Non sequitur

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Si vous voulez cet article en français, il peut être trouvé à Non sequitur (français).
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Non sequitur: when a train of thought proceeds from A to B and back again to Q.
—Bill Griffith, Zippy the Pinhead[1]

Non sequitur is a Latin phrase that means “that which does not follow”. It means that the conclusion reached does not follow from the premise(s). Often examples of non sequitur arguments are hilariously disconnected, but those encountered in the wild can be subtle and may not be easily uncovered. The reason that such arguments are fallacious in logic should be fairly obvious.

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[edit] Blatant examples

Examples of non sequitur arguments:

  • All men are mortal, and Socrates is a man, therefore Socrates likes women.
  • I sit at a table to read, and the elements are organized in a table, therefore reading is elementary.
  • This baloney is smiling. Your argument is invalid.

[edit] Subtle examples

A fine example of a non sequitur argument. The premise (a cat pushes a watermelon out of a lake) is unlikely to have anything to do with the argument (unless the debate is over whether a cat could push a watermelon out of a lake).

[edit] Men and machines

Non sequiturs can also be more subtle than the above, such as the following one, formulated by Alan Turing:

  • P1: If each man had a definite set of rules of conduct by which he regulated his life he would be no better than a machine.
  • P2: However, there are no such rules.
  • C: Therefore, men cannot be machines.

While this may seem plausible on the surface, it is still a non sequitur because 1) the first premise says nothing about whether or not men can be machines, but whether they would be better than (or, depending on how you unpack this term, greater than, more than, beyond, or simply more capable than) machines; and 2) it ignores the possibility that men could be machines without such definite rules. Therefore, the conclusion does not follow from the premises.

[edit] Jesus is savior

Another example can crop up among the careless of faith:

  • P1: If the Bible is correct, then we must accept Jesus as our personal savior.
  • P2: Jesus is my personal savior
  • C: Therefore, the Bible is correct.

Here, the fallacy is that P1 and P2 may in fact be correct, but C is not a valid conclusion based on P1 and P2. It does not follow that the Bible must be correct because one person finds that Jesus is their savior, even if to a believer this seems like the case. More specifically, this is affirming the consequent.

[edit] Belief in belief

A subtle but easy use of the fallacy is related slightly to belief in belief. It could be formed like this:

  • P1: Religion gives great comfort to people and causes them to do good things.
  • P2: These people are comforted and are good people.
  • C: Therefore God is real.

The non-sequitur here is a failure to distinguish between what the act of believing can accomplish, and the (almost) completely unrelated question of the truth value of that belief. Someone could hold a belief in God and do good things (P1) but, because this is due to the belief itself, their good works and feelings of comfort (P2) would still be true whether God exists or not. Without external evidence to say otherwise, you cannot tell the difference between someone being comforted because God exists, or because they just believe God exists. In short: the premises say nothing about the existence of God, only that some people believe in it.

[edit] The ad hominem attack

  • P1: You're a beautiful woman.
  • C: Therefore you're wrong.

The non-sequitur here is obvious: your status as a beautiful woman in no way affects the logical soundness or truth value of your arguments. One can be the most odious person in the history of humankind and yet make logically sound arguments based on demonstrably true premises.

[edit] See also

[edit] External links


[edit] Footnotes

  1. Zippy the Pinhead is a core resource for rationalists with a sense of humour, who from time to time like to let their minds off the leash for a period of delightful unhingeditude. Arbitrary donuts optional.
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