"Elementary, my dear Watson"

I recently watched the movie "Enola Holmes 2" with one of our daughters (actually, she fell asleep and missed the ending).  The movie brought back a lot of childhood memories of stories I heard about the world's greatest detective, Sir Arthur Conan Doyle's character Sherlock Holmes.  Except here's the thing, Holmes first appeared in print in the 1887 novel A Study in Scarlet, and since that time the character has been the subject of numerous television shows, movies, cartoons, and numerous other adaptations and derived works outside the traditional Holmes canon (Doyle wrote 4 full-length novels and 56 short stories featuring the great detective).  It's no wonder that what we think we know or heard about Holmes and what actually appeared in the canon can be completely different.  For example, Holmes technically never said "Elementary, my dear Watson" in any of the stories or novels by Doyle (the quote came from one of the movies).    Is this another great example of the so-called "Mandela Effect" (see my post, "I once knew a gold robot with a silver leg").  Maybe or maybe not.

Here's another interesting point.  The Wikipedia article on Sherlock Holmes states that "Holmes was known his proficiency with observation, deduction, forensic science and logical reasoning that borders on the fantastic, which he employs when investigating cases for a wide variety of clients, including Scotland Yard."  But technically, is Holmes really using his powers of deduction here?  I always forget the difference between "induction" and "deduction", but if I am correct, Holmes was brilliant at induction, not deduction.

Deduction moves from an idea (or theory) to observation to conclusion, while induction moves in the opposite direction from observation to idea.  Deduction involves the scientific method in which we use our powers of observation to confirm or refute a hypothesis or theory.  We move from a general framework to the specific observations of what is happening in the real world.  

One common form of deduction is known as a syllogism.  Here is an example.  "A spider has eight legs."  "A black widow is a spider."  Therefore, it follows that "a black widow has eight legs."  We are using the two true statements to deduce, if you will, that a black widow has eight legs.  

Conversely, with induction, we are moving from the specific to the more general.  We make several observations, notice a pattern (hopefully), and then draw our conclusions, often forming a theory about the world around us.  As an example, if we notice that there are fireflies in our backyard every summer, we may conclude that we will see more fireflies in our backyard this summer.  

So back to our brilliant detective.  Let's look at a few of his famous quotes (which actually appear in the canon).  In the novel A Study in Scarlet, Holmes said, "It is a capital mistake to theorize before one has data.  Insensibly one begins to twist facts to suit theories, instead of theories to suit facts."  Here he is talking about induction ("theories to suit facts") rather than deduction ("twist facts to suit theories"), right?  Similarly, in The Hound of the Baskervilles  Holmes argues that "the world is full of obvious things which nobody by any chance ever observes.”  Again, observations lead to theories and not the reverse.  Sherlock Holmes, in other words, used his brilliant powers of induction!