PHIL 242
Homework Set 4
Due Monday September 28 by 9
a.m.
[All problems are worth 2 points each]
1. Problems 6.9, 6.12, 6.14, page 162
2. Problem 6.19, p. 166.
3. Problem 6.24, p.172.
4. Problem 6.40, p. 175
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Strategies for Proofs
1. If you need to prove a sentence having
the form of a conjunction, prove each conjunct
separately, then use H Intro.
2. If you have a premise of the form of a disjunction and you need to prove a sentence D, try
deriving D from a sequence of subproofs, each of which has one of the disjuncts as its
assumption. Then use the rule of H Elim to get D as your main conclusion.
3. To prove a sentence of the form HD, begin a subproof with D as its assumption, derive a
contradiction, then use H Intro to get HD as the first line after the end of the subproof.
Alternatively, it is sometimes useful in deriving D to begin a subproof with HD as its
assumption, derive H D by the above strategy, and then use H Elim to get D.
4. At any point in a proof or subproof, if you can derive a contradiction H you can use the rule
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 Spring '08
 KNIGHT
 Logic, somewhat different topics, previous midterm examination

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