arguments and fallacies

structuring arguments

basic structure: syllogisms

  • syllogism: an argument with premises and a conclusion
  • two kinds of syllogisms   * categorical     * "all A are B,"      * "some A are B",      * "no A are B" or      * "some A are not B”   * hypothetical     * “if, then” statements
  • categorical syllogisms   * eg. all huskies are dogs.   * all dogs are mammals.   * therefore, all huskies are mammals.
  • hypothetical syllogisms   * use of if, then statements to prove a point     * “if statement P is true, then statement Q must also be true.     * letters stand for variables/phrases that can be inserted into the syllogism     * must also follow certain rules of

fallacies

  • fallacy: a mistaken belief, especially one based on unsound argument

  • modus ponens fallacy—denying the antecedent (invalid)   * if it’s raining, then the street will be wet   * it’s not raining   * therefore, the street is not wet   * why isn’t this valid?     * there may be other things wetting the street, eg. a street sweeper, the neighbor washed his car, the fire-dept. was cleaning out the hydrants, etc.

  • modus tollens fallacy—affirming the consequent (invalid)   * if it’s raining, then the street will be wet   * the street is wet   * therefore, it must be raining   * why isn’t this valid?     * again, street sweeper, neighbor washed his car, fire-dept. cleaning out the hydrants, etc.

  • ad hominem (attack against the person)   * an attempt to discredit the argument by discrediting the person making it   * “You like kale? Oh yeah? Well, you’re an idiot!”

  • ad populum (appeal to the masses)   * saying an argument is true because many people believe it   * “Everyone I talked to agrees that the brontosaurus is the best kind of dinosaur ever. Clearly you’re wrong.”

  • improper appeal to authority   * saying an argument is true because some authority says that it is   * “Well, Richard Dawkins says there is no God, and he teaches at Oxford!”

  • false dichotomy   * forces a choice between two options when there are really more available   * “If you don’t support lower taxes then you must hate the free market you stinkin’ Commie!”

  • straw man arguments   * instead of arguing against the opponent’s real argument, one argues against a weakened version of it which may be difficult to identify   * “You only want free healthcare so you can have the government pay for all of your problems

  • non sequitur (does not follow)   * an argument in which the conclusion does not really follow the premises   * “Of course Congress is a mess.  DC was designed by a Frenchman.”

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