Category
Formal fallacies
A formal fallacy is a mistake in the shape of an argument. The premises, even if every one of them is true, don’t guarantee the conclusion, and the reason is visible without knowing what the argument is about. Replace the subject matter with letters (P, Q) and the flaw is still there.
Formal fallacies are the most clear-cut errors on this site. There’s no “it depends on context”: an argument form is either valid or it isn’t.
Entries (2)
How to spot one
Look for a counterexample: a case with the same shape where the premises are obviously true and the conclusion is obviously false. If you can find one, the form doesn’t work.
If it’s a dog, it’s a mammal. It’s a mammal. So it’s a dog.
Swap in “cat” and both premises stay true while the conclusion fails. Any argument with that shape is unreliable, including ones where the conclusion happens to be right.
A true conclusion doesn’t rescue it
A formally fallacious argument can reach a true conclusion by luck. The fallacy is that the argument doesn’t show it. Equally, spotting a formal fallacy doesn’t show the conclusion is false. It shows only that this argument gives you no reason to believe it.
Valid is not the same as true
The opposite of a formal fallacy is a valid form, where true premises guarantee a true conclusion. Affirming the antecedent (modus ponens) is the standard example. Validity says nothing about whether the premises are true; an argument that is valid and also has true premises is called sound.
Formal and informal
A formal fallacy is wrong because of its structure. An Informal fallacy can have a perfectly valid structure and still be a bad argument, because of what it’s about, what it assumes, or how it’s being used.
Other categories: Informal fallacies · Cognitive biases