Inductive vs. Deductive Reasoning
Sherlock Holmes is quite fond of saying something along the lines of "I deduced it." Impressive powers of logic and reason are well known characteristics of the famous detective. His great skill at deduction is mentioned in almost all of the Holmes stories by his friend and assistant Dr. Watson. (Interestingly, the phrase, "Elementary, my dear Watson," never appears in any of Doyle's books, only in the movies.) Although his powers of deduction are legendary, his reasoning skills are very often inductive. So what is the difference?
Deduction and induction will arrive at the same conclusion usually, but the conclusion is arrived at in two very different ways. They are actually considered to be opposite from each other. Induction argues from observation and specific instances. Repeated observations gradually build up to an understanding of how something works. Induction is not a valid method of proof, but does help in the forming of ideas. Using inquiry as an approach to teaching science is done by inductive processes. Inquiry is defined as the process of gathering information and observations to solve problems of interest. Deduction argues from already known laws or principles. If the conclusion is correctly based on already proven facts, then the conclusion must also be true. Deduction is the direct application of prior knowledge to the creation of new knowledge.
The difference between deductive and inductive reasoning in a science class may be illustrated this way. A student drops a piece of granite into a tank of water and observes that it sinks. After doing this several times with various sizes of granite, he will notice that the granite sinks each time and will conclude that granite will always sink in water. This is inductive. Another student may say that granite is denser than water and therefore it will always sink. This is deductive. It is based on a prior knowledge of the density of granite and of water. However, the first student may then conclude that all rocks will sink in water, when, in fact, they do not. Pumice, for example, is less dense than water. So inductive reasoning may not always lead to a valid conclusion. The student with prior knowledge of density would know that rocks would have various densities and would not have drawn the conclusion that all rocks will sink in water.
Now back to Holmes' deduction. As I stated before, Holmes' deductive abilities are typically the focus of interest in those stories. However, much of what he does is inductive reasoning. For example, in "A Scandal in Bohemia," Holmes sets up Irene Adler with the threat of a fire, which is staged. His purpose in doing this is because he believes women will immediately, in event of a fire, go to retrieve their most prized possession. For mothers, it would obviously be the children. In the absence of children, other women would go straight to gather that most valued object before leaving the premises. While Ms. Adler did indeed go to retrieve the object Holmes was in search of, his reasoning would not be a valid method of proof for all women's behavior in the event of a fire. In Holmes' initial observations of new clients, the clients are generally shocked at the information he gains about them based solely on their appearance at that moment. Most of those initial inferences, however, would be inductive in nature, and not deductive. In the cases where Holmes has knowledge of a particular type of soil or a particular type of cloth, his reasoning would be deductive, but that kind of prior scientific knowledge only pertains to a portion of his analysis of people and situations.
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