Definition
A logical fallacy is a defect in an argument: a step in reasoning that fails to provide the support its surface form seems to promise. Fallacies divide into two broad families. Formal fallacies are violations of the rules of inference — the argument has a recognisable shape that does not in fact license its conclusion (affirming the consequent, denying the antecedent). Informal fallacies are defects of content or context rather than form — the argument may be structurally well-formed but rests on a smuggled assumption, a manipulated emotion, or an equivocation between two meanings of a word.
The label "fallacy" applies only when the defect is hidden well enough to deceive. An argument that openly admits its conclusion does not follow is not a fallacy; it is a confession.
Why it matters
How it works
Formal fallacies are recognised by abstracting from content to structure. The argument "if P then Q; Q; therefore P" looks like modus ponens but is affirming the consequent — the conclusion does not follow, as can be confirmed by a one-row counterexample on a truth table. Once the abstract shape is seen, the fallacy is undeniable and the dispute is over.
Informal fallacies require a different kind of analysis because the defect is not in the shape but in what is being assumed or implied. A straw man substitutes a weaker version of the opponent's claim and refutes that instead. An ad hominem attacks the arguer rather than the argument. An equivocation slides between two senses of a key term in mid-argument. A false dilemma forces a binary where richer options exist. Each requires the analyst to identify the implicit move and surface it. The cure is the same in every case: state the hidden assumption plainly, and the rhetorical force collapses. Recognising fallacies is therefore not just an academic exercise — it is the practical core of critical thinking.