Logical Fallacies — the Non Sequitur


This cat has one eye... your argument... ...is invalid
This post will deal with a common error in 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 where the conclusion doesn’t follow from the premises, or any argument where a logical connection between premises is implied that just isn’t there.

This is one of the 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 mysticism 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 obviously ridiculous example would be:

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

There is also the fallacy of Denying the Antecedent, taking the form of: If C is true, then D is true. C is false. So, 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. So, I’m not in Greece.

…And lastly the 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. So, she wants to get me transferred to another project.

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

One problem people sometimes have with this fallacy is that it can be subtle, and they are often too proud to speak out when they cannot see how an argument follows, or too polite to point out its lack of relevance to the speaker.

It’s 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.

Talotaa frang.

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