Definition · Plain-language
Non sequitur
A non sequitur is an argument in which the conclusion does not logically follow from the premises that are meant to support it.
The step most authors miss
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The broken link between premise and conclusion
In a valid argument, the premises provide genuine support for the conclusion. A non sequitur breaks that link: the conclusion simply does not follow, even if the premises are true. "She is wearing a red coat, so she must be a doctor" is a non sequitur because the premise gives no reason for the conclusion. The error is one of relevance and logical connection rather than of false facts — the problem is the inference itself, not the truth of the individual statements.
Broad and narrow senses
In the broad sense, any argument whose conclusion does not follow is a non sequitur, which makes it an umbrella term covering many specific formal fallacies such as affirming the consequent and denying the antecedent. In a narrower, everyday sense, "non sequitur" labels a conclusion so disconnected from the premises that it does not fit any recognised fallacy pattern at all — a remark that seems to come from nowhere. Both senses share the defining feature: the conclusion is not warranted by what precedes it.
Formal fallacies as non sequiturs
Formal fallacies are non sequiturs caused by a defective argument form. Affirming the consequent ("if P then Q; Q; therefore P") and denying the antecedent ("if P then Q; not P; therefore not Q") are classic examples: their structure does not guarantee the conclusion, so true premises can accompany a false conclusion. Recognising these patterns is central to evaluating deductive arguments, because they can appear superficially similar to valid forms such as modus ponens while lacking their logical guarantee.
Key facts
At a glance
- Definition: a conclusion that does not follow from the premises
- Latin: non sequitur ("it does not follow")
- Type: in the broad sense, the genus of formal fallacies
- Key flaw: a broken inferential link, not necessarily false facts
- Examples: affirming the consequent, denying the antecedent
- Contrast: valid forms such as modus ponens preserve the link
Common misconceptions
What people often get wrong
Often heard: A non sequitur is an argument with false premises.
Actually: A non sequitur can have entirely true premises. The flaw is that the conclusion does not follow from them. The problem lies in the logical connection — the inference — not in whether the individual statements are factually true.
Often heard: Non sequitur just means a random or off-topic remark.
Actually: That is the loose everyday sense. In logic, a non sequitur specifically denotes an argument whose conclusion is not supported by its premises, which includes structured formal fallacies, not only disconnected comments.
Often heard: If the conclusion happens to be true, the argument is not a non sequitur.
Actually: A non sequitur can reach a true conclusion by accident. The fallacy is about whether the conclusion follows from the premises, not about whether it is true. A true conclusion drawn from irrelevant premises is still a non sequitur.
Going deeper
