Quiz Entry - updated: 2026.07.14
What is the difference between a formal and an informal fallacy?
A formal fallacy is broken by its logical structure alone; an informal fallacy is broken by its content — irrelevant or unwarranted premises dressed up as support.
The distinction is about where the error lives:
| Formal fallacy | Informal fallacy | |
|---|---|---|
| Where the flaw is | The argument's form/structure | The argument's content/context |
| Detectable by | Logic alone, ignoring meaning | Reading what's actually said |
| Example | "If P then Q; Q; therefore P" (affirming the consequent) | Ad hominem, straw man, slippery slope |
A formal fallacy is invalid no matter what you plug in — e.g. "If Portland is the capital of Maine, it's in Maine; Portland is in Maine; therefore Portland is the capital" — the shape is wrong (Portland is in Maine but Augusta is the capital). An informal fallacy looks fine structurally; the defect is that a premise is irrelevant, biased, or smuggles in the conclusion. Most of the famous named fallacies (ad hominem, red herring, appeal to emotion) are informal.