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Logic operator

  1. Jun 17, 2006 #1
    So, there is this "inference rule" in my textbook:
    from P v Q, we can assume P

    but does this hold for P v Q', i.e. given this, can I say we have P?

    My intuition says "no", but I'd rather have a reasoned answer....
    Thanks in advance.
  2. jcsd
  3. Jun 17, 2006 #2


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    From P v Q, you may not infer P. You can assume P anytime you like, so long as you discharge all assumptions by the end of your proof. By "v", you do mean "OR", right? From P & Q you certainly may infer P. And if, by Q' you mean not-Q, then you may infer P from P & Q' just as you may infer it from P & Q.
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