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Arguments with Hypothetical or Disjunctive Premises

 

Formalize the argument. Name the argument form. State whether the argument is valid or invalid.

 

  1. He told me that if he could save $1000 (S), he was heading for Jamaica (J).   He just sent me a postcard from Jamaica so I guess he did save up that $1000.

    2.  

     

     

     

  2. He told me that if he stayed up late studying (L), he would do well on the test (W). Of course, he was asleep by 9:00 P.M. He is sure to fail the test now.

    3.  

     

     

     

  3. If John won the lottery W), he would be rich (R).   But he certainly isn't rich. This means he obviously didn't win the lottery.

    4.  

     

     

     

  4. That tree is either a white oak (W) or a live oak (L).   Live oaks don't grow this far north so it must be a white oak.

    5.  

     

     

     

  5. Smith will either run for the Senate (S) or become a university professor (U). If Smith runs for the Senate, he will lose (L).  If he loses, he will become a university professor.  Therefore, Smith will be a university professor.

 

 

 

 

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Valid Argument Forms

Modus Ponens

Modus Tollens

Disjunctive Syllogism

Hypothetical Syllogism (P>Q, Q>R, thus P>R)

Invalid Argument Forms - Formal Fallacies

Fallacy of Affirming the Consequent

Fallacy of Denying the Antecedent

Return to Logic Syllabus

 

Author: Hollace Graff
Oakton Community College
Last Updated: October 10, 2000