What is the difference between post hoc fallacy and non sequitur fallacy?

  • Post hoc fallacy infers causation from mere order of events, non-sequitur infers a conclusion that lacks logical connection to the premise. The latter is much more generic, any fallacy is a non-sequitur. – Conifold Nov 28 '20 at 23:48

Saying something is a non-sequitur typically implies that a formal fallacy has been committed; that a deductive argument has been made which has an invalid form. The term can also be used more casually to refer to something which more generally just doesn't make sense (especially by way of the last part of a statement/assertion being unrelated to what came before it). But in any more technical capacity, the first sense would be preferred.

The post hoc fallacy is an informal fallacy; while it may represent faulty reasoning in some way, it does not actually constitute a formally, logically invalid argument. "Post hoc ergo propter hoc" assumes that because one event followed another, it must have been caused by it. This may or may not be the case; some things appear to be caused by some preceding event(s), but not everything which follows an event seems to be "caused" by it. Specious reasoning? Yes. Logically invalid? Not quite.

Tl;dr: non-sequitur = formal fallacy, post-hoc = informal fallacy.

  • thank you! Brilliant answer! I'm stilling getting a grasp on all this. Can you give me examples to help me understand the points you are making? – Frank McCain Nov 28 '20 at 22:37
  • Non-sequitur example: All odd numbers contain the letter 'E'; seven is an odd number; therefore seven does not contain the letter 'E'. An argument is "logically valid" if it takes a form that would make it impossible for the premises to be true but the conclusion to still be false. In my example, the two premises seem to be true, but the conclusion false, meaning that that syllogism would not be logically valid. "Logical validity" is one of the central ideas in formal logic. – adHawk Nov 28 '20 at 22:57
  • Post hoc example, in two parts: 1. I did a rain dance yesterday, and it rained today, therefore the dance caused the rain. 2. I studied thoroughly for my test, and ended up acing it, therefore my studying must have been what caused my success. I'm actually not completely certain myself on the formal logic breakdown of these arguments, but informally it should be evident that one event following another doesn't necessarily imply nor automatically preclude a causal link between the two. The error seems to be less with the form of the argument, but rather something else. – adHawk Nov 28 '20 at 23:07
  • Ok great. thank you. I understand the post hoc better now because of your help. I am still not super clear on non-sequitur though. – Frank McCain Nov 29 '20 at 1:22
  • Have a look at the "Formal Fallacy" Wikipedia page, or perhaps this wonderful article from the Stanford Encyclopedia of Philosophy. I'm sure other quality resources will do a much better job than I could, even without a 600 character limit 😆 – adHawk Nov 29 '20 at 1:32

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