What are the common informal fallacies, and why do they make reasoning fail?
Recognising and explaining informal fallacies in everyday argument
The main informal fallacies including ad hominem, straw man, false dilemma, appeal to authority, slippery slope and begging the question, with examples and why each one weakens reasoning.
Reviewed by: AI editorial process; not yet individually human-reviewed
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What this dot point is asking
This dot point asks you to detect faulty reasoning in real arguments and explain the flaw. It supports criterion 2, using logic to analyse arguments, and criterion 3, exploring contemporary issues.
What makes a fallacy informal
Formal fallacies are errors of structure, such as affirming the consequent. Informal fallacies are different: the form may look fine, but the reasoning goes wrong because of misleading content, irrelevance or hidden assumptions. They are persuasive precisely because they often feel convincing, which is why critical reasoning trains you to spot them.
Fallacies of relevance
These introduce material that does not bear on the truth of the conclusion.
- Ad hominem attacks the person rather than their argument. Saying a politician is wrong about climate policy because they are wealthy ignores whether their reasoning is correct. Note that questioning a person's reliability can sometimes be legitimate, but only when their character is genuinely relevant.
- Appeal to authority cites a source who is not actually an expert in the relevant field, such as a famous actor endorsing a medical claim. Citing a real expert is not fallacious; the fallacy is misplaced or false authority.
- Appeal to emotion, such as appeal to pity or fear, substitutes feeling for evidence.
Fallacies of misrepresentation
- Straw man distorts an opponent's position into a weaker version that is easier to attack. If someone argues for moderate immigration controls and is accused of wanting closed borders, the responder is attacking a straw man rather than the real claim. The charitable principle, associated with thinkers like Daniel Dennett, requires representing a view in its strongest form before criticising it.
- False dilemma, or false dichotomy, presents only two options when more exist, as in the slogan that you are either with us or against us. It ignores middle positions.
Fallacies of weak inference
- Slippery slope claims that one step will inevitably lead to an extreme outcome without justifying the chain. It is fallacious only when the intermediate steps are not supported; sometimes a slope really is slippery and the argument is sound.
- Hasty generalisation draws a sweeping conclusion from too few cases, such as judging an entire country from one rude tourist.
- Post hoc reasoning assumes that because one event followed another, the first caused the second.
Fallacies of presumption
- Begging the question assumes in a premise the very conclusion it is trying to prove, as in claiming that a holy text is true because the text says it is true. The argument moves in a circle.
- Equivocation shifts the meaning of a key word partway through, trading on ambiguity.
Evaluating well
In the exam, do not just label a fallacy. State which fallacy it is, explain the general flaw in that pattern, then apply that explanation to the specific case in front of you. The best answers also note exceptions: many fallacies are context-dependent, so a careful thinker checks whether the reasoning really is defective rather than reaching for a label.