LogicalMANIPULATIVE

Non Sequitur

What it is

An argument whose conclusion does not follow from its premises — the premises may be true and relevant-sounding, yet nothing in them supports the conclusion drawn. The genus of which most formal fallacies are species.

How it works

Non sequitur (“it does not follow”) is the general failure of inference: a conclusion presented as flowing from premises when no valid or strong link exists. Formal fallacies such as affirming the consequent and denying the antecedent are non sequiturs with a named shape; the unnamed ones are the ones that slip through. The lever is that listeners supply missing links automatically — Grice's (1975) cooperative principle means we assume a speaker's premises are relevant and reconstruct whatever connection would make them so, so a fluent juxtaposition of premise and conclusion is heard as an argument. Fluency finishes the job: a sentence that reads smoothly feels valid. Weston (2009) recommends laying arguments out in premise-conclusion form for exactly this reason. The honest version is a speaker with a real argument in mind who has left out the step that carries it — an enthymeme with a false or missing premise. The knowing version exploits juxtaposition: “the economy grew after the tax cut,” “crime rose under the new mayor,” “our customers are the most satisfied — buy now,” each presented as reasoning while the link is never stated because, stated, it would be visibly weak. The tell is that the argument cannot be written in standard form without adding a premise the speaker did not say.

Real-world examples

  • •Presidents of both parties point to job growth during their term as demonstrating the success of their policies; the inference requires a premise about causation that is never stated, and that economists generally cannot supply for any single administration.
  • •Advertising runs on non sequiturs: a car shown on a mountain road, a person laughing over a soft drink. The premise (this product, this scene) and the conclusion (buy it) are juxtaposed, and the audience supplies the link.
  • •“He has a law degree from Harvard, so his tax plan will work” joins a true premise to a conclusion it does not reach; the credential bears on competence, not on the plan.
  • •The 2003 case for war in Iraq included the premise that Saddam Hussein had defied UN resolutions and the conclusion that he possessed weapons of mass destruction; the premise was true, and the conclusion, later shown false, did not follow from it.
  • •A meeting concludes that “the pilot got good feedback, so we should roll out nationally” — a true premise about the pilot and a conclusion about scale that requires premises about representativeness and cost that nobody stated.

Ethical guidelines

  • ●Lay your argument out in standard form before presenting it; if you need to add a premise to make it follow, say that premise out loud and defend it.
  • ●Do not rely on juxtaposition to imply an inference you would not state; if the link is unstated because it is weak, the argument is dishonest.
  • ●When you present a credential, an association, or a sequence of events, say what you claim it shows and why.
  • ●Prefer arguments that survive being written down; fluent speech hides gaps that a page reveals.

How to defend against it

  • ►Write it in standard form: premises numbered, conclusion last. Then ask which premise carries the conclusion; if none does, ask the speaker for the missing one.
  • ►Say “it does not follow” and mean it literally: “I accept the premise; show me the step to the conclusion.”
  • ►Ask the “so what?” question after each premise: “That is true — what does it establish about the conclusion?”
  • ►Check for the enthymeme: identify the unstated premise the argument needs, state it, and ask whether the speaker will defend it. Often they will not.
  • ►In advertising and political messaging, notice juxtaposition: when a claim and a conclusion sit side by side without a connective, you are being invited to build the argument yourself.

From the Defense Playbook

Argument Mappingtakes practice

Draw the argument as a diagram, with the main claim at the top, the reasons for and objections against it beneath, and the support for each of those beneath them, so that gaps, repetition, and unsupported leaps become visible instead of being carried along by fluent prose.

The Toulmin Checkminutes

Test any single argument by asking Toulmin's six questions of it: what is the claim, what are the grounds, what warrant connects them, what backs the warrant, how strongly is the claim qualified, and under what conditions would it fail?

"Which Step, Specifically?"minutes

When told that one action will lead through a chain of events to a dramatic outcome, lay the chain out link by link, ask what makes each link likely, and remember that the probability of the whole chain is lower than that of its weakest link.

Name the Missing Premiseminutes

Most everyday arguments leave out the premise that does the real work; find it by asking what would have to be true for the stated reason to support the conclusion, say it aloud, and see whether anyone is still willing to assert it.

Every playbook entry states how strong its evidence is and when not to use it. Browse the full playbook.

References

  1. Copi, I. M., Cohen, C., & McMahon, K. (2011). Introduction to Logic (14th ed.). Pearson
    The definition of non sequitur as the general failure of premises to support a conclusion, and its relation to named formal fallacies.
  2. Weston, A. (2009). A Rulebook for Arguments (4th ed.). Hackett Publishing
    The rule of laying arguments out in premise-conclusion form so that missing links become visible.
  3. Grice, H. P. (1975). Logic and conversation. In P. Cole & J. L. Morgan (Eds.), Syntax and Semantics, Vol. 3: Speech Acts (pp. 41-58). Academic Press
    The cooperative principle and maxim of relevance that lead listeners to reconstruct missing inferential links.
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