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.”