Fallacies: Formal and Informal
Formal versus informal
A formal fallacy is a defect in logical form, such as affirming the consequent or denying the antecedent. An informal fallacy is a defect in content, context, or relevance, such as ad hominem, straw man, false dilemma, begging the question, equivocation, composition, division, post hoc, hasty generalization, and appeal to inappropriate authority.
Worked example
“If the library is open, the lights are on. The lights are on. Therefore the library is open.” This affirms the consequent. Symbolically, P → Q; Q; therefore P is invalid, because the lights could be on while the library is closed. By contrast, “You cannot trust her argument about taxation because she is not an accountant” attacks the person rather than the reasoning: an ad hominem.
Traps that cost marks
- Calling every bad argument a formal fallacy; many are informal.
- Confusing straw man (misrepresenting an opponent) with ad hominem (attacking the person).
- Treating post hoc correlation as proof of causation.
- Missing equivocation when a key term shifts meaning.
- Accepting a false dilemma that ignores a third option.
- Begging the question: the conclusion is assumed rather than supported.