十姉妹－松さんの おぼえた日記 - 2013年8月31日(土)

我爸爸体重有九十公斤。
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★　帰納法・演繹法

"But Holmes! How could you possibly have figured all that out just from glancing at the lady's hand?"
"Elementary: her wedding band hasn't been polished in years (except for the inside, indicating it is removed often), and the other ring is a common type of 10th anniversary gift."

"I knew you came from Afghanistan. From long habit the train of thoughts ran so swiftly through my mind that I arrived at the conclusion without being conscious of intermediate steps. There were such steps, however. The train of reasoning ran, 'Here is a gentleman of the medical type, but with the air of a military man. Clearly an army doctor, then. He has just come from the tropics, for his face is dark, and that is not the natural tint of his skin, for his wrists are fair. He has undergone hardship and sickness, as his haggard（げっそりした） face says clearly. His left arm has been injured: He holds it in a stiff and unnatural manner. Where in the tropics could an English army doctor have seen much hardship and got his arm wounded? Clearly in Afghanistan.' The whole train of thought did not occupy a second. I then remarked that you came from Afghanistan, and you were astonished."
— The Trope Namer（命名者、指名者）, Sherlock Holmes

A device used to introduce a detective character and his skills. The detective mentions some fact about the person he's just met, something that is not immediately obvious and he has no way of knowing ("Quitting cigarettes appears to have been good for you", "How's the wedding planning going?", "You've holidayed in Italy recently"). The other character looks skeptical or surprised, then the detective describes his reasoning from a set of minor clues (state and style of clothes, marks on skin, tan, etc.) and consequent assumptions. May be involved in a Hannibal Lecture, especially if delivered by The Chessmaster or the Magnificent Bastard.

This is often not connected directly to the main plotline（plot：筋、構想）, but just to show "This is how the detective's mind works, and yes, the detective is That Good." The obvious subversion（転倒、打倒、破壊） is to play this out, then the detective to admit that he'd been told the fact. Or for the other person to insist the detective is utterly wrong.

This is often cited as a demonstration of deductive reasoning (reaching a conclusion via a connected, logical chain of observations) when in fact, this is technically an example of inductive reasoning (reaching a conclusion via a series of generalized observations) and is far less logically sound. (Then again, plenty of people use the term "deductive reasoning" to simply mean "using logic to figure stuff out")

A common parody of this is to have the detective note these details before drawing attention to the blindingly（盲目的に、むやみに） obvious clue.

Compare the Scarily Competent Tracker, who is like a Sherlock Scan done on footprints, and the Batman Cold Open. See also Hyper Awareness and Awesome by Analysis. When this kind of reasoning makes no sense but still works, it's a Bat Deduction. When the above subversion of performing one of these before noting an obvious hint occurs, it's Clue Evidence And A Smoking Gun. If there's a "psychic" bent to the scan, it's Cold Reading.