Lecture 5: Informal Fallacies

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Lecture 5Informal Fallacies

• Marc A. Moffett

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What is a Fallacy?

• Definition 5.1. A fallacy is an error in an
  argument that makes it unacceptable.
• Errors can be of two kinds
• Logical errors (i.e., errors which bear on the
  validity of the argument).
• Factual errors (i.e., errors which bear on the
  soundness of the argument).
• Note that factual errors are not necessarily
  errors in the sense of mistakes. They are often
  made on purpose for rhetorical effect.

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Overlooking Alternatives

• Considering only a small number of salient
  possibilities without looking for less obvious
  ones.
• This is the main error behind many argumentative
  fallacies.
• Consider, for example, false dilemma.

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Hasty Generalization

• Generalizing on the basis of too little evidence.
• Roughly, inductive inferences generalize from the
  properties of a subgroup to the properties of a
  group.
• If you do not have enough data, the data you do
  have might not be representative.

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False Cause

• Assuming that because one event follows another
  event, the first event must cause the second
  event.
• Post hoc, ergo, propter hoc.
• Other alternatives
• Common Cause
• Coincidence

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• Note that causes really do precede their effects.
  What makes the difference between good causal
  reasoning and false cause?
• One possibility is fitting into an explanatory
  scientific framework.

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Causal Reduction

• Occurs when someone reduces a complex set of
  causes into a single, simple cause.
• E.g., Guns dont kill people people kill people.

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Fallacy of Equivocation

• Occurs when a word is used with two different
  meanings in the course of the same argument.
• e.g., Representative Largent of Oklahoma, arguing
  in Congress on July 11, 1996 in favor of the
  "Defense of Marriage Act", a bill stipulating
  that 'marriage' be between a man and a woman
  only, excluding homosexual marriages "Let me
  just say first of all that this is not about
  equal rights. We have equal rights. Homosexuals
  have the same rights as I do. They have the
  ability to marry right now, today. However, when
  they get married, they must marry a person of the
  opposite sex, the same as me."

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Straw Man Fallacy

• Occurs when someone attacks a less defensible
  position than the one actually being put forth.
• "Advocates of legalized abortion predicted it
  would solve our social problems. Instead, this
  destruction of one-fourth of a generation has
  left a more violent society in its wake Child
  abuse has exploded, from 167,000 estimated cases
  in 1973 to 2.4 million in 1989, according to the
  National Center of Child Abuse and Neglect - a
  1,400 increase. Teen suicide, among non-aborted
  and thus presumably "wanted" children, has
  doubled. Violent crime has more than doubled." -
  "The Post Abortion Report", published by
  Fresno/Madera Right to Life

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Slippery Slope Fallacy

• Occurs when someone claims that a given act will
  result in a series of events whose culmination is
  plainly unacceptable when, in fact, the series of
  events is very unlikely to occur.

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• E.g., "I think that the use of marijuana as a
  medical treatment shouldn't even be considered.
  If we make drugs legal in a few cases, then we
  might eventually have to completely legalize them
  - which is even crazier than Proposition 215. If
  we want to help people out by letting them do
  illegal things, then let's just get rid of all
  our laws." - letter to editor of Newsweek,
  November 11, 1996.

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Naturalistic Fallacy

• Occurs when one infers normative or evaluative
  claims from purely descriptive ones.