Fallacy Theory – Evaluating Arguments
Formal Fallacy (Structure Flaw)
Affirming the Consequent:
•
if A then B, B, thus A
•
invalid version of Modus Ponens (affirming the antecedent)
•
Courtney will go to the movie only if she finishes her homework. She finished her
homework, so she will go to the movie.
Denying the Antecedent:
•
if A then B, not A, thus B
•
Invalid version of Modus Tollens (denying the consequent)
•
Courtney will go to the movie only if she finishes her homework. She didn’t go to
the movie, so she didn’t finish her homework.
Exclusive Or Fallacy:
•
occurs when an “or” that is inclusive is mistakenly read as being exclusive
•
Courtney will either go to the movie or she will finish her homework. She went to
a movie, so she didn’t finish her homework.
Illicit Predicate Instantiation I
•
All S’s are P’s, x is a P, so x is an S
•
All humans are mortal, Socrates is mortal, thus Socrates is a human
Illicit Predicate Instantiation II
•
All S’s are P’s, x is not an S, so x is not a P
•
All humans are mortal, Socrates is not a human, thus Socrates is not mortal
Illicit Universal Syllogism I
•
All A’s are B’s, all B’s are C’s, thus all C’s are A’s
•
All dogs are mammals, all mammals are animals, so all animals are dogs
Illicit Universal Syllogism II
•
All A’s are B’s, all A’s are C’s, so all C’s are B’s
•
All executives are well paid, all executives are stockholders, so all stockholders
are well paid.