fallacious

Fallacious is an adjective used to describe reasoning, arguments, or conclusions that rely on a fallacy—a defect in reasoning that undermines the argument's validity, soundness, or credibility. A fallacious argument may appear convincing but rests on mistaken assumptions, irrelevant evidence, or logical errors.

Fallacies are typically categorized as formal or informal. Formal fallacies derive from an error in logical form, such that the conclusion does not follow from the premises, regardless of content. Informal fallacies arise from the content or language of an argument, including deceptive rhetoric, irrelevant premises, or implicit biases. Common informal fallacies include ad hominem, straw man, false dilemma, begging the question, and appeal to authority when the authority is not credible.

In discourse, labeling an argument as fallacious signals a critique of its reasoning rather than its conclusion. Fallacious conclusions can be true or false; the issue is the faulty reasoning. In academic and professional contexts, identifying fallacies helps promote clear thinking and rigorous evaluation of evidence. The term derives from Latin fallacia, meaning deceit.

Notes: the designation fallacious does not automatically determine truth or falsity, and a fallacy may be present in a valid argument if its premises are incorrect. It also distinguishes fallacious reasoning from invalid argument structure and from unsound arguments.