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rdavidjr84 November 13, 2019

Further clarification required

Why can we not connect the contrapositive from P3 with the contrapositive from P1? P3: SKP ==> not PRBA P1: not AEGC and not PRBA ==> not DRHS to conclude ~DRHS???? thanks Rick

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Irina November 13, 2019

@rickyd,

Because your first premise involves a conjunction:

~ AEGC ^ ~ PRBA -> ~ DRHS

It means that BOTH of these variables have to be true - ~ AEGC AND ~ PRBA for us to conclude that ~DRHS. Since premise 3 only tells us that ~ PRBA is true, we cannot logically conclude ~ DRHS. Now, if the first premise looked like this:

~AEGC v ~PRBA -> ~DRHS

then , since this is an OR statement, only one of the variables have to be true for consequent to be true, and we could logically conclude that because ~PRBA is true, ~DRHS is true as well.

Let me know if this makes sense and if you have any other questions.