Evaluating Arguments

Worth of argument depends on 2 considerations:

1.     truth or falsity of its premises

2.     how much support its premises provide the conclusion

Deductive arguments
·        meant to be valid
 

Nondeductive arguments
·        premises are meant to confer some high degree of probability on the conclusion
 

Validity
·        Validity determines whether a certain relationship holds between the premises and conclusion

A valid argument can come in these various forms:

1.     false premises and a true conclusion

·        The moon is made of green cheeses, and everything made of green cheese orbits the earth.  Therefore, the moon orbits the earth.

2.     false premises and a false conclusion

·        Neptune is made of green cheese, and everything made of green cheese orbits the earth.  Therefore, Neptune orbits the earth.

3.     true premises and a true conclusion

·        If it is 100% humidity, it will rain.  It is 100% humidity.  Therefore, it is raining.

 You cannot have the combination of true premises and a false conclusion
To sterilize instruments, they must be boiled in water.  The water is the pot is 212°F.  Therefore, the instruments will not be sterile.

A sound argument:

·        Even though an argument is valid doesn’t mean it’s sound. 

·        For an argument to be sound, all of the premises must be true.

o       The last person to see the victim alive must have been the murderer, and Wainsworth was the last person to see the victim alive.  Thus, Wainsworth must have committed the murder.

For example:
All Democrats are liberals.
All liberals want more government spending.
Thus, all Democrats want more government spending.

Validity and Added Premises

·        If an argument is valid, additional information will not change the validity. 

Checking Validity

·        One quick way to check whether or not an argument is valid is to see if there are any loopholes in the argument. 

Nondeductive arguments

·        premises are meant to confer some high degree of probability on the conclusion

·        Share some basic characteristics:

1.     The premises are meant to make their conclusions probable or likely.  The premises provide some support for the conclusion.

2.     While validity is not a matter of degree, probability is.  Nondeductive support for a conclusion is based on a matter of degree.  Nondeductively successful indicates that an argument is more likely than not.  Nondeductively unsuccessful indicates that the conclusion is less than likely to be the case.  Essentially you want to determine the degree to which the premises of an argument support the conclusion.

3.     The addition of new information in the premises in a nondeductive argument may considerably change the overall support for the conclusion.

Some varieties of nondeductive arguments:

·       Example of a statistical syllogism:
Most people who read The New Republic are liberals.
Freedman reads The New Republic.
FC-Freedman is a liberal.

2.  Inductive Generalization

·       Example of an inductive generalization:
In the phone survey, 58% of the registered voters intending to vote in the election said thy planned to vote for Larson. 
FC-Larson will receive about 58% of the votes in the election.

3.     Causal Arguments –Arguments based on causal relationships—that if A results in B, then A causes B

4.     Arguments by Analogy- Something is similar to something else, therefore some argument is likely to be the case

·       Example: Dogs can think.  Haven’t you ever seen a dog get excited when the owner picks up the car keys?
Premise: The dog gets excited when the owner picks up the car keys
Intermediate conclusion: The dog recognizes what that means—I get to go for a ride
Final conclusion: The dog can think

5.     Plausibility Arguments –These are case-building arguments.  Often times they don’t have a particular pattern

Example:
We have probable cause to believe that Joe committed the crime.  His fingerprints were on the weapon, his DNA was found on the victim, and two independent witnesses testified that they saw him stab the victim.

Complex Arguments