Logical Fallacies
Logical Fallacies / Propositional Fallacies

Propositional Fallacies

A propositional fallacy is one where an error in logic takes place due to compound propositions--a proposition is a 'statement'. In order for a compound proposition to have a true value, each component proposition must also be true.

Logic formulas:

  • Affirming a disjunct: A or B; A; therefore not B
  • Affirming the consequent: A, then B; B, therefore A
  • Denying the antecedent: If A, then B; not A, therefore not B

Example of Propositional Fallacies

  • William Shatner is Captain Kirk or he is in Miss Congeniality. William Shatner is in Miss Congeniality. Therefore he is not Captain Kirk. It is possible for William Shatner to be in both of these movies (in fact he was).
  • I am going to bed or reading a book. I feel really tired tonight, so I will go to bed. So, I cannot read.It is possible to go to bed, as well as read a book.


Alternative Name: Denying the Antecedent