Definition and basic idea
Non sequitur is a Latin phrase meaning "it does not follow." In logic and critical thinking it describes an inference in which the conclusion is not supported by the stated reasons. As a type of logical fallacy, a non sequitur can arise when a purportedly deductive claim fails to preserve truth from premises to conclusion. An argument may contain true premises yet still reach a conclusion that does not follow logically; when that happens, the inference is invalid and termed a non sequitur.
Typical examples
Simple examples make the error clear. Consider a mistaken categorical pattern drawn from a syllogism:
- All birds have wings.
- This creature has wings.
- Therefore, this creature is a bird.
The conclusion does not follow because other animals also have wings. Other common forms include drawing a causal conclusion from mere correlation or introducing an unrelated assertion as if it were a consequence of earlier claims.
Types and related distinctions
In practice "non sequitur" covers several situations. It is often used as a catch‑all label for unnamed formal errors (invalid deductive moves), but it also overlaps with informal fallacies where relevance or sufficient support is missing. Related terms include ignoratio elenchi (arguing a different point), affirming the consequent, and denying the antecedent — specific invalid patterns that illustrate how conclusions can fail to follow.
Everyday uses and rhetorical contexts
Outside strict logic, the term describes abrupt remarks that appear unrelated to the preceding conversation — a comedic device in writing and speech known as a "comic non sequitur." In rhetoric and debate, calling an opponent's claim a non sequitur highlights a gap between evidence and conclusion and requests clarification or additional premises.
How to detect and avoid non sequiturs
To test an argument, make its premises explicit and ask whether accepting them would make the conclusion very likely. If not, the inference is weak or invalid. Formal methods involve checking argument form for validity; informal checks look for missing steps, hidden assumptions, or irrelevant information. Teaching clear premise–conclusion links and spotting unstated defeaters are practical ways to reduce non sequiturs.
Further reading
For introductions to related concepts, consult resources on argument structure, deductive logic, and catalogues of common fallacies. Explanations of premises and logical support are available in many critical‑thinking guides and textbooks; see materials linked under premises and introductory lists of fallacious patterns at logical fallacy inventories. For historical and rhetorical notes, see treatments of medieval Latin terminology and how the phrase entered modern usage in logic and literary comedy at syllogism studies and general discussions about birds as classic examples in teaching logic.