🧠 Argument & Fallacy
Telling a sound chain of reasoning from a persuasive-sounding one.
“I never guess. It is a shocking habit — destructive to the logical faculty.”
⚠️ The common misconception A convincing argument and a correct one are different things. A valid argument can lead to a false conclusion, and a true conclusion can rest on rubbish.
Validity versus soundness
Logicians separate two questions people usually blur. An argument is valid if its conclusion follows from its premises — a property of form alone. It is sound only if it is valid and its premises are actually true. "All fish can fly; a trout is a fish; therefore a trout can fly" is perfectly valid and utterly false, because a premise is wrong. Persuasion lives in the gap between the two.
The catalogue of fallacies
Most bad reasoning is not random; it repeats. Attacking the person instead of the claim (ad hominem), offering only two options where more exist (false dilemma), generalising from a handful of cases (hasty generalisation), treating authority as proof (appeal to authority), or sliding from a first step to catastrophe (slippery slope). Naming a fallacy is not point-scoring — it is locating exactly where a chain fails to hold.
Steelmanning before striking
The disciplined reasoner states the opposing argument in its strongest form before answering it. This is not politeness; it is self-defence against fooling yourself, and it is why Holmes so often voices Watson\'s theory in full before dismantling it. You cannot reliably refute what you have not first understood.
How to practise it
Moriarty\'s Web maps any argument for you — the central claim, the premises holding it up, the strength of each connection, and the specific fallacies in the weave, each named, explained, and quoted from the text. It is a mind-map for reasoning, and a fast way to learn to see structure beneath rhetoric.
Practise it
Related techniques
Where Holmes uses it
Concrete moments from the canon — how the method actually plays out in the cases. Contains spoilers.
The reasoning pivot: any scheme this elaborate about a man so unremarkable must be about his premises, not his person — the absurd premise is the misdirection to analyse, not dismiss.
A model of hypothesis discipline failing gracefully: the blackmail theory fit every stated fact and was still wrong — the story exists to show that fitting is not proving.
The salary is priced against the duties and found absurd; as in Wilson's league, Holmes asks what the employer buys, and the answer is the clerk's vacancy, not his work.
The Strand interview of minds: Moriarty states Holmes's moves back to him — 'you crossed my path on the fourth of January' — a duel conducted entirely in mutual inference.
The thumbprint is the fallacy of the gilded lily: evidence that appears after Holmes has already seen the clean wall proves manufacture — a frame betrayed by its own eagerness.
The story argues a hard syllogism: the law protects the blackmailer, the blackmailer destroys the innocent, therefore the just act is illegal — and Holmes reasons himself, and the reader, into agreement.
Holmes reasons at the scale of statecraft — feeding an adversary plausible falsehoods so consistent that acting on them cripples him — deception as strategy rather than trick.
Holmes argues that a man the law cannot reach may still be stopped by exposing his own record — moral reasoning that licenses the theft of the diary.
As with Wilson and Pycroft, the absurd pretext is analysed, not dismissed: an elaborate scheme about a nobody's surname must really concern his premises — the vacancy is the point.