Lecture Notes.
 
  

 
Module Eleven: Session Nine

Formal Fallacies

Invalid Disjunctive Syllogisms

P or Q. P. Therefore, it is not the case that Q.

Example:
Either Bill will be at the party or Donna will be at the party. Bill is at the party. Therefore, Donna is not at the party.

P or Q. Q. Therefore, it is not the case that P.

Invalid Disjunctive Syllogism Invalid Disjunctive Syllogism
Invalid DS
Invalid DS


Example:
Either Bill will be at the party or Donna will be at the party. Donna is at the party. Therefore, Bill is not at the party.

At first glance these seem alright. However, it is possible, given the meaning of the word "or" that Donna and Bill could BOTH be at the party. So it is possible for the premises to be true and the conclusion to be false.

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