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.”