Argument Completion Drills

P: A -> B P: B-> C P: A does not exist C: No valid conclusion. Why is it "no valid conclusion," and not "C does not exist"? Shouldn't the fact that A does not exist consequently mean that neither B nor C can exist?

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AnthonyH June 17, 2020

Hey, I had the same question! My understanding is that by invoking the third premise, "A does not exist", we can no longer draw a valid conclusion without breaking the "Don't Just Reverse Rule"/ or breaking the "Don't Just Negate Rule" applied the positive argument.

Basically, Not C-> Not B-> Not A would mean we're assuming the necessary condition allows us to conclude the sufficient.

If the third premise said, "A exists" then we COULD conclude "A->B->C" and the contrapositive
"Not C -> Not B-> Not A".

@Mehran, can you clarify this for us?