Disjunctive Syllogism

Date: 09/25/98 at 10:51:45
From: Lauren Ray
Subject: Disjunctive Syllogism

I am enrolled in a Elementary Logic and Set Theory class, and we have 
an assignment to find out definitions of the words: Modus Ponens, 
Modus Tollens, and Disjunctive Syllogism. I have already found the 
first two, but am stuck on the last one. So my question is, what does 
it mean?  

Thank you,
Lauren

Date: 09/25/98 at 11:28:48
From: Doctor Mike
Subject: Re: Disjunctive Syllogism

Hi Lauren,  
   
See Stefan Waner and Steven R. Costenoble's "Introduction to Logic" on 
the Web. It has the definition of Disjunctive Syllogism and other logic 
terms.
 
   http://www.hofstra.edu/~matscw/logic/logic4.html   
   
The disjunctive syllogism is  (P v Q) and notQ --> P. In words:

   "If one or the other is true, and one of them is false, the other 
   must be true."

Here is an example:

   -  Given: Either Congress meets in Washington D.C. OR pigs can fly.
   -  Given: Scientific evidence shows conclusively that pigs cannot 
             fly.
   -  Conclusion: Congress meets in Washington D.C.   
   
I hope this helps. 
   
"Thanks for writing."  OR  "Pigs can fly."
    
- Doctor Mike, The Math Forum
  http://mathforum.org/dr.math/