Bad arguments do not announce themselves. They arrive sounding sensible, often from people who are confident and articulate, and they exploit habits of thought that usually serve us perfectly well. A logical fallacy is the name for the specific way such an argument goes wrong, and having names for them turns a vague feeling that something is off into a precise diagnosis.
Philosophers divide them into two families. Formal fallacies are structural: the argument’s shape is invalid, so the conclusion does not follow no matter how true the premises are. Informal fallacies are failures of content: irrelevant appeals, misrepresented positions, loaded language, missing evidence. Almost every fallacy you meet in an actual disagreement is informal, because real arguments rarely fail on pure logic; they fail on relevance and honesty.
The study is old. Aristotle’s Sophistical Refutations describes thirteen ways an argument can merely look like a refutation, split between those that depend on language and those that do not. Medieval logicians took the subject up again, which is why so many fallacies still carry Latin names. No list is final: the Stanford Encyclopedia of Philosophy notes that the definitions of individual fallacies are often a matter of contention. The groups below follow the common textbook split into relevance, ambiguity and presumption, with causal, statistical and formal fallacies given groups of their own.
Every entry has the same three parts: what the fallacy does, an example (invented for this page), and how to answer it. Where the Atlas or the dictionary has a fuller page, the last column links to it.
Group 01 · 13 fallaciesFallacies of relevance: the argument changes the subject.
These swap the question for something else: the speaker, their friends, the crowd, a feeling. The reply may be true and still have nothing to do with whether the claim is right.
FallacyWhat it does · example · how to answer itGo deeper
Ad hominem“To the person”Attacks the person making an argument and treats that as if it defeated the argument.Example“You can’t trust her case for a four-day week. She’s never run a business.”Answer itAsk which step of the argument the remark actually touches. If none, the argument is still standing.Ad hominem, explained →
Tu quoque“You too” · a kind of ad hominemDismisses an argument because the person making it doesn’t follow it themselves.Example“My doctor told me to cut down on sugar, but she eats biscuits all day, so I’ll ignore her.”Answer itHypocrisy is a fact about the speaker. The advice is right or wrong on its own evidence.
Guilt by associationAlso: association fallacyRejects a view, or a person, because of the company they keep or because someone disliked holds the same view.Example“That diet must be nonsense. A celebrity I can’t stand swears by it.”Answer itAsk what the view’s own evidence is. People you dislike hold some true beliefs, and people you admire hold some false ones.Godwin’s law · dictionary →
Poisoning the wellA pre-emptive ad hominemDiscredits someone in advance, before they have spoken, so whatever they say is dismissed.Example“Before you hear from the landlord, remember that people like him will say anything to keep rents up.”Answer itAgree to judge the testimony on its content once it has been given.
Genetic fallacyJudging by originJudges a claim by where it came from rather than by whether it is true.Example“The idea for the new bus timetable came from a pub conversation, so it can’t be any good.”Answer itA doubtful origin is a reason to check a claim, not a reason to reject it. Ask about the merits.
Straw manAlso: straw-man argumentSwaps the other side’s real position for a weaker, distorted one, and attacks that instead.Example“You want a 20 mph limit outside the school? So you’d ban cars altogether.”Answer itRestate what you actually said, then ask them to answer that version.Straw man, explained →Straw man · dictionary →
Red herringAlso: irrelevant conclusionBrings in an irrelevant point that pulls attention away from the question at issue.ExampleAsked about the overspend on the new bridge, the councillor talks about how much he loves the town’s history.Answer itAcknowledge the point, then return to the question: “That may be so. The question was the overspend.”Red herring · dictionary →
Appeal to authorityLatin: ad verecundiamTreats someone’s say-so as proof when they are not an expert on this question, or when the experts disagree.Example“A famous physicist says this supplement sharpens your memory, so it must work.”Answer itAsk whether the authority is an expert in this specific field, and what that field broadly agrees on. Deferring to real expertise is usually good sense, not a fallacy.Appeal to authority, explained →
Appeal to popularityLatin: ad populum · also: bandwagonClaims something is true because lots of people believe it, or false because few do.Example“Millions of people use this horoscope app. There must be something in it.”Answer itPopularity shows that a belief is common, not that it is correct. Ask what evidence convinced all those people.Bandwagon effect · dictionary →
Appeal to emotionIncludes appeals to fear and to pityAsks you to accept a claim because of the feelings it stirs (fear, pity, pride, outrage) rather than because of reasons.Example“Think how let down your family will feel if you don’t buy the extended warranty.”Answer itAcknowledge the feeling, then ask what reason there is apart from it.
Appeal to traditionAlso: traditional wisdomArgues that something is right because it has always been done that way.Example“We’ve always marked exams by hand, so there’s no need to look at anything else.”Answer itAsk why the tradition began, and whether those reasons still hold.
Appeal to consequencesLatin: ad consequentiamArgues that a claim is false because believing it would be unpleasant, or true because it would be nice.Example“They can’t be cutting the bus route. The whole village would be stranded.”Answer itSeparate “is it true?” from “would it be bad?”. Both questions matter, but they are different questions.
Two wrongs make a rightA kind of ad hominemDefends a wrong by pointing to someone else’s wrong.Example“He parked in my space last week, so I’m allowed to take his today.”Answer itAsk whether the act would be acceptable if the first wrong had never happened.
Group 02 · 4 fallaciesFallacies of ambiguity: the words do the cheating.
Here the reasoning only works because a word or a sentence quietly changes meaning halfway through.
FallacyWhat it does · example · how to answer itGo deeper
EquivocationOne word, two meaningsUses the same word in two different senses within one argument.Example“The sign says ‘fine for parking here’, so parking here must be fine.”Answer itPin down what the slippery word means, and check that the meaning holds steady from start to finish.
AmphibolyAmbiguous grammarRelies on a sentence whose grammar allows two readings.Example“The minister promised on Monday to resign.” Was the promise made on Monday, or is the resignation due on Monday?Answer itAsk which reading is meant, and rewrite the sentence so that only one is possible.
CompositionFrom parts to the wholeAssumes that what is true of the parts must be true of the whole.Example“Every player in this squad is a star, so it will be a great team.”Answer itAsk whether the property really carries over. Wholes have qualities their parts lack, and the reverse.
DivisionFrom the whole to the partsAssumes that what is true of the whole must be true of each part.Example“It’s a rich country, so everyone who lives there must be well off.”Answer itAsk for evidence about the particular case, not the average.
Group 03 · 10 fallaciesFallacies of presumption: the conclusion is smuggled in.
These start from an assumption that has not been earned: a question that presumes its answer, a menu with options missing, a rule that conveniently excludes the counter-example.
FallacyWhat it does · example · how to answer itGo deeper
Circular reasoningAlso: begging the question · petitio principiiUses the conclusion, or something that already assumes it, as one of its premises.Example“This news site is trustworthy, because it says it only reports the truth.”Answer itAsk for a reason that someone who doubted the conclusion would accept.Circular reasoning, explained →
False dilemmaAlso: false dichotomy · either/orPresents two options as the only ones when others exist.Example“Either we close the library or we raise council tax.”Answer itName a third option. One is enough to break the frame.False dilemma, explained →
Loaded questionAlso: complex questionAsks a question that smuggles in an assumption, so any direct answer accepts it.Example“Have you stopped wasting money on that gym membership?”Answer itReject the assumption before answering: “I don’t accept that it’s a waste.”
No true ScotsmanAn ad hoc rescueProtects a generalisation from a counter-example by redefining the group so the example no longer counts.Example“No real runner walks during a marathon.” “My sister walked part of hers.” “Then she isn’t a real runner.”Answer itAsk what would count as evidence against the claim. If nothing could, the claim isn’t telling you anything.
Shifting the burden of proofAlso: misplaced burden of proofMakes a claim, then demands that other people disprove it.Example“This water filter cures headaches. Prove it doesn’t.”Answer itThe person making the claim supplies the evidence. You don’t owe them a disproof.
Appeal to ignoranceLatin: ad ignorantiamTreats a lack of proof as proof of the opposite.Example“Nobody has proved the new phone mast is safe, so it must be harmful.”Answer itAbsence of evidence counts for little unless someone has looked properly. Ask what the people who looked actually found.
Special pleadingA double standardApplies a rule to everyone else but exempts one case, often the speaker’s own, without a relevant reason.Example“Everyone should pay into the office coffee fund. Except me, because I set up the machine.”Answer itAsk what makes this case different in a way the rule actually cares about.
Sweeping generalisationAlso: the fallacy of accidentApplies a general rule to a case it was plainly never meant to cover.Example“The sign says no running by the pool, so the lifeguard shouldn’t have run to help that swimmer.”Answer itAsk what the rule is for. Most rules have exceptions, and their purpose tells you where.
False equivalenceA form of suppressed evidencePresents two sides as equally well supported when the evidence on one side is far weaker.ExampleA documentary gives equal airtime to space engineers and to a man who thinks the Moon landings were staged, as if the evidence were evenly split.Answer itAsk how much evidence sits on each side, not just how many sides there are.
Is–ought fallacyOne sense of the “naturalistic fallacy”Moves straight from how things are to how they ought to be, with no argument for the step.Example“People have always competed for status, so that’s how society should be run.”Answer itAsk for the missing premise that turns a fact into a value, and whether you accept it.
Group 04 · 6 fallaciesCausal fallacies: the wrong thing gets the credit.
Most of these are versions of what the IEP calls false cause: concluding that one thing caused another on evidence that cannot show it. The last two get the chain of causes wrong in a different way, by exaggerating it or by shrinking it to one link.
FallacyWhat it does · example · how to answer itGo deeper
Post hocLatin: post hoc, ergo propter hoc · “after this, therefore because of this”Assumes that because B followed A, A caused B.Example“I wore my lucky socks and we won, so the socks work.”Answer itAsk how often B happens without A, and A without B.Post hoc, explained →
Correlation as causationLatin: cum hoc, ergo propter hoc · “with this, therefore because of this”Assumes that two things which occur together must be cause and effect.Example“Students who sit at the front get better marks, so moving to the front will raise yours.”Answer itAsk what else could link the two: chance, a third factor, or causation running the other way.
Ignoring a common causeAlso: neglecting a common causeTreats one effect as the cause of another when both come from a third thing.Example“Umbrella sales and wellington-boot sales rise together, so umbrellas must make people buy boots.”Answer itLook for the hidden third factor. Here it is the rain.
Reversed causationCause and effect swappedGets the direction of cause and effect backwards.Example“People who own lots of running gear are fit, so buying running gear will make me fit.”Answer itAsk whether the supposed effect could be producing the supposed cause.
Slippery slopeAlso: the domino fallacyClaims that one step will set off a chain ending in disaster, without showing that each link is likely.Example“If we let staff work from home on Fridays, soon nobody will come in at all and the company will fold.”Answer itAsk for the evidence for each step. A slope is only slippery if every link in the chain holds, and some genuinely do.Slippery slope, explained →
Oversimplified causeAlso: the single-cause fallacyTreats one factor as the whole explanation for an outcome that had many causes.Example“We lost the league because of one missed penalty in March.”Answer itAsk what else contributed, and roughly how much each factor mattered.
Group 05 · 8 fallaciesStatistical and probability fallacies: the numbers mislead.
These are errors about samples, chance and evidence. Several are studied mainly as cognitive biases, but in an argument they work exactly like fallacies.
FallacyWhat it does · example · how to answer itGo deeper
Hasty generalisationAlso: jumping to conclusionsDraws a broad conclusion from too few cases, or from cases that aren’t representative.Example“The two plumbers I’ve hired were both late, so plumbers are never on time.”Answer itAsk how big the sample is, and how it was chosen.Hasty generalisation, explained →
Anecdotal evidenceOne vivid storyLets a single memorable story outweigh systematic evidence.Example“Seat belts can’t matter much. My uncle crashed without one and walked away.”Answer itOne case can’t show a rate. Ask what the larger studies found.
Cherry-pickingAlso: suppressed evidencePresents only the evidence that supports a conclusion and leaves out what counts against it.ExampleAn advert quotes the one glowing review from a page full of complaints.Answer itAsk for the whole set: how many results there were, and how many were left out.
Texas sharpshooterDrawing the target round the bullet holesPicks out a pattern in random results after the fact, then treats it as meaningful.Example“Of the twenty predictions in last year’s horoscope column, three came true. The stars know.”Answer itCount the misses too, and ask whether the pattern was predicted before the results came in.Apophenia · dictionary →
Gambler’s fallacyAlso: the Monte Carlo fallacyBelieves that in a truly random process a run of one outcome makes the other one “due”.Example“Red has come up six times in a row, so black is bound to be next.”Answer itIndependent events have no memory. The odds on the next spin are the same as on the first.Gambler’s fallacy · dictionary →
Hot hand fallacyContestedThe belief that someone on a streak is more likely to succeed next time. It was long taught as a textbook error, but that is now disputed.Example“She’s scored her last four shots. Give her the ball.”Answer itHandle with care. A 2018 study by Miller and Sanjurjo found a bias in the measure used by the classic 1985 study, and correcting it reverses that study’s conclusion. A person’s form can change in a way a roulette wheel’s can’t.Hot hand fallacy · dictionary →
Survivorship biasOnly the winners are visibleDraws conclusions only from the cases that made it through some filter, ignoring the ones that didn’t.Example“Plenty of famous founders dropped out of university, so dropping out is a good route to success.”Answer itAsk about the cases you can’t see, such as the dropouts nobody has heard of.Survivorship bias · dictionary →
Sunk cost fallacyThrowing good money after badKeeps putting money, time or effort into something because of what has already gone in, even though that can’t be recovered.Example“The film is dreadful, but I paid £15 for the ticket, so I’m staying to the end.”Answer itAsk: if I were starting fresh today, would I choose this? What’s already spent is gone either way.Sunk cost fallacy · dictionary →
Group 06 · 3 fallaciesFormal fallacies: the shape is broken.
Formal fallacies fail on structure alone. Swap in any content you like and the conclusion still doesn’t follow, however true the premises are.
FallacyWhat it does · example · how to answer itGo deeper
Affirming the consequentIf A then B; B; so AReasons from “if A, then B” and “B” to “A”. B could be true for some other reason.Example“If it rained, the pavement would be wet. The pavement is wet, so it rained.” (A street cleaner may have been past.)Answer itFind another way B could be true without A.
Denying the antecedentIf A then B; not A; so not BReasons from “if A, then B” and “not A” to “not B”.Example“If you’re a dentist, you know what a molar is. You’re not a dentist, so you don’t know what a molar is.”Answer itShow that B can hold without A.
Undistributed middleAll A are C; all B are C; so all A are BConcludes that two groups overlap just because both share a feature.Example“All cats have four legs. All dogs have four legs. So all cats are dogs.”Answer itDraw the circles. Two groups can sit inside a third without touching each other.
Group 07 · 1 fallacyAnd one about fallacies: where this vocabulary goes wrong.
The last entry is the one people commit most often once they have learnt the other forty-four.
FallacyWhat it does · example · how to answer itGo deeper
The fallacy fallacyAlso: argument from fallacyConcludes that a claim is false because an argument for it was fallacious.Example“Your case for the cycle lane was a slippery slope, so the cycle lane is a bad idea.”Answer itA bad argument leaves its conclusion unproven, not disproven. Look for a better argument, on either side.
The cautionNaming a fallacy is not winning.
There is a failure mode that arrives fast once you learn this vocabulary, and it deserves its own warning. Identifying a fallacy shows that this particular argument fails. It does not show that the conclusion is false. Someone can defend a true claim with terrible reasoning, and dismissing the claim because the reasoning was bad is itself an error: the fallacy fallacy, the last entry above. The point of learning these patterns is to reason better, not to acquire a faster way to dismiss people.
The second caution concerns charity. Many apparent fallacies dissolve when you state the other person’s argument as strongly as you can before responding, the discipline this whole site is built on. A slippery slope with a genuine causal mechanism is not a fallacy; it is an argument. An appeal to a relevant expert is not a fallacy; it is ordinary good practice. What makes each version fallacious is a specific missing element, and knowing which element is missing is the actual skill.