How To Spot a Logical Fallacy Part 3

Informal fallacies arguments that are logically unsound for lack of well-grounded premises.

Argument to moderation (false compromise, middle ground, fallacy of the mean, argumentum ad temperantiam) – assuming that a compromise between two positions is always correct.

  • One person saying that the sky is blue, while another claims that the sky is in fact yellow—and conclude that the truth is that the sky is green
  • So you are saying your car is worth $20k.  I think it is worth $1, so let’s just compromise and say it is worth $10k
  • Exception: When the two extremes are equally distanced from the “correct” value — and there actually is a correct, or fair, value between the two proposed values.

Divine fallacy (argument from incredulity) – involves arguing for a conclusion on the grounds that it is unimaginable for it not to be true.

  • I cannot imagine how P could be true; therefore P must be false.
    • Arguments from incredulity can sometimes arise from inappropriate emotional involvement, the conflation of fantasy and reality, a lack of understanding, or an instinctive ‘gut’ reaction
  • Back to the Future
    • Marty: Doc, I’m from the future. I came here in a time machine that you invented. Now, I need your help to get back to the year 1985.
    • Doc: I’ve had enough practical jokes for one evening. Good night, future boy!
  • Explanation: Clearly Marty is making an extraordinary claim, but the doc’s dismissal of Marty’s claim is based on pure incredulity. It isn’t until Marty provides the Doc with extraordinary evidence (how he came up with the Flux Capacitor) that the Doc accepts Marty’s claim. Given the nature of Marty’s claim, it could be argued that Doc’s dismissal of Marty’s claim (although technically fallacious) was the more reasonable thing to do than entertain its possibility with good questions.
  • Exception: We can’t possibly entertain every crackpot with crackpot ideas. People with little credibility or those pushing fringe ideas need to provide more compelling evidence to get the attention of others

Motte-and-bailey fallacy – conflating two positions with similar properties, one modest and easy to defend (the “motte”) and one more controversial (the “bailey”). The arguer first states the controversial position, but when challenged, states that they are advancing the modest position.

  • Person A: “I don’t understand why people believe in astrology, there’s no scientific evidence to support it.”
  • Person B: “The moon has enough pull to cause tides every day on Earth, but it has no effect on people? Are you trying to say humans are literal gods unaffected by nature? I guess evolution isn’t real, either!”
    • Here, Person B has substituted an easy-to-defend motte:
  • “Human beings are affected by natural forces, including the Moon’s gravity.”
    • for a controversial bailey claim:
  • “Astrology’s use of the positions of celestial bodies in the sky to make predictions about people’s personality, characteristics, and behavior is scientifically valid.”

Fallacy of composition – assuming that something true of part of a whole must also be true of the whole.

  • This fallacy is related to the fallacy of hasty generalization
    • If someone stands up out of their seat at a cricket match, they can see better. Therefore, if everyone stands up, they can all see better.
    • If a runner runs faster, he can win the race. Therefore, if all the runners run faster, they can all win the race.
  • Hydrogen is not wet.  Oxygen is not wet.  Therefore, water (H2O) is not wet.
  • Exception: If the whole is very close to the similarity of the parts, then more assumptions can be made from the parts to the whole.  For example, if we open a small bag of potato chips and discover that the first one is delicious, it is not fallacious to conclude that the whole snack (all the chips, minus the bag) will be just as delicious
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