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.
Exam-style practice questions
Practice questions written in the style of TASC exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
TCE 20225 marksIdentify the informal fallacy in the following argument and explain why it weakens the reasoning: "You cannot trust her case for the new tax, because she is rich and would benefit from the changes."Show worked answer →
A 5 mark short response needs the correct label, the general flaw, and its application to the case.
Label. This is an ad hominem fallacy, of the circumstantial kind, since it attacks the arguer's situation rather than her argument.
General flaw. Whether a conclusion is well supported depends on the premises and reasoning offered, not on facts about the person presenting them. Attacking the person leaves the actual argument untouched.
Application. Even if she is wealthy and would benefit, that does not show her premises are false or her inference invalid. The argument against the tax could still be good or bad on its own merits, so the attack is irrelevant and the reasoning fails to engage it.
Markers reward the right name plus a clear explanation that the attack is irrelevant to the truth of the conclusion, ideally noting that an interest in an outcome is not the same as an error in reasoning.
TCE 202414 marks"Labelling an argument as a fallacy is enough to refute it." Critically discuss this claim, using at least two informal fallacies to support your answer.Show worked answer →
A 14 mark extended response should argue a thesis, not just list fallacies.
Exposition. Define an informal fallacy as a pattern of reasoning that is persuasive but defective for reasons of relevance, meaning or evidence rather than form. Explain that naming the pattern is the first step of analysis.
Argument against the claim. Show that labelling is not sufficient. First, a fallacy identifies a failure to support the conclusion, not that the conclusion is false (a fallacious argument can have a true conclusion). Second, many informal fallacies are context-dependent: an appeal to authority is legitimate when the authority is a genuine expert, and a slippery slope is sound when the intermediate steps are supported. So a bare label can misfire.
Develop with two examples. Use ad hominem (sometimes a person's reliability is genuinely relevant, as with a witness known to lie) and slippery slope (some slopes really are slippery). In each, show that you must demonstrate the inference is unsupported in this case, otherwise you commit the fallacy fallacy of dismissing a sound argument by mislabelling it.
Judgement. Conclude that identifying the fallacy is necessary but not sufficient: a successful refutation must explain why, in the specific case, the reasoning fails. Markers reward a defended thesis, accurate use of at least two fallacies, the point that a fallacy does not show the conclusion false, and a reasoned conclusion.