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.”