Sherlock Holmes and Deduction

With all the new hype surrounding the figure of Sherlock Holmes in current media projects, such as the excellent new Mini-series British Television show ‘Sherlock’ by Steven Moffat and Mark Gatiss, or the 2009 movie ‘Sherlock Holmes’, or even the popular medical/comedy show ‘House’, there comes as well an unfortunate misunderstanding about what deduction really is. Sherlock is famously and popularly recognized to be a champion of deduction, with such phrases as “the power of deduction” being anything but alien to his character’s lips. However, what Sherlock Holmes actually does isn’t deductive reasoning at all. It is strictly inductive reasoning! In fact, to be more specific, it is abductive reasoning (which basically means it is the methodological employment of informed guessing).

Deductive reasoning, if done correctly, yields a conclusion which cannot possibly be false while the premises are true. However, it is elementary to think about any number of logically possible worlds in which any one of Holmes’ brilliant inductive guesses happens to be false while all the evidence which pointed him in the direction of his guess remains veridical. For instance, take the following example:

 In “A Scandal in Bohemia” Holmes deduces that Watson had got very wet lately and that he had “a most clumsy and careless servant girl”. When Watson, in amazement, asks how Holmes knows this, Holmes answers:

It is simplicity itself … My eyes tell me that on the inside of your left shoe, just where the firelight strikes it, the leather is scored by six almost parallel cuts. Obviously they have been caused by someone who has very carelessly scraped round the edges of the sole in order to remove crusted mud from it. Hence, you see, my double deduction that you had been out in vile weather, and that you had a particularly malignant boot-slitting specimen of the London slavey.
~ Wikipedia, Sherlock Holmes

However, in some logically possible world, something else accounted for the inside of the left shoe having six almost parallel cuts. For instance, Mr. Watson may have somehow caused those cuts himself, whether intentionally or not. Perhaps Dr. Moriarty did it knowing Holmes would guess it was a slave and anticipating Watson’s delight at being able to correct Sherlock, just to put him off his game. Perhaps Aliens did it, or perhaps Holmes was having a psychotic break and forgot that he himself had arranged to put those cuts on Watson’s shoes. The alternative doesn’t have to be plausible, it just has to be logically possible. Deductive reasoning works differently. In a deductive argument, the conclusion’s truth follows logically and necessarily from the truth of the premises. Thus:

Socrates is a woman
all women are immortal______
Therefore, Socrates is immortal

Is a valid deductive argument. If the premises were true, the conclusion would follow. In other words, in all logically possible worlds in which all the premises were true, the conclusion would also be true. This is rarely (I dare say never) the case with Sherlock Holmes’ reasoning.

Therefore, if you have read this post, spread the word and get it right – Sherlock Holmes, despite what the television tells you, does not in fact use deductive reasoning primarily, or even most of the time, or even much of the time, or even any of the time.

Moreover, as a final thought, it would be nice to see this well recognized correction make its way into some media portrait of Holmes. Some might think that this would take something away from Holmes’ character, but I beg to differ. Consider the 1980’s film “Without a Clue” where Dr. Watson is actually the brilliant detective who has simply used Sherlock Holmes as a cover, allowing Holmes to take all the credit while Watson remains the mastermind (though, of course, Dr. Moriarty knew that Holmes was not the genius the public thought him to be). That movie certainly might be viewed as non-canonical by avid Sherlock Holmes fans, but it is beyond contest that what it proposes about his character does far more damage to the traditional image than this proper correction would.


About tylerjourneaux

I am an aspiring Catholic theologian and philosopher, and I have a keen interest in apologetics. I am creating this blog both in order to practice and improve my writing and memory retention as I publish my thoughts, and in order to give evidence of my ability to understand and communicate thoughts on topics pertinent to Theology, Philosophy, philosophical theology, Catholic (Christian) Apologetics, philosophy of religion and textual criticism.
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5 Responses to Sherlock Holmes and Deduction

  1. “How often have I said to you that when you have eliminated the impossible, whatever remains, however improbable, must be the truth?”

    Sherlock Holmes Quote

    -The Sign of Four

  2. Pete says:

    I’m not sure this is correct, in economics induction is about producing generalisations not hypotheses from accumulated information, deduction is the production of testable hypotheses based on observation, insight or intuition. From these definitions, what Sherlock does is closer deduction rather than induction… Also bear in mind that induction and deduction are not opposites.

  3. Philip Alawonde says:

    There is a difference between how we think to arrive at deductions and strictly logical thought, if that is even humanly possible. It is not, for we are not machines. We are humans, shaped by a lot of other things — experience, culture, etc. — that, together with logical thought, help us AS A COLLECTION in solving our problems. Thus, no human can think purely logically. Even in mathematics, they are human too, and the methods by which they arrive at their conclusions is not always a linear logical path, as most people think, although they are so presented. Indeed, mathematics is highly creative: we conjecture, play around, scribble, seemingly waste lots of time, paper and try different approaches before finally hitting it. And it is when we want to present our argument or proof to others — and here’s the difference — that we structure them in the formal way we have agreed upon, which people see. That does not, by any means, imply that we got our result in the same fashion: it might have been the whole way round, or intricately windy, for all I care (the human thought process is incredibly complex, and cannot be reduced to a few perfect logical links). So, humanly speaking, either Holmes used deductive thinking (as we all do, although in varying degrees) or no one else does (if you take it to mean, purely dedcutive). Thank you.

  4. Pingback: Twitter’s favourite fictional mathematicians | The Aperiodical

  5. Pingback: Sherlock Holmes: rei da dedução ou indução?

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