You can now judge an argument's link: validity for deduction and strength for induction. Some bad links show up so often, and fool people so well, that they have names. These are the fallacies.
Key idea
A fallacy is a common pattern of bad reasoning that can seem convincing. A formal fallacy is a flaw in the form, visible with the words replaced by letters. An informal fallacy is a flaw in the content: you must read what the premises say to see it.
Why it works
Naming a pattern makes it easier to notice. "Everyone believes it, so it's true" sounds fine until you have a name for it (appeal to the people) and remember why popularity is not proof. The names are tools for asking a sharper question, not labels to win with.
| Formal fallacy | Informal fallacy |
|---|---|
| "If it rained, the grass is wet. The grass is wet. So it rained." The shape "If P then Q; Q; so P" is broken, whatever P and Q are. | "Everyone at school has this game, so it's the best game ever." You must read the premise to see that popularity is not quality. |
Worked example: A fallacy with a true conclusion
"Most people I know think the Pacific is the largest ocean, so it is." Is the conclusion false?
- The Pacific is in fact the largest ocean, so the conclusion is true.
- But what most people think is not what makes it true; measurement is.
- The argument is bad; its conclusion is true anyway.
Answer: No. The conclusion is true, but this argument does not show it.
Common mistake
Calling any false statement a fallacy, or deciding that a fallacious argument has a false conclusion. "Spiders have six legs" is false, but it contains no reasoning, so it is not a fallacy. And a bad argument can reach a true conclusion; it just fails to support it.