Supply the missing premise that makes the conclusion follow logically:P: X–some–YP: ?C: Z–some–Y

KBurky September 8, 2017

Not/Not

I am confused how one of these is correct and one is incorrect. Don't they ultimately express the same truth? Z -> X Not Z -> Not X

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Mehran September 15, 2017

@KBurky absolutely not! Remember, DON'T JUST NEGATE!

When you are creating the contrapositive, you need to reverse and negate.

So here, the missing premise is:

not Z ==> not X

The contrapositive of this principle is:

X ==> Z

Hope that helps! Please watch our video lesson on Sufficient & Necessary Conditions for a more in-depth discussion of these concepts.