Twenty logical fallacies, with the sentence that gives each away

What a fallacy is, and is not

Logicians distinguish formal fallacies, where the shape of the argument is invalid regardless of content, from informal fallacies, where the content fails to support the conclusion even though the shape may look fine. Almost everything in everyday argument is informal. A fallacious argument can still have a true conclusion; what it lacks is support. Spotting a fallacy tells you that this particular argument does not establish its claim, not that the claim is false.

Fallacies of relevance

The support offered has nothing to do with whether the claim is true.

• Ad hominem: attacking the person instead of the argument. "His plan cannot work; he failed economics."
• Tu quoque: dismissing advice because the adviser does not follow it. "My doctor is overweight, so I can ignore her advice."
• Appeal to authority: citing someone whose expertise is elsewhere. "A famous actor recommends this supplement."
• Appeal to popularity: treating wide use as proof. "Everyone uses this app, so it is the best."
• Appeal to emotion: substituting a feeling for a reason. "Think of the children; that settles it."
• Appeal to ignorance: treating absence of disproof as proof. "Nobody has shown it does not work, so it works."
• Red herring: changing the subject. "Why worry about the deficit when the roads have potholes?"
• Genetic fallacy: judging a claim by its origin. "Of course the study found that; the researchers wanted it to."

Fallacies of presumption

The argument quietly assumes something it has no right to assume.

• False dilemma: presenting two options as the only ones. "Ban all cars or accept endless gridlock."
• Slippery slope: asserting a chain of consequences without support for the links. "Allow one retake and degrees become worthless."
• Hasty generalization: concluding from too few cases. "Two rude people, so the whole city is rude."
• Begging the question: assuming the conclusion in the premise. "The policy is right because it is the correct thing to do."
• Loaded question: a question that presupposes its answer. "Have you stopped cutting corners?"
• No true Scotsman: redefining a group to exclude counterexamples. "No real fan would criticise the club."

Fallacies of structure and cause

The argument's parts do not connect the way it claims.

• Straw man: refuting a distorted version of the position. "A five percent cut means she wants us defenceless."
• False cause: mistaking sequence or correlation for cause. "Sales fell after the manager arrived, so the manager caused it."
• Equivocation: sliding between two meanings of a word. "Fine for parking here."
• Composition: attributing a part's property to the whole. "Every part is light, so the machine is light."
• Division: attributing the whole's property to each part. "The best team, so every player is the best."
• Texas sharpshooter: choosing the pattern after seeing the data. "Look how many winners live in this postcode."

A three-question method

• What exactly is the claim? Write it as one sentence.
• What is offered as support? Write that as a separate sentence.
• Does the support bear on the claim, is it enough, and is it honestly stated? A no to any of the three points to a family: relevance, presumption or structure.

The method works on your own arguments as well as other people's, which is where it is most useful and least comfortable.

Why naming a fallacy is not winning

Pointing out a fallacy shows that one argument fails. It does not show that the conclusion is false, and treating it that way is itself a mistake sometimes called the fallacy fallacy. The productive move after spotting one is to ask what a good argument for the same claim would look like, and whether it exists. The companion test on this site gives you twenty sentences to practise the noticing part.