Non sequitur means, in Latin, “  who does not follow the Prémisse s  ”. In formal logic, an argument is a not sequitur if the conclusion does not follow the premises. It is notable that in a not sequitur , the conclusion can be either true or distorts, but the argument is a Sophisme, because the conclusion does not follow the premises. All sophisms are in fact of the kinds different of not sequiturs .

Any argument which takes the following form is a not sequitur :

1. If has is true, B is true.

2. B is true.
3. Thus, has must be true.

Example:

1. If I am in Tokyo (A), then I am in Japan (B).

2. I am in Japan.
3. Thus, I must be in Tokyo.

Whereas actually, I could be anywhere in Japan.

The following form is also a not sequitur :

1. If has is true, then B is true.

2. has is false.
3. Thus, B is false.

Example:

1. If I am in Tokyo (A), then I am in Japan (B).

2. I am not not in Tokyo.
3. Thus, I am not in Japan.

This sophism is resulting from errors on the mathematical concept of implication (or sufficient condition )
The assertion: " If has is true, then B is vraie" can say " Has implies B" in mathematics (and formal language: " WITH => B " )

To be in Tokyo is a sufficient condition to say that one is in Japan, but it is not necessary , (no matter what can say certain tourist guides). Thus one can be in Japan without being in Tokyo, (or differently formulated: the fact of not being in Tokyo does not mean that one is not in Japan.)

The error of reasoning (desired or not by its author) makes us give to this implication more force than it really does not have any. He allotting by error the force of a equivalence , or condition " necessary and sufficient ". Equivalence being a logical bond much stronger and much easier to use between two assertions.

The term of nonsequitur has a special application in Droit, under a formal legal definition.

See too

• Sophism
• Quaternio terminorum

Zh-yue: 不當結論