Logical Fallacies
Common errors in reasoning — formal and informal — with the standard catalogue and its caveats.
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A logical fallacy is an error in reasoning that makes an argument weak, unsound, or misleading. The term covers two families. Formal fallacies are arguments whose form is invalid — the conclusion does not follow from the premises no matter what the content is, as in affirming the consequent and denying the antecedent, the invalid cousins of modus ponens and modus tollens. Informal fallacies depend on content and context: the reasoning fails because of what is said or how, not because of a bad logical shape.
The most frequently cited informal fallacies include:
| Fallacy | Pattern | Example |
|---|---|---|
| Ad hominem | Attack the person instead of the argument | "You cannot trust his climate claims; he drives an SUV." |
| Straw man | Misrepresent the opponent's position, then refute it | "She wants lower taxes, i.e., no services for anyone." |
| Appeal to authority | Cite an authority outside their expertise | "A celebrity recommends this diet, so it works." |
| False dilemma | Force two options when more exist | "Either we cut wages or the company closes." |
| Slippery slope | Assert a chain of events without evidence | "Legalizing one drug will end in total legalization." |
| Post hoc ergo propter hoc | Infer causation from mere sequence | "I wore this shirt and won; the shirt brings luck." |
| Bandwagon | Treat popularity as proof | "Everyone believes it, so it must be true." |
| Begging the question | Assume what you are trying to prove | "The book is true because it says it is inspired." |
The list is a teaching tradition as much as a science: the classic catalogue descends from Aristotle's Sophistical Refutations and was systematized in medieval and early-modern logic textbooks. Modern argumentation theory treats fallacies less as rules to memorize than as recurring ways arguments can fail, and it emphasizes two caveats. First, context matters: an appeal to authority can be perfectly reasonable when the authority is genuine and in their domain, and a slippery slope can be a legitimate warning when the mechanism of escalation is specified. Second, fallacy labels can themselves be misused — the "fallacy fallacy" is the error of concluding that because an argument contains a fallacy, its conclusion must be false; a bad argument can still have a true conclusion.
Studying fallacies is therefore not a parlor trick but part of critical thinking: recognizing the patterns helps in evaluating claims, in adversarial settings such as law and debate, and in resisting manipulation in politics and advertising. The goal is not to win arguments by labeling but to notice when reasoning has gone wrong and to ask what evidence would settle the question.
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critical thinking logic reasoning