We balance probabilities and choose the most likely. It is the scientific use of the imagination.”

Sherlock Holmes in The Hound of Baskervilles


I have been a lifelong fan of Sherlock Holmes, and logical reasoning is essential in his work just as it is in clinical medicine for clinicians, especially with complex cases. Logical reasoning is typically described in the following two types:

Deductive reasoning is the process of presenting a generalized statement or hypothesis and supporting it with specific scenarios or observations, thus leading to a logical conclusion in a “top-down” approach. The scientific method uses deductive reasoning to test hypotheses and theories. Deductive reasoning has its roots in Aristotles’ syllogisms. In short, deductive reasoning is dependent on rules and facts and has a general-to-specific (theory-to-observations) flow.

Inductive reasoning, also known as “cause-and-effect reasoning”, is the process of using specific scenarios or observations in leading to generalized hypotheses or theories in a “bottom-up” approach. In short, inductive reasoning is dependent on patterns or trends and has a specific-to-general (observations-to-theory) flow.

A third type of reasoning that must be included in reasoning in science and medicine is abductive reasoning (or retroduction). This type of reasoning derives the most likely explanation out of an incomplete set of observations, and is very useful for formulating hypotheses or diagnoses with information that is known. It is, in short, an “educated guess”. It is this reasoning that will be leveraged in the future of AI in medicine.

While Sherlock Holmes is often praised for his unique skill in the science of deduction, some would argue that Holmes uses mainly inductive rather than deductive reasoning since he often takes specific clues to derive at theories. While inductive reasoning is considered to be the opposite of the deductive reasoning, science and medicine (as well as Sherlock Holmes) sometimes have an interplay between inductive and deductive inferences. It is even appropriate (see the aforementioned quote above) to surmise that both Sherlock Holmes as well as astute clinicians use abductive reasoning in their work quite often.

Finally, an interesting factoid is that Sherlock Holmes never proclaimed “Elementary, my dear Watson” in Sir Arthur Conan Doyle’s works even though this phrase is often said in the films!