PHI 102 - Chapter 10, Homework Solutions

Part A:

2. Valid - Denying the Consequent (Modus Tollens) 5. Invalid - Denying the Antecedent 8. Valid - Affirming the Antecedent (Modus Ponens) 11. Invalid - Affirming the Consequent

Some extras: 1. Invalid- Affirming the consequent 4. Invalid- Denying the antecedent 7. Valid- Affirming the Antecedent (Modus Ponens) 10. Disjunctive Syllogism (Affirming a disjunct)- Valid only for "exclusive" or 13. Invalid- Denying the antecedent

Part B:

2. Valid - Denying the Consequent	If I take statistics, then I'd take it in the morning. I can't take statistics in the morning. I won't take statistics.
5. Valid - Hypothetical Syllogism	If I wait till Wednesday, my suit won't be ready till Saturday. If my suit won't be ready till Saturday, then I won't be able to wear it on Thursday. / If I wait till Wednesday, then I won't be able to wear my suit on Thursday.
8. Valid - Affirming the Antecedent (Modus Ponens) *Note that this is an enthymeme; the conclusion hasn't been explicitly stated.	If salmon reliably return to the streams in which they were spawned, then they must be sensitive to subtle differences in their chemical compositions. Salmon reliably return to the streams in which they were spawned. / [Salmon are sensitive to those subtle chemical differences]*
11. Valid - Denying the consequent (Modus Tollens)	[If Robin is embarrassed, she'll get a blotch.]* Robin didn't have a blotch. /Robin wasn't really embarrassed.
15. Invalid - not an appropriate form for hypothetical syllogism.	If you are not serious about logic, then you will not master it If you have read this far, then you must be serious. /If you have read this far, you will be able to master logic.

Part D

2. Invalid - Denying the antecedent.	If the charges were true, I'd resign The charges are not true. /I won't resign.
5. Valid - Denying the consequent (Modus Tollens)	If faith were reason, then religion would be part of philosophy. Religion is assuredly not part of philosophy. /Faith is not reason.
8. Valid - Denying the consequent (Modus Tollens)	If imitation is an important factor in language learning, then we'd have evidence of its importance. We don't have any evidence of its importance. /Imitation is not an important factor in language learning.
11. Invalid - Denying the antecedent.	If epidemiological studies hinted that insects played a role in spreading AIDS, we'd do more detailed studies. Epidemiological studies have not hinted that insects play a role in spreading AIDS. /[We'll not do more detailed studies.]