logical deductions

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logical deductions

Postby lsand2525 on Sun Jun 29, 2014 2:15 am

I'm trying to understand why the following is not a sound inference rule:

Given that NOT P IMPLIES NOT Q, P implies Q.

However, this is a sound rule:

Given NOT P IMPLIES NOT Q, Q implies P.

Thanks.
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Re: logical deductions

Postby nona.m.nona on Tue Jul 01, 2014 11:24 am

lsand2525 wrote:Given that NOT P IMPLIES NOT Q, P implies Q.

What did you see when you did truth tables for each of "~P => ~Q" and "P => Q"? Did they have the same truth values, given the same inputs?

What real-life examples did you try out? For instance, one could say that P stood for "the street is wet" and Q stood for "it is raining now". Then assume that any time the street is dry (~P), it therefore also is true that it is not raining right now (~Q). Does that necessarily mean that, if the street is wet (P), it must be raining right now (Q)? Or might the neighbor's sprinkler be wetting the road, or maybe the fire department opened a hydrant for a seasonal test?

What happens if you apply this example to the other statement?

Please reply with your thoughts. Thank you.
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