Is It ‘Deduction’ or ‘Induction’, My Dear Watson?

Sherlock Holmes often talks about ‘deductive reasoning’ but was he really using deduction or induction? Although by definition these two approaches appear to be opposites, in practice, the differences between the two can be subtle.

A simplified contrast between deductive and inductive reasoning is that deduction is reasoning from the top down and induction is reasoning from the bottom up. However, the modern definitions of these philosophies have many nuances, which address issues with both of these oversimplified descriptions and blur the lines between the two. Nonetheless, as a natural scientist, I view deduction as the formation of generalized rules that help prediction. Some have argued that deduction does not allow for a conclusion to be false, but that would not be science. It has been my observation that scientists using deductive reasoning make use of exceptions when they are discovered to refine understanding. For example, in this scene, Sherlock over-extends a generalization and learns a lesson that he probably won’t forget:

Inductive reasoning seems less comfortable with prediction but provides better specifics about what is known and what is unknown. In other words, deduction has a problem with ‘not knowing what you don’t know’ whereas induction is more cautious by stating ‘what is known and how well it is known.’ Clearly these two philosophies have their respective strengths and can be used in concert with one another. However, it is interesting how deduction has come in and out of favor over time.

During the Age of Enlightenment (18-19th centuries), deduction was a popular mode of science. Because Sherlock (as well as the books’ author, Sir Arthur Conan Doyle) lived during the later part of this time, it makes sense that this would be the term chosen. Intriguingly, sometime between the 19th century and today, there has been a shift in scientific emphasis from the deductive to inductive approach. I suspect two potential causes for this.

First, in the 19th and early 20th centuries, science got into some hot water by making generalizations about humans. At the time, this wasn’t considered offensive and it was used improperly to support racist philosophies that were prevalent. However, today we do not tolerate racism (rightfully so!), and anything that was associated with racism has fallen out of favor (see environmental determinism).

The second reason for the shift, I believe, is due to the momentum of the ‘quantitative revolution’ in science. This transition in science is less of a revolution than it is often heralded to be. It also took various forms and happened at diverse times in different disciplines. In any case, it was a paradigm shift that put greater emphasis on the quantitative measurement of data and firmly placed a line between ideas that were supported by the data and those that were mostly speculative. Because speculative “arm waving” was fairly common in scientific literature prior to the ‘quantitative revolution’, I believe deduction suffered from its association. In addition, even though deductive approaches did regularly rely on quantitative measurements, the ‘quantitative revolution’ increased the focus on the data (fact collection). I think this is a false shift (deduction can be just as quantitative as induction), but the spirit of this ‘revolution’ brought induction into favor.


So, was Sherlock using deduction? Yes, but his methods would be questionably scientific by today’s standards. Not because of his strategy, but because the generalizations he used to make his conclusions were based on experience (perhaps we could call this anecdotal evidence), as opposed to fully tested experiments using quantitative data to prove (or disprove) the generalization. Although in all fairness, Sherlock did say, “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” (from A Scandal in Bohemia) as well as “Data! Data! Data! I can’t make bricks without clay” (from the Adventure of the Copper Beeches).

Here is how Sherlock described his process, “In solving a problem of this sort, the grand thing is to be able to reason backwards. That is a very useful accomplishment and a very easy one, but people do not practice it much. In the everyday affairs of life, it is more useful to reason forwards, and so the other comes to be neglected…Let me see if I can make it clearer. Most people, if you describe a train of events to them, will tell you what the result will be. They can put those events together in their minds, and argue from them that something will come to pass. There are few people, however, who, if you told them the result, would be able to evolve from their own inner consciousness what the steps were which led up to that result. This power is what I mean when I talk of reasoning backwards, or analytically.” (from A Study in Scarlet)

More quotes from Sherlock Holmes on this topic

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