Today I'm pleased to have a guest blogger, J. Conrad Beech. The author of The Eldamunde Cunningham Mysteries (Gneiss Press http://www.gneisspress.com/eldamundecunningham.htm), Mr. Beech lives near Victoria, BC, Canada.
Sherlock Holmes was skilled in the art of deductive reasoning, but what does this mean?
There are many kinds of logic, but the two most basic forms are inductive and deductive. Inductive logic involves gathering specifics and reasoning to a conclusion. Deductive logic starts with a premise and from that derives the details. Another way of putting it is that inductive logic argues from the particular to the general, whereas deductive logic argues from the general to the particular.
The clearest display of both these approaches, as they apply to detection, is in the story Silver Blaze. A horse trainer is killed and a valuable horse goes missing. The police gather clues and arrest a man. His scarf was found in the dead man’s hand, he was seen hanging around the stables, he tried to give something to the maid, he needed betting information about the horse – all facts showing that he had motive and opportunity for murdering the trainer.
Based on the evidence, the police induce that the man is guilty.
Holmes takes a different approach.
He does not gather evidence to reach a conclusion, but rather uses evidence to support a theory. Holmes takes the facts of the crime and asks, “In order for this set of circumstances to occur, what had to happen?” He forms a premise or hypothesis about how the crime was committed, then compares the evidence to the theory. If the evidence fits, then the theory is correct; if the evidence doesn’t fit, then the theory has to be revised.
Instead of gathering random bits of information and trying to make sense of them, Holmes uses the theory to direct his search and indicate what kind of information is relevant and what to look for. In Holmes’ theory, there must have been a light at the crime scene, so he paws through the mud until he finds a spent match. The police missed it because they were not looking for it, but Holmes knew that, if his theory was correct, it had to be there.
Holmes knows that the police have the wrong man, not because the evidence doesn’t add up, but because he sits back and thinks, “If this man committed the crime, what had to happen?” Given the circumstances, the tout could not have killed the trainer because of, among other things, the mysterious incident of the dog in the night-time. Holmes slots different people into his scenario, including the dead man himself, until he finds one whose actions fit the facts.
What Holmes is actually using is the scientific method, which was developing into its modern form in the late 19th century.
Rather than randomly mixing chemicals together to see what will happen, the scientific method starts with a hypothesis, “If we mix A and B together we will cause reaction C and the result will be D,” and then designs an experiment to test that hypothesis, just as Holmes concocts a theory and then sets out to test it against the evidence and suspects. The Sherlock Holmes mysteries demonstrate the power of the scientific method for extracting truth from seemingly impenetrable circumstances.
In the scientific method, a hypothesis is formed based on prior observation, and this is precisely how Holmes operates. By observing minute details about a person’s clothing and manner, he forms a theory about the person’s life and character, then tests it by presenting his conclusions and having the subject either confirm and deny them.