A fallacy is any error in reasoning that undermines the support an argument offers for its conclusion. Fallacies divide into two broad kinds. A formal fallacy is a flaw in the logical structure of an argument, often identifiable by its symbolic form. Two classic examples are affirming the consequent (if \(P\) then \(Q\); \(Q\); therefore \(P\)) and denying the antecedent (if \(P\) then \(Q\); not \(P\); therefore not \(Q\)). Both patterns resemble valid deductive forms but commit the structural error of reversing or negating the wrong term. In contrast, an informal fallacy arises from errors in content, context, or language rather than in formal pattern.
Many informal fallacies concern relevance—reasons offered in place of actual evidence. The ad hominem attack dismisses an argument by attacking the person who made it. Tu quoque, the "you too" fallacy, rejects criticism by accusing the critic of similar wrongdoing. The straw man misrepresents an opponent's view to make it easier to attack. A red herring introduces an irrelevant topic to divert attention. The genetic fallacy judges an idea by its origin rather than its merits. Poisoning the well discredits a speaker in advance so that anything they say is dismissed. Whataboutism deflects criticism by pointing to others' faults. None of these moves engages the substance of the argument at issue.
Other informal fallacies arise from weak inductive support. Hasty generalization draws a broad conclusion from too few examples; anecdotal evidence substitutes a personal story for systematic data. The post hoc fallacy assumes that because \(B\) followed \(A\), \(A\) must have caused \(B\); the broader correlation/causation fallacy treats any statistical association as a causal relationship. The gambler's fallacy mistakenly believes that past random events influence future independent ones. Cherry-picking selects evidence that supports a conclusion while ignoring contradicting evidence, and the Texas sharpshooter fallacy draws a conclusion from data by highlighting clusters after the fact. A further cluster of fallacies stems from ambiguity: equivocation uses a word with multiple meanings inconsistently; amphiboly arises from ambiguous sentence structure. The composition fallacy assumes what is true of parts is true of the whole, while the division fallacy commits the reverse error. The false cause fallacy, more generally, misidentifies a cause.
A final family of informal fallacies involves false presuppositions and illegitimate appeals. A loaded question smuggles in an unjustified assumption, as in "Have you stopped lying?" Begging the question assumes the very point at issue, and circular reasoning embeds the conclusion in a premise. The false dilemma presents only two options when more exist; the slippery slope claims that one step will trigger an inevitable chain of disasters without justification. Special pleading exempts oneself from a standard applied to others, and moving the goalposts shifts the criteria of an argument once they have been met. The no true Scotsman fallacy arbitrarily redefines a category to exclude counterexamples. Appeals to authority, popularity (ad populum or bandwagon), tradition, ignorance, nature, consequences, and emotion—particularly fear—all substitute psychological pressure for evidence. The middle-ground or fallacy of moderation assumes that the truth lies between two positions simply because they are opposed, and the perfect solution fallacy rejects good solutions because they are not flawless. The gish gallop overwhelms opponents with many rapid, weak arguments. Recognizing these patterns is essential because they are common in everyday reasoning, advertising, and political debate.