LFotW: Non-Sequitur

In Latin this term translates to “doesn’t follow.” This refers to an argument in which the conclusion does not necessarily follow from the premises. In other words, a logical connection is implied where none exists. This is the most basic type of logical fallacy, and in fact many of the fallacies listed below are also non-sequiturs but are an identifiable and common type.

Subtype: Magical Thinking
Magical thinking is perhaps the most common type of non-sequitur. The logical connection in these fallacious arguments are often a vague magical principle not based upon logic or evidence. For example, Feng Shui proponents might argue that it is better to place your bed in a certain position within your bedroom so that luck will flow into your home.

Here are two types of non sequitur of traditional noteworthiness:

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

1. If A is true, then B is true.
2. B is stated to be true.
3. 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.

An example of affirming the consequent would be:

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

"I" could be another type of mammal without being a human. While the conclusion may be true, it does not follow from the premises. This argument is still a fallacy even if the conclusion is true. It is a non sequitur (note that it is the exact same argument form as in example 1 - the form is always a non sequitur).

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 anywhere else 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 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.