A fallacy is an error in reasoning, especially a defect in an argument offered to support a conclusion. Studied in logic and philosophy, fallacies concern how claims are justified, rather than simply whether those claims are true. A fallacious argument can reach a true conclusion, while a mistaken belief need not result from a fallacy. Definitions differ: some emphasize arguments that appear convincing despite being defective; others identify violations of standards governing evidence, inference, or discussion. (plato.stanford.edu)
Historical development
Aristotle provided the earliest surviving systematic treatment of fallacies in Sophistical Refutations. He distinguished thirteen kinds, separating those dependent on linguistic expression from those arising independently of it. His analysis addressed apparent refutations in disputation, including ambiguity, begging the question, and irrelevant conclusions. This framework influenced subsequent traditions of argument analysis. (plato.stanford.edu)
Modern study expanded beyond the classification of deceptive arguments. Charles L. Hamblin’s Fallacies (1970) challenged conventional textbook treatments and became an important reference for informal logic. Later approaches examined everyday reasoning, the justification of beliefs, and the rules governing exchanges between arguers. These developments produced competing accounts rather than a universally accepted definition or classification. (plato.stanford.edu)
Formal fallacies
A formal fallacy is a defect identifiable through an argument’s logical structure. In deductive reasoning, validity requires that the conclusion cannot be false when all premises are true. This differs from soundness, which additionally requires true premises. An invalid argument therefore fails to establish its conclusion deductively, even when its premises and conclusion happen to be true. (iep.utm.edu)
Two familiar examples occur in propositional logic:
- Affirming the consequent: from “If P, then Q” and “Q,” infer “P.” For example: “If a figure is a square, it has four sides; this figure has four sides; therefore it is a square.” A nonsquare rectangle supplies a counterexample.
- Denying the antecedent: from “If P, then Q” and “Not P,” infer “Not Q.” The corresponding example incorrectly concludes that a nonsquare figure cannot have four sides. (iep.utm.edu)
These patterns resemble, but differ from, the valid rules modus ponens and modus tollens. The error is treating a sufficient condition as though it were also necessary. Neither fallacy establishes that the conclusion is false; each shows that the stated premises do not guarantee it. (iep.utm.edu)
Informal fallacies
An informal fallacy requires attention to content, meaning, evidence, or argumentative context, rather than logical form alone. Common groupings include relevance, ambiguity, unjustified presumption, and insufficient support, although classifications overlap. Representative patterns include: (iep.utm.edu)
- Ad hominem: using an irrelevant feature of a speaker to dismiss the speaker’s claim or argument.
- Straw man: replacing an opponent’s position with a distorted or weaker version and attacking that substitute.
- Equivocation: shifting a term’s meaning during an argument while treating it as unchanged.
- Begging the question: relying on premises whose acceptance already depends on accepting the disputed conclusion.
- False dilemma: presenting alternatives as exhaustive when relevant additional possibilities exist.
- Hasty generalization: drawing a broad conclusion from insufficient or unrepresentative evidence.
- Post hoc reasoning: treating temporal succession alone as adequate evidence of causation.
- Appeal to ignorance: treating failure to establish a proposition as sufficient proof of its opposite. (iep.utm.edu)
For illustration, “Every component is inexpensive, so the complete system must be inexpensive” transfers a property from parts to a whole without accounting for their number or assembly costs. Such examples expose missing support; their assessment depends on what the premises actually establish. (plato.stanford.edu)
Context and standards of assessment
Not every argument resembling a named fallacy is defective. An appeal to authority can provide reasonable, defeasible support when the authority has relevant expertise. Likewise, information about a witness’s reliability can matter when evaluating testimony, although an unrelated personal attack does not refute an independently supported claim. Fallacy assessment therefore concerns the relevance and weight of a consideration, not merely its presence. (plato.stanford.edu)
The standard of inference also matters. Inductive reasoning offers support without guaranteeing its conclusion, while abductive reasoning compares possible explanations. Their failure to meet deductive validity is not, by itself, a fallacy. An explanation may resemble affirming the consequent yet remain reasonable when offered provisionally and assessed against alternatives. (plato.stanford.edu)
One influential informal-logical approach evaluates premises for acceptability, relevance, and sufficiency. Dialogue-based approaches additionally examine whether argumentative moves obstruct a critical discussion—for example, by improperly shifting the burden of proof. These frameworks distinguish failures of justification from failures in the conduct of an exchange. (plato.stanford.edu)
Identification and conceptual limits
Identifying a fallacy requires reconstructing the premises and conclusion, considering unstated assumptions, and explaining the specific defect. A label alone does not establish that reasoning is faulty. Nor does identifying one defective argument settle whether its conclusion could be supported by another argument. (iep.utm.edu)
Fallacies also differ from cognitive biases. Biases concern tendencies in judgment and help explain why errors occur; fallacy analysis concerns the standards the reasoning fails to satisfy. A person can commit a fallacy unintentionally, and defective reasoning need not reveal dishonesty. The distinction separates psychological explanation from normative assessment of arguments. (plato.stanford.edu)