| name | fallacy-detector |
| description | Analyze text for logical fallacies using Zakery Kline's framework from Chapter 3 of How to Think. Use when someone says 'check this argument', 'find the fallacies', 'is this reasoning valid', 'analyze this debate', 'what's wrong with this argument', 'logical fallacies', 'is this logically sound', 'audit this essay', 'check my reasoning', or 'fallacy check.' Scans for all 10 named fallacies, quotes the specific passages, and shows how to fix each one. |
Fallacy Detector
Analyze any piece of text — an argument, article, essay, debate transcript, op-ed, social media thread, or speech — for logical fallacies using Zakery Kline's taxonomy from Chapter 3 of How to Think.
When to Use
The user has a specific piece of text they want examined for logical errors. They want to know: Where is the reasoning valid? Where does it break down? And how much of the argument's persuasive force comes from fallacies versus legitimate reasoning?
How to Run This
Ask the user to paste the text they want analyzed. Get the full text if possible, plus context: What is the argument trying to prove? Who is the intended audience? Is this the user's own writing (improvement mode) or someone else's (evaluation mode)?
Then scan the text against all 10 fallacies in order. For each one found, quote the passage, name it, explain why it fails, and show how to fix it. After the scan, identify any reasoning that LOOKS like a fallacy but isn't. Close with the overall health assessment.
The 10 Fallacies
1. Ad Hominem
What it is: Attacking the person making the argument instead of the argument itself. The character, motives, or identity of the speaker are treated as evidence against the claim.
The logical violation: The truth of a claim is independent of who says it. A liar can state facts. A saint can be wrong. Discrediting the messenger does not discredit the message.
What to look for:
- "You only say that because you're a..."
- Questioning motives instead of addressing evidence
- Dismissing a position by labeling the person (elitist, uneducated, biased)
- "Of course THEY would say that"
How to fix it: Strip the speaker from the claim. Restate the argument without attribution and evaluate it on its own merits. If the argument falls apart without the personal attack, it was never an argument — it was name-calling.
2. Straw Man
What it is: Misrepresenting someone's position to make it easier to attack. Instead of engaging the actual argument, you construct a weaker version and demolish that instead.
The logical violation: You have not refuted a position until you have refuted the position as stated by its strongest advocates. Defeating a caricature proves nothing about the original.
What to look for:
- "So what you're really saying is..."
- Extreme restatements that the original arguer would not recognize
- Responding to an implication rather than the stated claim
- Attacking a position nobody actually holds
How to fix it: Restate the opponent's position so accurately that they would say "yes, that's exactly what I mean." Then respond to THAT. This is the steel man test — if you can't pass it, you haven't understood the argument well enough to criticize it.
3. Appeal to Authority
What it is: Accepting or rejecting a claim solely because of who says it. The authority's credentials replace evidence and reasoning.
The logical violation: Authority is not evidence. Expertise makes a claim more likely to be true, but does not make it true. The claim still needs to stand on its own reasoning.
What to look for:
- "Dr. X says it, so it must be true"
- Citing credentials instead of evidence
- "The experts agree" without specifying what evidence convinced them
- Using authority to shut down questioning rather than to inform it
IMPORTANT — The legitimate version: Not all appeals to authority are fallacious. Citing a relevant expert's published, peer-reviewed findings in their area of expertise is legitimate. The fallacy occurs when: (a) the authority is outside their domain, (b) the authority replaces evidence rather than pointing to it, or (c) the appeal is used to end inquiry rather than inform it.
How to fix it: Replace "X says it" with "X's research shows Y, based on evidence Z." The authority becomes a pointer to evidence, not a substitute for it.
4. False Dilemma
What it is: Presenting only two options when more exist. Forces a binary choice on a situation that has a spectrum of possibilities.
The logical violation: Logical alternatives are not limited by rhetorical framing. The existence of a third (or fourth, or fifth) option invalidates the forced choice.
What to look for:
- "You're either with us or against us"
- "We can either do X or accept Y" (when Z is also available)
- "If you don't support A, you must support B"
- Any binary framing on a complex, multi-dimensional issue
How to fix it: Name the missing options. "This is framed as X vs. Y, but options C, D, and E also exist." The false dilemma collapses the moment a third possibility enters the room.
5. Begging the Question
What it is: Assuming the conclusion in the premise. The thing that needs to be proven is smuggled in as a starting assumption.
The logical violation: An argument must derive its conclusion from independent premises. If the conclusion is already embedded in the premises, the reasoning is circular — it proves nothing.
What to look for:
- "Clearly, X is the case, which means X"
- Definitions that embed the conclusion ("Sensible people agree that...")
- Arguments where the premise and conclusion are restatements of each other
- Loaded terms that presuppose the answer ("the obviously correct policy")
How to fix it: Identify the unstated assumption and make it explicit. Then ask: "Is this assumption itself supported by evidence, or is it the very thing we're trying to prove?" If it's the latter, the argument hasn't started yet.
6. Post Hoc Ergo Propter Hoc
What it is: Assuming that because B followed A, A caused B. Temporal sequence is treated as causal proof.
The logical violation: Correlation is not causation. Sequence is not causation. Two things can co-occur or follow each other without one producing the other.
What to look for:
- "Ever since we did X, Y has happened"
- Before/after comparisons without controlling for other variables
- "It worked for me" (personal anecdote treated as causal evidence)
- Policy claims based on timing alone ("We passed the law and crime dropped")
How to fix it: Ask three questions: (1) Is there a plausible mechanism connecting A to B? (2) Have other explanations been ruled out? (3) Does the pattern hold across multiple instances, or just this one? If you can't answer yes to all three, you have correlation at best.
7. Appeal to Popularity
What it is: Claiming something is true because many people believe it. Consensus is treated as evidence.
The logical violation: Truth is not determined by vote. Millions of people can be — and historically have been — wrong about verifiable facts. Popularity measures social agreement, not accuracy.
What to look for:
- "Everyone knows that..."
- "Most people believe..."
- "It must be true — look how many people support it"
- Using poll numbers or follower counts as evidence for a factual claim
- "The majority can't be wrong"
How to fix it: Replace the headcount with evidence. "Many people believe X" becomes "The evidence for X is..." If the only support for a claim is that lots of people accept it, the claim is unsupported.
8. Equivocation
What it is: Shifting the meaning of a key term mid-argument. The same word is used in two different senses, making the argument appear valid when it is not.
The logical violation: A valid argument requires consistent definitions. If term A means one thing in the premise and a different thing in the conclusion, the logical chain is broken even though the words look connected.
What to look for:
- Key terms used loosely ("freedom," "rights," "natural," "fair")
- Arguments that seem to follow logically but feel slippery
- Conclusions that only work if you blur the definition of a central concept
- Switching between technical and colloquial meanings
How to fix it: Pin down the definition. "When you say 'freedom' in premise 1, do you mean X or Y? Because in your conclusion, you're using it to mean Z. Those are different claims."
9. Slippery Slope
What it is: Claiming that one action will inevitably lead to extreme consequences without providing evidence for each step in the chain.
The logical violation: Each link in a causal chain requires its own evidence. Asserting A leads to B leads to C leads to catastrophe does not prove it — it imagines it. The burden of proof applies to every step, not just the first.
What to look for:
- "If we allow X, next thing you know..."
- Chain reactions presented as inevitable without evidence for any link
- Extreme endpoints used to discredit modest proposals
- "Where does it end?" (as if the answer is always "the worst possible place")
IMPORTANT — The legitimate version: Some slopes really are slippery. If there is documented evidence for each causal step — historical precedent, institutional mechanisms, measurable feedback loops — then the argument is not a fallacy but a prediction supported by evidence. The fallacy is in asserting inevitability without that evidence.
How to fix it: Break the chain into individual claims. "You say A leads to B. What's the evidence? You say B leads to C. What's the evidence?" If any link is speculative, the chain breaks there.
10. Appeal to Ignorance
What it is: Claiming something is true because it has not been proven false, or false because it has not been proven true. Absence of evidence is treated as evidence.
The logical violation: The inability to disprove a claim does not constitute proof of it. The burden of proof rests with the person making the positive claim. "You can't prove I'm wrong" is not an argument — it's an admission that you have no evidence.
What to look for:
- "You can't prove it's NOT true"
- "There's no evidence against it, so it must be right"
- "Nobody has been able to disprove..."
- Treating "we don't know" as support for a specific claim rather than as genuine uncertainty
How to fix it: Restate the actual epistemic state. "We don't have evidence for or against X" is an honest position. "Therefore X is true" is the fallacy. Separate uncertainty from conclusion.
Phase 1: The Scan
Read the full text. For each fallacy found, produce an entry:
FALLACY: [Name]
PASSAGE: "[Exact quote from the text]"
DIAGNOSIS: [One-sentence explanation of why this specific passage commits this specific fallacy]
THE LOGICAL BREAK: [What principle of valid reasoning does it violate?]
THE FIX: [How could the author reformulate this passage to make the same point without the fallacy? Provide a concrete rewrite.]
If a fallacy appears multiple times, log each instance separately. If a fallacy is not present, skip it — do not force findings.
Phase 2: The False Positives
After the scan, review any reasoning that RESEMBLES a fallacy but is actually valid:
- Legitimate authority citations — referencing a domain expert's published findings is not an appeal to authority fallacy
- Evidence-based causal chains — a well-documented slippery slope with evidence at each step is not a slippery slope fallacy
- Real dilemmas — some situations genuinely have only two options; calling it a false dilemma requires showing the missing third option
- Strong consensus — when scientific consensus rests on independently replicated evidence, citing it is not an appeal to popularity
For each false positive identified, explain why the reasoning is valid despite surface resemblance to a fallacy.
Phase 3: The Reasoning Health Score
Deliver an overall assessment:
REASONING HEALTH REPORT
TOTAL FALLACIES FOUND: [N]
TOTAL FALSE POSITIVES (valid reasoning that looks like fallacy): [N]
FALLACY DENSITY: [Low / Moderate / High / Severe]
- Low: 0-1 fallacies, argument stands on evidence and logic
- Moderate: 2-3 fallacies, core argument may be sound but presentation is weakened
- High: 4-6 fallacies, significant portions of the argument rest on invalid reasoning
- Severe: 7+, the argument's persuasive force is primarily fallacious
STRONGEST REASONING: [Quote the best-reasoned passage and explain why it works]
WEAKEST REASONING: [Quote the most fallacious passage and explain why it fails]
OVERALL ASSESSMENT: [One paragraph. How much of this argument's persuasive force comes from fallacies vs. legitimate reasoning? If you removed every fallacious passage, would the argument still stand?]
What This Is NOT
This is a logical analysis tool, not a fact-checker or a political position. A fallacy-free argument can still have false premises. A fallacious argument can still reach a true conclusion (by accident). This skill evaluates the STRUCTURE of reasoning — whether conclusions follow from premises through valid logical steps. It does not evaluate whether the premises themselves are true. That is a different question requiring different tools.
Based on Chapter 3 of How to Think by Zakery Kline.
