fallacy

A fallacy is a flaw in reasoning that weakens an argument. Fallacies can arise from mistakes in logic, poor evidence, or deceptive rhetoric, and they may be intentional or unintentional. Although a fallacious argument can be persuasive, its conclusion is not reliably supported by its premises.

Fallacies are commonly divided into formal and informal categories. Formal fallacies involve an error in the logical form of an argument, so the conclusion fails to follow from the premises regardless of the content. Informal fallacies depend on language, context, or the content of the argument and are not tied to a specific logical form. They often arise from ambiguous terms, irrelevant premises, or defective inductive reasoning.

Common informal fallacies include ad hominem (attacking the person instead of the argument), straw man (misrepresenting an opponent’s position), appeal to authority (relying on authority rather than evidence), appeal to ignorance (claiming something is true because it cannot be proven false), false dilemma (presenting only two options when more exist), circular reasoning (the conclusion is assumed in the premises), and hasty generalization (drawing a broad conclusion from limited evidence). Other examples are post hoc ergo propter hoc (assuming causation from sequence) and slippery slope arguments.

In critical thinking, identifying fallacies helps assess the strength of arguments. Fallacies do not always invalidate a claim, but they undermine the argument’s reasoning. Context matters: some apparent fallacies may be rhetorical devices, while others reflect genuine logical weaknesses.