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Tom
Registered Member
Date Joined Jul 2001
Total Posts : 10353
| Posted 11/20/2003 3:45 AM (GMT -6) |   | Alan-
I ran accross the following law of "transposition" in a small intro to logic that I'm ready. The author presents this:
(p --> q) = (~p --> ~q)
This seems wrong to me. p --> q means (to my brain) 'wherever p obtains, q will obtain'. It does NOT mean that q can ONLY obtain in the event of p. There may be circumstances OTHER than p in which q might, or does, obtain. All it means is that if p DOES obtain, q WILL obtain.
But this cannot mean ~p --> ~q. p may not obtain. Big deal. q may still obtain under other circumstances. p --> q doesn't mean 'q if and only if p'.
So why would an intro to logic say this?
I must be unlucky enough to buy the WORST intros on logic. Sheesh.
Tom  | | Back to Top | | |
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Tom
Registered Member
Date Joined Jul 2001
Total Posts : 10353
| Posted 11/20/2003 3:48 AM (GMT -6) | | Don't tell me this has something to do with the parentheses.
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Alan Rhoda
Registered Member
Date Joined May 2002
Total Posts : 680
| Posted 11/20/2003 6:39 AM (GMT -6) | | Magneton's right. Transposition is where you switch antecedent and consequent and negate both. Thus,
p -> q = ~q -> ~p
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Tom
Registered Member
Date Joined Jul 2001
Total Posts : 10353
| Posted 11/20/2003 9:33 AM (GMT -6) | | Magneton, you've been holding out on us, you anal-retentive-logical-lingo-expert you!
I just glanded at the quote in the book again. You guys are right. The author DID transpose p and q the second time. They just look so much alike. I didn't read it correctly.
I am NOT to be trusted! Arghghgh.
Take care,
Tom | | Back to Top | | |
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