This article is about the logical fallacy. For the comic strip, see Non Sequitur (comic strip).

Non sequitur is Latin for "it does not follow." An argument is called non sequitur if the conclusion does not follow from the premises. It should be stressed that 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 premises. All logical fallacies are actually just specific types of non sequiturs.

Here are two types of non sequiturs of traditional noteworthiness:

1) Any argument that takes the following form is a non sequitur:

1. If A then B. (e.g. If I am a cat, I am a mammal.)
2. B. (e.g. I am a mammal.)
3. Therefore, A. (Therefore, I am a cat.)

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.

2) Another common non sequitur is this:

1. If A then B. (e.g. If I am in Tokyo, I am in Japan.)
2. Not A. (e.g. I am not in Tokyo.)
3. Therefore, not B. (e.g. Therefore, I am not in Japan.)

The speaker could be in all kinds of other places in Japan. 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.)

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 'deduction'. Another example: "If I read a book it will rain."

Here is another humorous example, attributed to Tim Vine:

One in every five people is Chinese.

There are five people in my family.

There's me, my mum and my dad, my brother Colin, and my brother Ho-Cha-Chu.

(I think it's Colin.)

Non sequitur can also be used to mean a seemingly disconnected or random comment that is not particularly relevant to the discussion, such as a random subject change.

Much humor is based on deliberate non sequiturs. Examples are the comic strips Zippy the Pinhead and Non Sequitur, the television series Monty Python's Flying Circus, the animated cartoon Family Guy, the fiction novels written by Douglas Adams and the stand-up comedy of Mitch Hedberg.

See also

• regression fallacy
• post hoc ergo propter hoc