Non sequitur
A non sequitur is a conclusion that does not follow from the statements offered to support it. The Latin phrase means literally "it does not follow", and in logic it names the most general failure an argument can have: whatever the premises establish, it is not the thing being claimed.
What a non sequitur is
A non sequitur is a break in the link between premises and conclusion, and every other fallacy is a named way of producing one. What distinguishes it from its siblings among the logical fallacies is its rank. Ad hominem, the straw man fallacy, the red herring and the false dilemma are all specific routes to the same destination, which is a conclusion the premises do not reach. This is why non sequitur is best treated as the parent category rather than as one more entry on a list of a dozen.
The phrase carries two senses in English and they are worth separating at the start. In logic, a non sequitur is a defective inference, and the sentences involved may be perfectly on topic. In ordinary conversation, a non sequitur is a remark with no visible connection to what preceded it, which is a fact about relevance rather than about argument. This page is about the first, and the second is where the word is met most often in media and everyday speech.
The shortest accurate definition, with the synonyms and the antonym
The shortest accurate definition is three words: the conclusion overreaches. The premises may be true, they may be relevant, and they still do not deliver what is being claimed.
The near synonyms are invalid inference, formal fallacy, fallacious reasoning and the plain English "that does not follow". They are not exact equivalents: formal fallacy is narrower, since some non sequiturs fail on relevance rather than on structure. As for an antonym, English does not supply a common one, though the rare word sequitur exists. The functional opposites in logic are valid inference and entailment, meaning a conclusion the premises make unavoidable. Pronunciation in English is conventionally "non SEK wit er", with the stress on the second syllable.
The form it takes: five classic patterns
The 5 patterns below account for most non sequiturs that arise from structure alone, and all 5 can be spotted without knowing anything about the subject.
- Affirming the consequent. If the seal failed, the tank leaks. The tank leaks. Therefore the seal failed. A cracked weld would produce the same leak.
- Denying the antecedent. If the seal failed, the tank leaks. The seal did not fail. Therefore the tank does not leak. Same problem, mirrored.
- Undistributed middle. All calibration rigs are inspected annually. This device is inspected annually. Therefore it is a calibration rig. Sharing one property does not make two categories the same.
- Affirming a disjunct. Either the belt is worn or the motor is misaligned. The belt is worn. Therefore the motor is aligned. Both faults can be present at once.
- Illicit scope shift. Every station has a supervisor. Therefore there is a supervisor who covers every station. Two very different claims share a sentence shape.
Compare these with the 2 valid forms they imitate. Modus ponens runs: if A then B, A, therefore B. Modus tollens runs: if A then B, not B, therefore not A. Both are sound; the five patterns above are near misses of them, which is exactly why they persuade.
A worked example, and the repair
Here is a worked example with the repair. A warehouse manager writes in a report: "Stock discrepancies dropped after we installed the new scanners, so the scanners fixed our counting problem."
Set it out in lines and the gap appears:
- Premise: discrepancies were lower after installation than before.
- Conclusion: the scanners caused the reduction.
What is missing is any premise linking the two, and several rival explanations are untouched. New equipment came with training. Staff knew counts were being watched. The comparison period included a quieter season. The repair is not to reject the conclusion but to name the missing premise out loud and then ask whether anyone can supply it: "For this to follow, nothing else that affects counting can have changed in the same window. Did anything?" A non sequitur is repaired either by supplying the missing link or by weakening the conclusion until the premises can carry it, and the second option is available far more often than people use it.
What a non sequitur is confused with
A non sequitur is confused with 2 things in particular: a red herring, which is a deliberate change of subject rather than a broken inference, and the word fallacy itself, which names the whole family rather than this member of it. 2 comparisons resolve the rest.
| Non sequitur | Red herring | |
|---|---|---|
| What it names | A conclusion the premises do not reach | A deliberate change of subject |
| Where the fault sits | In the inference | In the relevance of the reply |
| Intent | Usually accidental | Usually evasive |
| Category | The general class | One informal species within it |
On the second comparison, fallacy versus non sequitur: a fallacy is any faulty pattern of argument, and a non sequitur is the broadest fallacy of them all. Formal fallacies are non sequiturs whose failure is visible in the structure. Informal fallacies are non sequiturs whose failure is visible only once you know the context. Saying "that is a non sequitur" is accurate but unspecific, rather like saying a car has a mechanical problem, which is why the more precise names remain useful.
When a gap between premises and conclusion is not a fault
A gap between premises and conclusion is not automatically a fault, because most useful reasoning is not deductive. Three cases are legitimate.
- Argue inductively, and say so. "9 of the last 10 shipments were on time, so the next one probably will be" does not follow with certainty and was never claimed to. Weakening the conclusion with probably closes the gap honestly.
- Infer to the best explanation. A puddle under a parked van and a wet trail to the curb support the conclusion that it leaked, not because the inference is airtight but because the alternatives are worse. Occam's razor helps rank the candidates here, and it is worth remembering that it ranks, it does not prove.
- Reason from an assumption you have declared. "Assuming the meter is accurate, consumption rose by 20 percent" is a conditional conclusion with its condition attached, which is not a leap but a labeled step.
The honest limit: calling something a non sequitur proves only that this particular argument fails, not that its conclusion is false. Someone can defend a true claim with an argument that does not follow, and treating the bad argument as a refutation of the claim is itself a recognized error.
The test to run: state the missing premise
Run this test on any argument that moves quickly. Ask: what sentence would have to be added here for the conclusion to follow, and is anyone willing to assert it?
Write the missing sentence out in full, because vagueness protects a non sequitur and specificity kills it. Then ask the follow up, which is how to avoid a non sequitur in your own writing: could my premises all be true while my conclusion is false? If they could, the conclusion needs either more support or fewer words, and cutting the claim is usually the faster fix.
Non sequitur in everyday use, and in the context of fallacies
Two contexts use this term and they pull in different directions, so it is worth setting them side by side before you use the phrase in public.
In everyday use, a non sequitur is a remark that does not connect to what came before. A colleague answers a question about the printer with a comment about a film they watched. Nothing is being argued, so nothing is being argued badly. Comedy writers use the device deliberately, and a cartoon strip built on non sequiturs is doing craft, not committing a fallacy.
In the context of fallacies, a non sequitur is a defective inference, and the two sentences involved are usually about the same subject. "The warehouse is clean, so the stock count is accurate" is entirely on topic and still does not follow. That is the sense that matters when you are checking an argument, and it is the sense most glossaries and logic courses mean.
The practical advice is short. Use the everyday sense in conversation, where it is understood, and in an argument use a more specific name when one fits, since "affirming the consequent" or "hasty generalization" tells the other person what to fix and "non sequitur" only tells them that something is wrong.