Evaluating Arguments

1
Evaluating Arguments

• Deductive
• Validity
• Soundness
• Inductive
• Strength
• Cogency

2
Validity

• Relates to the form of the argument.
• Ask, is there any interpretation of the argument
  in which the premises could be interpreted as
  true and still lead to a false conclusion?
• If not the argument is valid.
• If yes, the argument is invalid.
• Validity is a binary attribute. Like death and
  virginity, an argument cannot be a little
  invalid.

3
Validity (cont)

• E.g.
• All television networks are media companies.
• NBC is a television network.
• Therefore NBC is a media company.
• Assume all premises to the true.
• Given that assumption is it possible for the
  conclusion to be false.

4
Validity (cont)

• All auto manufacturers are computer
  manufacturers.
• United Airlines is an automaker.
• Therefore, United Airlines is a computer
  manufacturer.
• All banks are financial institutions.
• Smith-Barney is a financial institution.
• Therefore, Smith-Barney is a bank.

5
Soundness

• An argument is considered sound if it is valid
  and all premises are true.
• All cats have hair.
• Josephine is a cat.
• Therefore, Josephine has hair.
• All auto manufacturers are computer
  manufacturers.
• United Airlines is an automaker.
• Therefore, United Airlines is a computer
  manufacturer.
• All college professors drink to excess.
• Shaun is a college professor.
• Therefore, Shaun drinks to excess.

6
Inductive Arguments

• Strong an argument in which the premises, if
  true, make it unlikely that the conclusion is
  false.
• Weak the conclusion is improbable, even if the
  premises are true.
• All dinosaur bones discovered to this day have
  been at least 50 million years old. Therefore,
  the next dinosaur bone found will be at least 50
  million years old.
• All meteorites found to this day have contained
  gold. Therefore, the next meteorite found will
  probably contain gold.
• When lighted match is slowly immersed in water,
  the flame is quenched. Gasoline is a liquid, just
  like water. Therefore, when a lighted match is
  immersed slowly in gasoline, the flame will
  probably be snuffed out.

7
Inductive Arguments (cont)

• Unlike validity which is a binary attribute,
  strength is a relative term.
• The truck contains 500 onions
• Three onions selected at random are rotten.
• Therefore, all 500 onions are probably rotten.
• The truck contains 500 onions
• Three hundred onions selected at random are all
  rotten.
• Therefore, all 500 onions are probably rotten.

8
Cogency

• An inductive argument is cogent when (1) it is
  strong (the premises, if true, make it unlikely
  that the conclusion is false) and (2) all the
  premises are true, and (3) no important factors
  have been left out.
• Surfing at Newport Beach is usually lots of fun.
  Today there is a good 5 foot Southwest swell.
  Therefore it is a good day to go surfing at
  Newport.