The Non sequitur reference article from the English Wikipedia on 24-Jul-2004
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Non sequitur

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Non sequitur is Latin for "it does not follow." To say that an argument is a non sequitur is simply to say that the conclusion does not follow from the premises. This term would apply to any argument that has a conclusion that doesn't follow from its premises. It is often used, however, to refer to particular types of arguments that clearly do not follow from their premises and never could.

Two types of non sequitur are traditionally identified. For example, any argument that takes the following form is a non sequitur:

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

It is clear that this argument does not follow. Even if the premises and conclusion were 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:

  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." There is no real connection. Advertising typically applies this kind of 'deduction'. Another example: "If I read a book it will rain."

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 non sequiturs. Examples are the comic strip Zippy the Pinhead, the television series Monty Python's Flying Circus, and the fiction novels written by Douglas Adams.