Logical Fallacies: 15 to Recognise and Name
Intro logic / rhetoric / AP Language · 12 flashcards · 7 quiz questions · updated 2026-08-19
A fallacy is an argument whose conclusion does not follow from its premises. Formal fallacies break the structure of the inference; informal fallacies keep a valid-looking shape but smuggle in something irrelevant.
Naming one precisely is what earns marks — 'that's a strawman' is only worth something if you can say what the person's actual position was.
Fallacies of relevance
| Fallacy | The pattern | Example |
|---|---|---|
| Ad hominem | Attack the person instead of the claim | "Her climate argument is worthless — she flies constantly." |
| Strawman | Refute a distorted version of the position | "You want a bike lane, so you want to ban all cars." |
| Appeal to authority | Cite an authority outside their field, or a disputed one | "A Nobel physicist doubts this diet, so it fails." |
| Appeal to popularity | Treat wide belief as evidence | "Millions use it, so it works." |
| Appeal to emotion | Substitute feeling for evidence | "Think of the children" in place of an argument |
| Tu quoque | Deflect criticism by charging hypocrisy | "You smoked at my age." |
Fallacies of weak induction
| Fallacy | The pattern | Example |
|---|---|---|
| Hasty generalisation | Generalise from too small a sample | Two rude drivers, therefore that city is rude |
| Post hoc ergo propter hoc | Treat sequence as causation | "I wore this shirt and we won." |
| Slippery slope | Assert a chain of consequences without support | "Allow this and society collapses." |
| False dilemma | Present two options as exhaustive | "Either cut spending or go bankrupt." |
| False analogy | Compare cases that differ in the relevant respect | "A country's budget is just a household budget." |
Fallacies of presumption and structure
| Fallacy | The pattern | Example |
|---|---|---|
| Begging the question | Assume the conclusion in a premise | "It is reliable because it never errs." |
| Equivocation | Shift a word's meaning mid-argument | "Only man is rational; no woman is a man." |
| Loaded question | Presuppose an unproven claim | "Have you stopped cheating?" |
| Affirming the consequent | If P then Q; Q; therefore P | "If it rained the ground is wet. It is wet, so it rained." |
| Denying the antecedent | If P then Q; not P; therefore not Q | "If it rained it is wet. It didn't rain, so it isn't wet." |
The pairs students confuse
- •Ad hominem vs tu quoque: both attack the speaker, but tu quoque specifically charges inconsistency.
- •Strawman vs false dilemma: a strawman misrepresents one position; a false dilemma pretends only two exist.
- •Slippery slope vs valid causal chain: the fallacy is asserting the chain without evidence for each link, not predicting consequences at all.
- •Appeal to authority vs legitimate expert testimony: citing a relevant, consensus expert is good reasoning. The fallacy needs an irrelevant or disputed authority.
- •Affirming the consequent vs modus ponens: modus ponens (if P then Q; P; therefore Q) is valid. Reversing it is not.
Valid forms to hold on to
- •Modus ponens: If P then Q. P. Therefore Q. Valid.
- •Modus tollens: If P then Q. Not Q. Therefore not P. Valid.
- •Hypothetical syllogism: If P then Q. If Q then R. Therefore if P then R. Valid.
- •Disjunctive syllogism: P or Q. Not P. Therefore Q. Valid.
- •Validity concerns structure; soundness adds that the premises are actually true.
Common mistakes
- ✗Calling every insult an ad hominem. It only counts when the insult is offered as the reason to reject the claim.
- ✗Labelling any prediction of consequences a slippery slope.
- ✗Confusing validity with truth — a valid argument can have false premises and a false conclusion.
- ✗Treating an appeal to a relevant expert as a fallacy.
- ✗Naming a fallacy without showing which premise fails to support the conclusion.
Flashcards
Tap a card to reveal the answer.
Ad hominem⌄
Rejecting a claim by attacking the person making it.
Strawman⌄
Refuting a distorted, weaker version of the opponent's position.
Post hoc ergo propter hoc⌄
Inferring causation from mere sequence.
False dilemma⌄
Presenting two options as if they were the only ones.
Begging the question⌄
Assuming the conclusion within one of the premises — circular reasoning.
Equivocation⌄
Using one word in two senses within the same argument.
Affirming the consequent⌄
If P then Q; Q; therefore P — invalid.
Denying the antecedent⌄
If P then Q; not P; therefore not Q — invalid.
Modus tollens⌄
If P then Q; not Q; therefore not P — valid.
Hasty generalisation⌄
Drawing a broad conclusion from an unrepresentative sample.
Tu quoque⌄
Dismissing criticism by pointing at the critic's own conduct.
Validity vs soundness⌄
Valid = the conclusion follows from the premises. Sound = valid and the premises are true.
Practice quiz
Answer first, then open the explanation.
1. "You can't trust his economic analysis — he's been bankrupt twice." This is:
- A. Strawman
- B. Ad hominem
- C. False dilemma
- D. Equivocation
Show answer
B. Ad hominem
The argument attacks the arguer's history instead of engaging with the analysis.2. "Either we ban the app entirely or we accept total surveillance." This is:
- A. Slippery slope
- B. False dilemma
- C. Appeal to emotion
- D. Hasty generalisation
Show answer
B. False dilemma
Two options are presented as exhaustive when others clearly exist.3. "If it rained, the ground is wet. The ground is wet, so it rained." This is:
- A. Modus ponens
- B. Modus tollens
- C. Affirming the consequent
- D. Disjunctive syllogism
Show answer
C. Affirming the consequent
The ground could be wet for other reasons; the inference reverses a conditional.4. "The medicine works because it cures illness." This is:
- A. Begging the question
- B. Post hoc
- C. Appeal to popularity
- D. Tu quoque
Show answer
A. Begging the question
The premise restates the conclusion instead of supporting it.5. Which argument form is valid?
- A. If P then Q; not P; therefore not Q
- B. If P then Q; Q; therefore P
- C. If P then Q; not Q; therefore not P
- D. If P then Q; therefore Q
Show answer
C. If P then Q; not Q; therefore not P
That is modus tollens.6. A politician says a colleague who wants stricter emissions rules 'wants to shut down every factory'. This is:
- A. Strawman
- B. Equivocation
- C. Appeal to authority
- D. Loaded question
Show answer
A. Strawman
The position is exaggerated into something easier to attack.7. Citing a cardiologist on heart surgery outcomes is:
- A. An appeal to authority fallacy
- B. Legitimate expert testimony
- C. Ad hominem
- D. Hasty generalisation
Show answer
B. Legitimate expert testimony
The fallacy requires an authority who is irrelevant or whose field is not the one at issue.
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