Informal fallacies
1 Informal Fallacies
Formal vs informal fallacies a fallacy is a defect in an argument other than its having false premises An informal fallacy is a defect in the content of an argument.(A formal fallacy is a defect in the structure of an argument. We have seen many valid rules of deduction Formal fallacies can be understood as a use of unacceptable rules
2 Formal Vs Informal Fallacies • A fallacy is a defect in an argument other than its having false premises. • An informal fallacy is a defect in the content of an argument. (A formal fallacy is a defect in the structure of an argument.) • We have seen many valid rules of deduction. Formal fallacies can be understood as a use of unacceptable rules
Examples of formal Fallacies 1. Affirming the consequent P→Q/Q∥/P
3 Examples of Formal Fallacies • 1. Affirming the consequent: • P Q / Q // P
2. Denying the antecedent P→Q/~P∥~Q
4 • 2. Denying the antecedent • P Q / ~P // ~Q
3. Commutation of conditionals P→Q∥Q→P
5 • 3. Commutation of conditionals: • P Q // Q P
4. Improper transposition P→Q∥P→~Q
6 • 4. Improper transposition: • P Q // ~P ~Q
5. Improper disjunctive syllogism PvQ/P∥^Q
7 • 5. Improper disjunctive syllogism: • P v Q / P // ~Q
Before discussing the formal fallacies concerning categorical syllogism, we should first learn what is meant by a distributed term If a categorical proposition tells us something about every member of a class referred by a term, the term is distributed in the proposition
8 • Before discussing the formal fallacies concerning categorical syllogism, we should first learn what is meant by “a distributed term”. • If a categorical proposition tells us something about every member of a class referred by a term, the term is distributed in the proposition
Consequently the following underlined terms are distributed in the propositions AllS are p No s are p Some s are p Some s are not p Now. we can discuss the formal fallacies of categorical syllogism
9 • Consequently the following underlined terms are distributed in the propositions: – All S are P. – No S are P. – Some S are P. – Some S are not P. • Now, we can discuss the formal fallacies of categorical syllogism
6. Undistributed middle Some p are m (some politicians are liars) Some M are s (some liars are thieves) Therefore, Some S are P. (some politicians are thieves
10 • 6. Undistributed middle: • Some P are M (Some politicians are liars) • Some M are S (Some liars are thieves) • Therefore, Some S are P. (Some politicians are thieves)
