The Science of Deduction

Lore & Background

Holmes observes the dress and attitude of his clients and suspects, noting skin marks, contamination such as ink stains or clay on boots, emotional state and physical condition, in order to work out their origins and recent history. The style and state of wear of a person's clothes and personal items, such as walking sticks, pipes and hats, are also relied on. Although the stories always call the method deduction, Holmes relies mainly on abduction, inferring an explanation for the details he observes, and he uses deductive reasoning as well.

Reader's Guide

The foundation of Holmes's method is acute observation. He notes details such as contamination on boots and the state of wear of a person's clothes. In 'A Scandal in Bohemia,' this skill allows him to deduce that Dr. Watson has been wet recently and that his servant girl is clumsy based on the state of his boots. Next comes the application of specialized knowledge. Holmes maintains an encyclopedic memory for facts relevant to crime, such as tobacco ash varieties or handwriting analysis. Finally, he employs logical elimination. Once all possible explanations are gathered, Holmes systematically removes those that contradict the observed facts. As he famously states, when one has eliminated the impossible, whatever remains must be the truth, no matter how improbable it seems.

Did You Know?

• Holmes uses tobacco ashes and cigarette butts to identify criminals, and often uses a magnifying glass at the scene.
• Holmes believes the mind has a finite capacity for information, so that learning useless things reduces one's ability to learn useful things.