64 LR Two Question 20
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what's wrong with D ? I have:
SC or notBSH ----> No Joke
It's close! But in (D) we're only told we believed the joke would show contempt for someone, so our rule about showing contempt for the subject of the joke cannot prove that (D) is true and the joke would be wrong.
(Those bastards!)
Why can't we do the contrapositive of the conditional here?
C or BSH -> /J
J -> /C & /BSH
Wouldn't that make B right?
First, the good news is that you can always use the contrapositive of any conditional statement, here and everywhere else all the time.
But (B) is wrong because our rule does not prove that it's true. We have a rule that would prove when it's wrong to play a practical joke (if it shows contempt or you believe it might cause significant harm), but nothing that would help us prove when it's not wrong to do so.
The short version is you can read a conditional from left to right, but never the other way around.
A slightly longer version is that you can use the sufficient condition to prove the necessary condition, but never the other way around.
Finally, an example to bring it home (you can do the symbols to clarify further if that would help):
If a person is a minor or a woman, then that person is not in the NBA. Dave Hall is not a minor, and he's not a woman. Thus, he must play in the NBA.
Same false reasoning is happening in (B). Make sense?