The Language of Argument

9 4

cH A Pt eR 5 ■ D e e p A n a l y s i s

father and no fathers are female. This example makes it obvious that validity

is not the same as truth. It also makes it obvious that another requirement of

a good argument is that all of its premises must be true.

Soundness

We thus make at least two demands of a deductive argument:

1. The argument must be valid.


2. The premises must be true.
   When an argument meets both of these standards, it is said to be sound. If it
   fails to meet either one or the other, then it is unsound. Thus, an argument is un-
   sound if it is invalid, and it is also unsound if at least one of its premises is false.
   All Premises True At Least One False Premise
   Valid Sound Unsound
   Invalid Unsound Unsound
   Soundness has one great benefit: A sound argument must have a true con-
   clusion. We know this because its premises are true and, since it is valid, it is
   not possible that its premises are true and its conclusion is false. This is why
   people who seek truth want sound arguments, not merely valid arguments.

Indicate whether each of the following arguments is valid and whether it is

sound. Explain your answers where necessary.

1. Most professors agree that they are paid too little, so they are.


2. David Letterman is over four feet tall, so he is over two feet tall.


3. Lee can’t run a company right, because he can’t do anything right.


4. Barack Obama is smart and good-looking, so he is smart.


5. Barack Obama is either a Democrat or a Republican, so he is a Democrat.


6. Since Jimmy Carter was president, he must have won an election.


7. Since Gerald Ford was president, he must have won an election.


8. Pat is either a mother or a father. If Pat is a mother, then she is a parent. If
   Pat is a father, he is a parent. So, either way, Pat is a parent. (Assume that
   this conclusion is true.)


9. People who live in the Carolinas live in either North Carolina or South
   Carolina. Hillary Clinton does not live in North Carolina or South
   Carolina. Hence, she does not live in the Carolinas.


10. If all of Illinois were in Canada, then Chicago would be in Canada. But
    Chicago is not in Canada. Therefore, not all of Illinois is in Canada.

exercise v

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