The fallacy families
Formal fallacies break the pattern. Informal ones break relevance, presumption, cause, or meaning.
Ask the four questions in order and stop at the first "no."
- Is the form valid? If the argument has a named invalid pattern, it is a formal fallacy (for example, affirming the consequent).
- Are the premises relevant to the conclusion? If they attack the person, the crowd, or someone's feelings instead of the claim, it is a fallacy of relevance.
- Do the premises assume something they haven't earned? If they assume the conclusion, only two options, or a hidden claim, it is a fallacy of presumption.
- Does a causal claim or a key word carry more than it can? A cause read off timing or correlation, or a slide down unsupported links, is a causal fallacy. A word or grouping that shifts meaning is a fallacy of language.
Naming the family is only half the job. Say in one sentence what the premises fail to do. A fallacy shows the support is bad, not that the conclusion is false.
Formal fallacies (a broken pattern)
An invalid argument form that looks like a valid one.
The invalid form: if P, then Q; Q; so P.
Example
"If it rained, the grass is wet. The grass is wet. So it rained." A sprinkler also wets grass.
The invalid form: if P, then Q; not P; so not Q.
The invalid form: P or Q; P; so not Q.
A syllogism whose middle term is not distributed in either premise.
The categorical-logic cousin of these. The same "the pieces don't connect" defect.
Fallacies of relevance (premises beside the point)
A fallacy whose premises, true or not, have nothing to do with the conclusion.
Term
ad hominem
Attacking the person making an argument instead of the argument.
Example
"Don't trust her budget plan. She failed algebra in school."
Term
tu quoque
Dismissing a claim because the person making it doesn't follow it.
Term
straw man
Attacking a distorted, weaker version of someone's position.
Term
red herring
Changing the subject to something that sounds related but isn't.
Treating "lots of people believe it" as a reason to think it's true.
Using feelings such as pity or fear in place of reasons.
Using a threat in place of reasons.
Citing someone whose expertise doesn't cover the question.
Judging a claim by where it came from instead of by the evidence for it.
Concluding something is true because it hasn't been disproved, or false because it hasn't been proved.
Fallacies of presumption (an unearned assumption)
A fallacy that quietly assumes something it has no right to assume.
Assuming, in the premises, the very conclusion you are trying to prove.
Presenting two options as the only ones when there are others.
Example
"Either we cancel the trip or we get soaked." You could also bring umbrellas.
A question that smuggles in an assumption, so any direct answer accepts it.
Drawing a general conclusion from too few or unrepresentative cases.
Applying a sound general rule to an exceptional case it was never meant to cover.
Causal fallacies (a cause that wasn't shown)
Term
post hoc
Concluding that A caused B only because B came after A.
Example
"I wore my lucky socks and we won, so the socks helped."
Term
false cause
Claiming a cause on evidence that doesn't support one.
Term
correlation
Two things tending to vary together.
A correlation alone never settles which way a cause runs, or whether a third factor drives both.
Naming one cause for an outcome that had several.
Claiming one step will set off an unstoppable chain of worse ones, without support.
Fallacies of language (a meaning that shifts)
A fallacy that works only because a word or sentence can be read two ways.
Switching between two meanings of the same word within an argument.
Example
"A feather is light. What is light can't be dark. So a feather can't be dark."
Term
amphiboly
An argument that relies on a sentence's ambiguous grammar.
Reasoning that what is true of each part must be true of the whole.
Reasoning that what is true of the whole must be true of each part.