# Disjunctive syllogism

Posted by robanaurochs
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 Disjunctive syllogism July 21, 2006 09:46PM IP/Host: ---.gprs.vodacom.co.za Registered: 13 years ago Posts: 1,424 Rating: 0
Has anybody been able to find this rule in the textbook?

The solution to Ass 2 Q2.2 relies on this rule to prove the conclusion. First of all, I didn't know that you could use anything other than the Boolean Intro and Elim rules in deductive proofs. Secondly, the solution says that the deductive syllogism rule and the modus tollens rule are found in chaps 3 and 4. I think I'm going blind because I can't find them at all. Please could sight-advantaged people please post the page numbers.

If I'd have known of this rule, I would have nailed the question. I just got stuck because I didn't know what to do with the "A v B v C" premise. I got from A to Ã‚Â¬B but I couldn't figure out how to get from Ã‚Â¬B to A.
 Re: Disjunctive syllogism July 22, 2006 03:04PM IP/Host: ---.onspeed.com Registered: 13 years ago Posts: 137 Rating: 0
Hi

Because we know that to get from "A V B" to say B we have to :
1) Prove that we can get from A to B
2) Prove that we can get from B to B 'easy'

I assumed that to get from "A V B V C" to say A we have to :
1) prove that we can get from A to A 'easy'
2) prove that we can get from B to A
3) prove that we can get from C to A

Ciao

 Re: Disjunctive syllogism July 26, 2006 01:45PM Moderator IP/Host: Moderator Registered: 13 years ago Posts: 17 Rating: 0
If you read you key to assignment 2 properly you can see that on page 5 the reference you are given is not of the current revised book you are using Language Proof and Logic by Barwise and Etchemendy, but their previous book, "The Languages of First-Order Logic"(1992). I wanted you to know that you can use such implication rules as well.However you could have succeeded in solving that proof without using those implication rules as well. What the second author has mentioned is correct, you can use disjunction elimination as well. Sent me your e-mail address so that l can sent you a different solution. Unfortunately l can not put it on Osprey because Osprey distorts characters.

Lecturer COS 261-C

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