Logical Fallacies — the Non Sequitur

Welcome, all. This post will deal with a common error in both formal and informal reasoning known as the Non Sequitur, or in English, It does not follow.

This term is often applied in a general sense to any sort of logical fallacy, more specifically referring to any chain of reasoning and argumentation where the conclusion doesn’t follow from the premises, or any argument where a logical connection between premises is implied that just ain’t there.

Most of the time, this is one of the easiest and most ‘generic’ fallacies to pick out and identify, often found alongside other forms of invalid reasoning in the very same statement. Here’s a couple of handy examples of the most common form:

• Our cult shall be feared by all, for Cthulhu is mighty.

• Scientists claim that it is impossible to create or destroy energy, so whatever brand of mystical spiritism (as opposed to spirituality) I propound on must be true.

But there are more subtle forms of this fallacy as well:

There is the formal Fallacy of the Undistributed Middle, in which a conclusion is invalidly drawn from two given or assumed premises, and takes the form of: All Xs are Cs. A is a C. Therefore, A is an X. An example would be:

• All birds have endothermic metabolisms. My kittehs have endothermic metabolisms. Therefore, my kittehs are birds.

There is also the formal and informal fallacy of Denying the Antecedent, taking the form of: If C is true, then D is true. C is false. Therefore, D is also false. A good example would be:

• If I am in Classical Athens, I’m in Greece. I’m not in Classical Athens. Therefore, I’m not in Greece.

…And lastly the formal and informal fallacy of Affirming the Consequent, which takes the form: If C is true then D is true. D is true. Therefore C is true. This last one has as an example:

• If my Senior Technician intends to transfer me to another project, she’ll have a talk with the Program Director. My Senior Technician is going to talk with the Program Director. Therefore, she wants to get me transferred to another project.

This last is clearly an example of invalid reasoning, because the Senior Tech could be seeing the Program Director for entirely different reasons than those given.

My only problem in encountering arguments using this fallacy is overcoming the temptation to lazily commit the knee-jerk response of identifying errors in reasoning with its generic form, rather than examining an argument for less low-hanging fruit so to speak.

It’s particularly important to more specifically pick out what is being said as a less general sort of fallacy, including the Non Sequitur’s aforementioned variants, as well as other forms of specious reasoning as dealt with in this series of posts.

Ahui hui.