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.