Guide
Logical fallacies
Logical fallacies are errors in reasoning that make an argument invalid or unsound. This guide names and defines the major formal and informal fallacies.
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Formal versus informal fallacies
Logicians sort fallacies into two families. A formal fallacy is an error in the logical structure of a deductive argument: the form itself is invalid, so even true premises need not yield a true conclusion. Affirming the consequent and denying the antecedent are classic formal fallacies. An informal fallacy, by contrast, has no structural defect you can spot from form alone; the problem lies in the content — irrelevant premises, unwarranted assumptions, ambiguous language or weak evidence. Most fallacies encountered in everyday debate, advertising and politics are informal. Recognising which kind you are dealing with guides how you expose it: for formal fallacies you analyse the argument’s shape, while for informal fallacies you examine relevance, assumptions and evidence.
Fallacies of relevance
Fallacies of relevance introduce material that does not bear on the conclusion. The ad hominem attacks the person rather than the argument. The straw man misrepresents an opponent’s position to attack a weaker version. The red herring diverts attention to an irrelevant topic. The appeal to emotion substitutes feelings for evidence, while the appeal to authority misuses the testimony of an irrelevant or unqualified source. The bandwagon fallacy (appeal to popularity) treats widespread belief as proof. In each, the premises offered — however persuasive — simply fail to support the conclusion they are attached to.
Fallacies of presumption and ambiguity
Fallacies of presumption smuggle in an unjustified assumption. Begging the question and circular reasoning assume the conclusion within the premises. The false dichotomy (false dilemma) presents only two options when more exist. The complex question packs a contestable assumption into a single question. Fallacies of ambiguity exploit unclear language: equivocation shifts the meaning of a word mid-argument, while the fallacy of composition or division wrongly transfers a property between a whole and its parts. The shared theme is that something contestable is treated as already settled.
Fallacies of weak induction and causation
Some fallacies offer evidence that is genuinely relevant but far too weak to support the conclusion. The hasty generalisation draws a sweeping claim from an inadequate or unrepresentative sample. The false cause fallacy — in its post hoc and cum hoc forms — infers causation from mere sequence or correlation. The slippery slope claims that one step inevitably leads to extreme consequences without showing that the chain holds. These are errors of inductive strength: the premises raise the probability of the conclusion hardly at all, yet the argument treats them as decisive.
How to spot and respond to a fallacy
To analyse any argument, first identify its conclusion and premises, then ask two questions: do the premises actually support the conclusion, and are the premises true? Formal fallacies fail the first test on structural grounds; informal fallacies fail it because the premises are irrelevant, presumptuous or too weak. When you spot a fallacy, name it and explain why the support breaks down rather than simply asserting "that is a fallacy". Beware the fallacy fallacy — the mistaken belief that because an argument for a claim is fallacious, the claim itself must be false. A poorly argued conclusion may still be true; it just has not been properly established.
Key facts
At a glance
- Definition: a flaw in reasoning that undermines an argument’s logic
- Two broad classes: formal (invalid structure) and informal (faulty content)
- Fallacies of relevance: ad hominem, straw man, red herring, appeals
- Fallacies of presumption: begging the question, false dichotomy
- Fallacies of weak induction: hasty generalisation, false cause, slippery slope
- Fallacy fallacy: a bad argument does not prove the conclusion false
Common questions
FAQ
What is the difference between a formal and an informal fallacy?+
A formal fallacy is a defect in the logical structure of a deductive argument — the form is invalid, so the conclusion does not follow even if the premises are true. An informal fallacy has a structurally acceptable form, but fails because of its content: irrelevant premises, unwarranted assumptions, ambiguity or weak evidence. Most everyday fallacies are informal.
What is the most common logical fallacy?+
There is no single agreed "most common" fallacy, but the ad hominem, straw man, false dichotomy, hasty generalisation and appeals to emotion and authority appear constantly in debate, advertising and politics. They are common because they are persuasive: each substitutes something psychologically compelling for genuine, relevant support.
If an argument contains a fallacy, is its conclusion false?+
No. Concluding that a claim is false merely because an argument for it is fallacious is itself an error, known as the fallacy fallacy (argument from fallacy). A fallacious argument fails to establish its conclusion, but the conclusion may still be true for other reasons. Spotting a fallacy refutes the argument, not necessarily the claim.
Going deeper
