Holmesian Deduction

Anyone who has read Arthur Conan Doyle’s Sherlock Holmes stories will be aware and probably in awe of his deductive and logical prowess. In the stories, it reads like something of a super-power, and many people wish or even fantasize about attaining similar skills themselves. Holmes’ talents frequently leave the other characters dumbfounded, and in “The Adventure of the Devil’s Foot” Dr. Leon Sterndale even denounces Holmes as “the devil himself.” This allegation is unfounded, and throughout the stories, we (along with Watson- essentially Holmes’ student) learn that his methods are firmly rooted in the study of logic.
Deductive arguments are one of the two major argument forms covered in the study of logic. A deduction is essentially a conclusion that is drawn from a series of premises. For example, from the facts that “Sherlock Holmes is a great detective” and “great detectives are experts in deduction,” we could deductively infer that “Sherlock Holmes is an expert in deduction.” This is a deductive argument because if the two premises are true, then the conclusion has to be true. The other type of argument is an inductive argument, where the premises provide good grounds for accepting a conclusion, but do not prove it beyond all doubt.
Sherlock Holmes is able to take this basic skill and make it into something spectacular. How he does this is by employing a mixture of deductive and inductive reasoning (which he refers to as “the balance of probability”) to reach a reasonable solution to a problem. Forming deductions in the way Holmes does is the process of deciding what can be reasonable extracted from a given set of facts.
To employ this skill in your life, you first have to think rationally. Imagine that several boxes of stock have disappeared from a shop. Matt, Sarah and Harry are suspects. Matt was fired from his last job for theft, but Sarah and Harry are the only staff members that have keys to the store-room. Matt and Sarah were both working on the day of the disappearance.
From this situation, it is tempting to suspect Matt. He has been in trouble for this precise thing before, and he was working on the day of the theft. You couldn’t, however, validly infer that this is true, because he doesn’t have the means to get into the store-room. For Matt to be guilty there would have to be an explanation of how he got in. To a rational mind, schooled in deductive reasoning, Sarah and Harry are more likely suspects.
Holmes knows you can only reason correctly from evidence, and likewise, when you make your own deductions, you have to focus on proof. In this example, you would search diligently for any further evidence, and then see if your theory still holds up. Deduction isn’t a super-power; it is an attainable skill, if you look to the master.

Posted on Jan. 30th 2012 7:26 PM | by MagicRai | in Deduction, The Art of Observation | 11 Comments »
  • Rocky

    ( sorry for my language )
    You use very common arugment’s, but this does not make this information false. But when you read Sherlock, you wonder how he know to get information from some cluess( on the shirt) How look? or when you find this cluess – you ask yourself “what this all mean?”
    So, think about this and keep this work up, you make great impact on me. 

  • Tazrai

    Thanks for your comments Rocky,
    The Art of Deduction details your questions. A synposis will follow soon. Thanks!

  • harish

    i have one question. I know that the conclusions we draw are based on the observations. But how to observe? I have tried it but people get suspicious when they find me staring at them especially the ladies. Is there a way to observe without the subject knowing

    • Annon

      Face their general direction, but not precisely at them. Look out of the corner of your eye, and learn to take in a lot of information in a small amount of time

    • Binish

      Hello Harish,
      This question of yours is a major problem i think among the starters its i think quite natural.Even i had this same problem .It is suggestive that you temporarily ignore that because once you start improving or once you become habitual in it you will find that it you have eventually eliminated your problem (the problem of people getting suspicious on you )

  • anonymous

    Matt is the one that seems obvious as he has committed a similar crime but Sarah has the key she would be thinking that Matt would be accused making it easy. but if the thief was clever he/she would have done it when they were not on a shift they would go in with the key make an excuse that they had left something in their office or something and steel the goods quickly. if they manage that without getting seen in the building then it makes it even easier as they are removed completely from the picture

  • Tim Symonds

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    Tim Symonds was born in London. He grew up in Somerset, Dorset
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  • D

    Validity is not something that is always apparent. Sarah and Harry are more likely the suspects. But this case lacks clues. Matt while fired for the same offense couldn’t have done it is what most people would think. However, we lack details as to if a key was missing? Matt might have lifted a key from one of them. Harry might have made a key on his day off (or any day for that matter. Premeditation.) Sarah and Harry might have conspiring to take the boxes of Stock, while knowing of Matt’s former offenses, blaming matt in the long run. Not to mention, cameras. Or even time schedules (bathroom breaks, common room, how long it would take to do something(depending on the job)). What might pop into a rational mind, might not always be the truth.

    • D

      Meant to say Sarah and harry are more likely the suspects to a normal, rational mind. Someone who doesn’t follow deeper..

  • Fel

    Actually, it was Matt.

    Unfortunately, the Logician forgot that a simple bump key can be used to pick any lock with no skill involved; and that hundreds of internet videos demonstrate its creation and usage.

    Holmesian logic is quite burdensome and often dismisses possibilities with no rational reason for doing so.

    Remember, if you don’t watch the delivery man and check the inventory as the stock is being delivered, you could easily get a fourth unknown suspect because you were too focused on the employees. The “list of suspects” for the reader to play with, if you will.

    In fact, Matt could be working with Sarah (yet another dismissed possibility). A check on Sarah is unlikely to turn anything up if Matt is the one holding / fencing the merchandise.

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