Category
Informal fallacies
An informal fallacy is an argument that fails for reasons you can’t see from its shape alone. The flaw is in the content: the reasons offered don’t bear on the conclusion, an assumption is doing work it hasn’t earned, or a word quietly changes meaning halfway through.
Unlike formal fallacies, most informal fallacies depend on context. Questioning someone’s reliability is a fallacy when it replaces a response to their argument, and entirely reasonable when you’re being asked to take their word for something. That’s why every entry here has a section on when it isn’t an error.
Entries (8)
- Appeal to natureTreating “natural” as if it meant good, safe or right, and “unnatural” as if it meant bad.
- Appeal to popularityTreating the fact that many people believe or do something as proof that it’s true or right.
- Argument from ignoranceTreating a lack of proof as proof of the opposite, as in “it hasn’t been disproven, so it’s true” or “it hasn’t been proven, so it’s false”.
- Attacking the personRejecting an argument by attacking the person who made it instead of their reasons.
- Chronological snobberyDismissing an idea because it’s old or from an earlier era, instead of asking whether it was ever shown to be wrong.
- False dilemmaPresenting a few options, usually two, as the only possibilities when there are others, or when more than one could be true.
- Red herringAnswering a claim by raising a different issue, as if discussing that settled the original question.
- Straw manRefuting a distorted or weaker version of someone’s position instead of the position they actually hold.
The traditional groups
Informal fallacies are usually sorted into three broad families. The boundaries are loose and textbooks disagree about where some fallacies go.
- Relevance. The premises don’t actually bear on the conclusion. Attacking the arguer instead of the argument (Attacking the person, or ad hominem) is the classic case.
- Presumption. The argument depends on an assumption that hasn’t been established, such as a false choice between two options or a conclusion smuggled into the premises.
- Ambiguity. A word or phrase is used in two senses, so the argument only seems to connect.
Naming it is not refuting it
Calling out an informal fallacy is a starting point, not a verdict. The useful follow-up is always the same question: does this flaw actually undermine the conclusion here? Treating “that’s a fallacy” as a winning move is its own reasoning error, and a common one.
Informal fallacies and biases
An informal fallacy belongs to an argument. A Cognitive bias belongs to a person’s judgment. They often travel together: a bias makes a certain bad argument feel convincing, and that argument then gets written down.
Other categories: Formal fallacies · Cognitive biases