Logical Fallacies

  1. Fallacies
    1. Formal Logical Background

      Formal logic is deductive. The conclusions drawn are inevitable based on the premises. The basic component of the formal logical argument is the syllogism which has two premises and a conclusion. Three types of syllogism are :

      1. Categorical

        Premises use the terms "all", "some", or "none" to separate items into categories. Example: All men are mortal / Socrates is a man / therefore Socrates is mortal. (A syllogism the Athenians tested). Conclusions may be drawn from set theory.

      2. Conditional

        These begin with an "if-then" statement. The "if" part is called the antecedent, and the "then" part is the "consequent".

        Arguments of this type are true if the antecedent is true, or if the consequent is denied. Denying the antecedent or affirming the consequent is not a valid form of argument.

        For example consider: If its pure snow, its color is white


        If the consequent is denied, "It's not white, so it can't be pure snow" the result is true.
        But affirming the consequent, "It's white therefore it must be snow" is not valid. Neither is "It's not pure snow, therefore it can't be white".

      3. Disjunctive

        These syllogisms start with an "either-or" statement. The argument asserts or rejects one of the parts and draws conclusions about the other part. The tricky part is that "or" has two definitions in English, exclusive (only one of the statements may be true, but not both) or non-exclusive (both statements may be true).

    2. Formal Logical Fallacies
      1. Fallacy of Propositional Logic
        1. Affirming a Disjunct

          (p or q) / p / therefore not-q

          This is ambiguous because of the meaning of "or". It is implied to be an exclusive "or" (only one may be true), but in fact, it may be "inclusive" (one or both may be true)

        2. Affirming the Consequent

          if p then q / q / therefore p

        3. Commutation of Conditionals

          if p then q / therefore q then p

          Aristotle in 'On Sophistical Refutations' gives the example, "If it has been raining, the ground is wet, therefore if the ground is wet, it has been raining". Which of course is not always the case.

        4. Denying the Antecedent

          if p then q / not p / therefore not q

        5. Improper Transposition

          if p then q / not p / therefore not q

      2. Fallacy of Quantificational Logic
    3. Informal Fallacies
      1. Argumentum ad Ignorantiam / Argument from Ignorance / Absence of proof is proof of absence

        Science News in the April 6 edition on p222 has an article entitled "American Pre-History / New World hunters get a reprieve".

        The text reads: "The number of spear points found at Clovis sites indicates that there were too few hunters to have wiped out entire species on their own...."

        My question is "How do we know that other Clovis sites [Prehistoric camp sites] don't also exist?" Dozens of other Clovis sites, with enough spear points to decimate boughs of species, may exist somewhere under malls and fields of wheat. We just don't know. Just because we can't see them, doesn't mean they aren't there.

      2. Appeal to alleged common knowledge

        Example From a spokesperson for SCO "Linux doesn't exist. Everyone knows Linux is an unlicensed version of Unix,"

      3. An activity is recommended because it has a good result.

        One study says, "Drinking one glass of wine a day is good for your heart." The implication is that its good to drink a glass a day. But although moderate drinking is good for your heart, it raises the risk of breast cancer. Just because an activity has a single good side effect, it doesn't mean that it lowers your total mortality risk, because it could be more detrimental in other aspects.

      4. Pejorative Language

        Using to create an emotional appeal to an argument.

      5. Leading with Information

        Before asking a question, giving information that causes bias

      6. False Dilemmas

        Question asked of our 5 year old: "Can you do the Monkey bars at School now because you are stronger or because You are taller?"
        Our daughter: "No, I'm braver."

      7. An example from Parenting Magazine using all of the above four techniques.

        The Trouble With Movies Kids & TV: A Get-Real Guide

        What Do You Think About Media Ratings?

        PG-13. TVY7-FV. E. Parental Advisory. What do these mean to you?

        Very little, say researchers at the National Institute on Media and the Family. Parents told them last year that rating systems for movies, TV, video games, and music are confusing and inconsistent and not strict enough.

        A group that includes the National Institute, the American Medical Association, and the National PTA is calling for a universal rating system and a monitoring committee made up of parents, experts, and entertainment officials.

        Senator Joe Lieberman (D-CT) and Representative Steve Israel (D-NY) proposed bills last spring that would allow the Federal Trade Commission to regulate the current rating systems as well as fine companies that market adult material to children. Both the Senate and the House bills are still in committee.

        Hollywood leaders, including the Motion Picture Association of America and the Recording Industry Association of America, oppose these measures. They say they've already made changes in response to criticism over marketing techniques and argue that each medium is so different that it would be impossible to establish a universal rating system. What's more, they say, an oversight panel would have trouble reviewing the 650 films, 1,300 computer and video games, and 40,000 music releases each year not to mention 2,000 hours of daily TV programming.

        [In the print version they followed this with a mail-in survey:]

        Where do you stand? (check one):

        1. There should be one rating system and an oversight board.
        2. The four systems in place can work, but entertainment is often mislabeled.
        3. The rating and monitoring systems are fine as they are.
        How does the above article use the methods mentioned above?
  2. Argumentation Techniques
    1. Overstate your opponent's case

      Go way beyond what your opponent is really saying and then destroy the argument (similar to 'straw man').

    2. Pathetic Anecdotal Case

      Find a single example of how your opponent's cause has or will hurt someone. For example, suppose a cure for all cancer was found that cost a dollar and tasted like chocolate. On the news, interview a person who had just gone into debt studying as a nurse and now has no job to feed their children and no way to repay the loan. It doesn't hurt if they break down and cry during the interview. Don't detail statistics on how the cure will help millions of people, just focus on this one pathetic case.

  3. Favorite Links
    1. Informal Fallacies
    2. 38 Ways to Win an Argument