Fallacia

Fallacia (fallacy) is a term for flaws in argumentation: patterns of reasoning that appear persuasive but fail to provide adequate support for their conclusion. The word derives from Latin fallacia, from fallere, to deceive.

Fallacies are commonly categorized as formal or informal. Formal fallacies involve the form of the argument and render it invalid or unsound regardless of its content (for example, affirming the consequent). Informal fallacies arise from issues with the argument's content, language, or assumptions, such as ambiguity, irrelevant reasons, or presuppositions.

Common informal fallacies include ad hominem (attacking the person rather than the argument), straw man (misrepresenting an opponent's position), false dilemma (presenting only two options), red herring (diverting with irrelevant information), appeal to authority, appeal to emotion, post hoc ergo propter hoc, hasty generalization, and begging the question. These patterns can undermine persuasive writing and speech even when the overall claim is plausible.

In practice, recognizing fallacies helps evaluate claims in law, journalism, science, and public discourse. Not all persuasive arguments are fallacious; a claim can be strong or sound even if it uses rhetoric. Conversely, a non-fallacious argument can be weak if the evidence is insufficient.

Historically, the study of fallacies traces to Aristotle and ancient rhetoricians, who described patterns of faulty reasoning. In modern logic, fallacies are systematically classified and used to teach critical thinking, argumentation skills, and the assessment of evidence.