<P> In the first Holmes story, A Study in Scarlet, Dr. Watson compares Holmes to C. Auguste Dupin, Edgar Allan Poe's fictional detective, who employed a similar methodology . Alluding to an episode in "The Murders in the Rue Morgue", where Dupin determines what his friend is thinking despite their having walked together in silence for a quarter of an hour, Holmes remarks: "That trick of his breaking in on his friend's thoughts with an apropos remark...is really very showy and superficial". Nevertheless, Holmes later performs the same' trick' on Watson in "The Adventure of the Cardboard Box". </P> <P> This methodology allows Holmes to learn a stranger's occupation and other details . He observes the dress and attitude of his clients and suspects, noting skin marks (such as tattoos), contamination (such as ink stains or clay on boots), emotional state, and physical condition in order to deduce their origins and recent history . The style and state of wear of a person's clothes and personal items are also commonly relied on; in the stories Holmes is seen applying his method to walking sticks, pipes, hats, and other objects . </P> <P> Holmes does employ deductive reasoning as well . The detective's guiding principle, as he says in The Sign of the Four and other stories, is: "When you have eliminated the impossible, whatever remains, however improbable, must be the truth". </P> <P> Despite Holmes' remarkable reasoning abilities, Conan Doyle still paints him as fallible in this regard (this being a central theme of "The Adventure of the Yellow Face"). </P>

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