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Diagnose & Evaluate · Free
Find the reasoning error, explain why it matters, and repair the argument. The tool checks your argument against 14 common fallacy patterns and asks you to do the actual diagnosis — it surfaces possible matches with transparent evidence, but the judgment is yours.
What it is: Attacking the person making the argument rather than the argument itself.
Pattern: The argument dismisses or undermines a claim by pointing to traits, motives, or character of the speaker rather than the merits of what they said.
Example: We can't trust her economic plan because she's never had a real job. Her ideas don't deserve serious consideration.
Repair: Engage with the actual argument. Even if the speaker has a relevant conflict of interest, name it as a reason to scrutinize the evidence — don't substitute it for engagement.
What it is: Misrepresenting an opponent's position to make it easier to attack.
Pattern: The argument paraphrases the opposing view in an extreme, simplified, or uncharitable form, then refutes that version.
Example: Opponents of the new bike lanes want children to be hit by cars. They'd rather see kids die than give up a parking space.
Repair: State the opposing view in the form your opponent would recognize and accept (the 'principle of charity'). Engage with that version, even if it's harder to refute.
What it is: Presenting only two options when more are available. Forces a binary choice that doesn't actually exist.
Pattern: The argument frames the issue as exactly two mutually exclusive options, ignoring middle-ground or alternative positions.
Example: You're either with us in supporting this bill or you're handing the country to our enemies. There is no middle ground.
Repair: List at least three plausible options. If only two truly exist, justify why no third option is available — don't assume it.
What it is: Drawing a sweeping conclusion from too small or too unrepresentative a sample.
Pattern: The argument moves from one example or a tiny sample to a universal or near-universal claim.
Example: My uncle smoked two packs a day and lived to 95. Clearly the warnings about smoking are exaggerated.
Repair: Either narrow the conclusion to fit the evidence ('this case is unusual' rather than 'the warnings are wrong'), or gather a larger and more representative sample before generalizing.
What it is: Claiming a small step will inevitably lead to a chain of catastrophic consequences without justifying each link.
Pattern: The argument predicts an extreme outcome from an initial action, with each step in the chain assumed rather than supported.
Example: If we allow same-sex marriage, soon people will marry their pets, then their household appliances, and the institution of marriage will collapse.
Repair: If you think the chain is real, justify each link with evidence. If you can't, drop the chain and argue against the actual proposal directly.
What it is: Treating a claim as true because an authority said so, especially when the authority isn't actually expert on the question.
Pattern: The argument's main support is that someone famous, important, or credentialed says the conclusion is right — without engaging with the underlying reasoning.
Example: Dr. Mehmet Oz says this supplement boosts immunity, so it must work.
Repair: Cite the authority as a starting point, then explain the underlying evidence or reasoning that supports the claim. The authority's testimony is suggestive, not conclusive.
What it is: Treating a claim as true because many people believe it (also called ad populum or bandwagon).
Pattern: The argument supports the conclusion by appealing to widespread belief, popular opinion, or majority view rather than evidence.
Example: Everyone knows organic food is healthier. Millions of buyers can't all be wrong.
Repair: Drop the appeal to numbers and look at the evidence directly. If the popular view is right, the underlying evidence will support it on its own.
What it is: Substituting an emotional reaction (fear, pity, anger, indignation) for evidence and reasoning.
Pattern: The argument uses emotionally charged language or scenarios to push a conclusion that isn't actually supported by the facts presented.
Example: Think of the children! Anyone who opposes this curfew is endangering kids' lives. We can't sit by while families suffer.
Repair: Make the emotional concern explicit ('this matters because…') but then offer evidence the policy will actually address it. Emotion can motivate; evidence has to do the inferential work.
What it is: Assuming the conclusion in the premises. The argument is circular — what it's trying to prove is already presupposed.
Pattern: The premise restates the conclusion in different words, or only makes sense if the conclusion is already accepted.
Example: Reading is good for you because reading improves the mind, and improving the mind is what's good for you.
Repair: Find premises that someone who doesn't already accept the conclusion would still find plausible. Independent evidence breaks the circle.
What it is: Diverting attention from the actual issue by introducing an irrelevant topic.
Pattern: The argument shifts focus from the question being debated to a different question that's emotionally or rhetorically related but logically distinct.
Example: Why are we worried about congressional ethics? The real problem is what's happening at the border.
Repair: Stay on the original question. If the new topic is genuinely connected, show the connection explicitly. If it isn't, set it aside.
What it is: Using a key word in two different senses within the same argument so that the inference only works because of the shift.
Pattern: The same word appears in multiple places, but the sense changes between them — one sense supports a premise, a different sense supports the conclusion.
Example: The end of life is its purpose. Death is the end of life. So death is the purpose of life.
Repair: Pick one sense of the ambiguous word and stick with it. If you need both senses, the inference doesn't go through.
What it is: Concluding that A caused B from the fact that A came before B (post hoc ergo propter hoc — 'after this, therefore because of this').
Pattern: The argument infers a causal relationship purely from temporal sequence, without ruling out coincidence, common causes, or reverse causation.
Example: Ever since the new mayor took office, traffic accidents have risen 12%. Clearly her policies are making the city less safe.
Repair: Provide a mechanism (how does A cause B?) and rule out alternatives: confounders, reverse causation, base-rate trends, regression to the mean. Pure correlation isn't causation.
What it is: Drawing a conclusion from an analogy where the two cases differ in ways that matter for the conclusion.
Pattern: The argument relies on a comparison between two cases. The cases share some features, but they differ in features that are actually relevant to the conclusion.
Example: Banning guns is like banning cars — both can kill people, and we don't ban cars, so we shouldn't ban guns.
Repair: Spell out the analogy explicitly: which features are shared, and why those features matter for the conclusion. If the relevant features actually differ, drop the analogy.
What it is: The conclusion does not follow from the premises. The inferential link is broken or missing entirely.
Pattern: Even if every premise is true, the conclusion isn't supported by them. There's no logical bridge from the reasons offered to the claim being made.
Example: I always get sick around exam time. Therefore, exams are bad for public health.
Repair: Either supply the missing premise that bridges the gap, or scale back the conclusion to something the premises can actually support.