bring up others that may not matter all that much (I think I forgot to eat lunch because I was so caught up in the drama; it's like that time I was reading The Hound of the Baskervilles for the first time, and forgot to eat, and then my head hurt, and I was in bed, and…).

The network of connections differs from person to person. My memories differ from yours which differ from Holmes's which differ from Watson's. So, in the process of rummaging around for relevant facts, each person will inevitably come up with some slightly different answer. But where Holmes understands the importance of sticking to the original line of thought, many a Watson tend to err on the side of inclusion, thinking that more can only be better. Even the kitchen sink couldn't really hurt.

And that is precisely where Holmes steps in to correct the errant reasoner. True, the tendency to include could be useful, bringing to light an overlooked detail, but more likely than not, it will hurt the broader purpose, obscuring the so-called essential by too much focus on the merely incidental--or on something that, while perhaps essential at another time, is incidental at the present moment (think back on The Hound of the Baskervilles: how often did Watson include details that were both irrelevant and misleading in his missives to Holmes? And how often, elsewhere, does Holmes fault Watson for noticing everything except for anything that is actually of use?). When it comes to logic, timing really does matter.

If the tendency to over-activate and over-include isn't checked, the activation can spread far wider than is useful for the purpose at hand - and can even interfere with the proper perspective needed to focus on that purpose. I can't focus on the significance of the dog and the horse if I am simultaneously trying to think about the crypt. The latter distracts from, instead of enhancing, the former. Yes, it is praiseworthy that I can remember all of these details. But I need to learn to divide them in my mind in order to maximize productive reasoning. I have to learn when not to think of them as well as when to bring them in.

Essentially, Holmes's message is one of focus. Learn to concentrate on one thing at a time, to develop a single idea in a sitting; otherwise, you may end up by getting exactly nowhere on any of the myriad ideas floating through your head.

Photo credit: Sherlock Holmes and Dr. Watson. By Sidney Paget (1860 - 1908) (Strand Magazine) [Public domain], via Wikimedia Commons

Previously in this series:

Don’t Just See, Observe: What Sherlock Holmes Can Teach Us About Mindful Decisions

Lessons from Sherlock Holmes: Paying Attention to What Isn’t There

Lessons from Sherlock Holmes: Cultivate What You Know to Optimize How You Decide

Lessons from Sherlock Holmes: Perspective Is Everything, Details Alone Are Nothing

Lessons from Sherlock Holmes: Don’t Underestimate the Importance of Imagination

Lessons from Sherlock Holmes: Confidence Is good; Overconfidence, Not So Much

Lessons from Sherlock Holmes: The Situation Is in the Mindset of the Observer

Lessons from Sherlock Holmes: The Power of Public Opinion

The views expressed are those of the author and are not necessarily those of Scientific American.

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