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.