In the 1870s, medical students at the University of Edinburgh used to crowd in to watch Dr Joseph Bell meet his patients. Bell had a habit of telling a stranger where they had come from, what they did for a living and sometimes where they had recently been, before they had said much at all. One of his students, who worked as his clerk, was a young man called Arthur Conan Doyle. Years later, Doyle wrote to Bell that it was “most certainly to you that I owe Sherlock Holmes”.<\/p>\n\n\n\n
What looked like magic was a method. Bell taught his students to look carefully, to notice small physical details, and to reason from them, always checking the conclusion against the facts. Holmes puts the heart of it in a single line to Watson in A Scandal in Bohemia<\/em>: “You see, but you do not observe.”<\/p>\n\n\n\n I call the method the Sherlock Scan. It has three steps, and each one has a typical mistake.<\/p>\n\n\n\n Observation means collecting facts without interpreting them. Your job is to be a camera, not a critic.<\/p>\n\n\n\n This is the step almost everyone skips, because our minds interpret automatically. We do not see “a desk with three coffee cups, a stack of papers and an open laptop”. We see “a messy desk”. The judgment arrives so fast that it replaces the facts, and once the facts are gone, every later step is built on an opinion.<\/p>\n\n\n\n The mistake: contaminated observation.<\/strong> Words like messy, nervous, lazy, rich<\/em> and angry<\/em> are not observations. They are conclusions.<\/p>\n\n\n\n Exercise: the camera rule.<\/strong> Pick a person, object or scene in front of you. For one minute, describe it, out loud or on paper, using only statements a camera could confirm. No words that carry judgment or emotion.<\/p>\n\n\n\n Instead of “He looks angry”, try “His eyebrows are lowered, his lips are pressed together and his arms are crossed.”<\/p>\n\n\n\n It feels slow and slightly ridiculous. That is the point. You are separating the data from the story.<\/p>\n\n\n\n Inference asks what the facts might mean. An inference is not a wild guess; it is a reasonable possibility that follows from what you observed.<\/p>\n\n\n\n A laptop covered in national-park stickers might mean its owner hikes. A shiny, worn patch on a jacket sleeve and ink on the side of a hand might mean someone who writes a lot by hand.<\/p>\n\n\n\n The mistake: the single story.<\/strong> We tend to grab the first inference that comes to mind and treat it as fact. One plausible explanation crowds out all the others.<\/p>\n\n\n\n Exercise: three stories.<\/strong> Take one clean observation from the camera exercise. Write at least three different inferences, each starting with “This could mean\u2026”<\/p>\n\n\n\n You are not trying to find the right answer yet. You are training your mind to remember that any single fact can carry several meanings.<\/p>\n\n\n\n The final step is choosing the explanation that best accounts for all the facts, not just the convenient ones.<\/p>\n\n\n\n Strictly speaking, Holmes rarely deduces anything in the logician’s sense, where the conclusion must follow from the premises. What he does is closer to what the philosopher Charles Sanders Peirce called abduction<\/em>: reasoning to the most likely explanation, while staying open to being wrong. That last part matters. A good conclusion is a hypothesis you are willing to test.<\/p>\n\n\n\n The mistake: ignoring contradictions.<\/strong> Once we like a story, we quietly drop the facts that do not fit it.<\/p>\n\n\n\n Exercise: build the case, then break it.<\/strong> Look at your observations and your list of inferences. First, build the case: write the single explanation that connects the most facts. Then switch sides and try to break it. Which observation does your explanation not account for? What other story would fit the same facts? If your case survives, hold it with more confidence. If it does not, you have just avoided a mistake.<\/p>\n\n\n\n Here is the full scan on an ordinary office morning.<\/p>\n\n\n\n Observe:<\/strong> A colleague who usually arrives at 9:30 is at their desk at 8:45. They are holding a cup from a caf\u00e9 that opened last week near the station. Their bag is a gym bag, not the usual backpack.<\/p>\n\n\n\n Infer:<\/strong> They could be trying a new route to work. They could have started going to a gym near the station. They could have an early meeting.<\/p>\n\n\n\n Deduce:<\/strong> The explanation that fits all three facts is a new routine: an earlier start, a gym visit and coffee on the way. Now try to break it. An early meeting would explain the time but not the gym bag. The new-routine story holds, for now. If you are curious, the best next step is not to announce your conclusion. It is to ask.<\/p>\n\n\n\n Two cautions. First, practise discreetly and kindly. The goal is to train your own attention, not to make confident claims about strangers. Second, remember that the method is most useful when it is turned on situations where you are already sure. The people, decisions and problems you think you understand are exactly where the camera rule, the three stories and the broken case will surprise you.<\/p>\n\n\n\n Most people stop at the first step, and many skip even that. With practice, the three steps become fast and almost automatic, and ordinary days start to look like what they are: full of information you were not reading.<\/p>\n\n\n\nStep 1: Observe<\/h2>\n\n\n\n
Step 2: Infer<\/h2>\n\n\n\n
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Step 3: Deduce<\/h2>\n\n\n\n
Putting it together<\/h2>\n\n\n\n
A word on using this well<\/h2>\n\n\n\n