Non sequitur (Latin for "it does not follow."), in formal logic, is an argument where its conclusion does not follow from its premises. In a non sequitur, the conclusion can be either true or false, but the argument is a fallacy because the conclusion does not follow from the premise. All formal fallacies are specific types of non sequitur. The term has special applicability in law, having a formal legal definition.
Here are two types of non sequitur of traditional noteworthiness:
- Any argument that takes the following form is a non sequitur:
If A is true, then B is true. B is stated to be true. Therefore, A must be true.
Even if the premises and conclusion are all true, the conclusion is not a necessary consequence of the premises. This sort of non sequitur is also called affirming the consequent.
- Another common non sequitur is this:
If A then B. Not A. Therefore, not B.
This sort of non sequitur is called denying the antecedent.
(If either of the above examples had "If and only if A, then B" as their first premise, then they would be valid and non-fallacious but unsound.)
Many other types of known non sequitur argument forms have been classified into many different types of logical fallacies. In everyday speech and reasoning, an example might be: "If my hair looks nice, all people will love me." However, there is no real connection between your hair and the love of all people. Advertising typically applies this kind of reasoning.
- An Onyx Sentinel referred to its classification of Spartan-III Ash-G099 as a Reclaimer as a non sequitur.