Definition
Informal Fallacy
An informal fallacy is an error in reasoning that arises from the content, context, or language of an argument, rather than from its logical form.
Contrast with Formal Fallacies
A formal fallacy is a defect in logical structure that can be detected without knowing what the argument is about. An informal fallacy may have a valid logical form but fail because a premise is irrelevant, ambiguous, unsupported, or misrepresented.
Sources
Informal fallacies typically involve:
- Relevance: premises that do not bear on the conclusion.
- Ambiguity: shifts in the meaning of terms within the argument.
- Presumption: assumptions that are unwarranted or unstated.
Examples
Ad Hominem
Attacking the person making the argument rather than the argument itself. The logical form may be valid, but the premise is irrelevant to the truth of the conclusion.
Motte-and-Bailey Fallacy
See motte-and-bailey fallacy. The speaker shifts between a strong claim and a weak claim, exploiting ambiguity to make the strong claim appear defended.