"If the forest continues to disappear at its present pace, the koala will approach extinction," said the biologist. "...

shunhe April 3, 2020

Hi @anchelle,

Thanks for the question! Let’s take a look at what the politician is saying. The politician says that “all that is needed to save the koala is to stop deforestation.” In other words:

stop deforestation —> save koala

Now note that given this statement, we can really only conclude one other statement from it, which is its contrapositive:

~ save koala —> ~stop deforestation

And to have something inconsistent with these claims (since we’re looking at both the original claim and its contrapositive) is to have the sufficient condition be true, but the necessary condition be false. In other words, we can show that either deforestation was stopped, but the koala was not saved and became extinct (which is what (B) tells us), or we can show that the koala was not saved (went extinct), and deforestation was stopped. (A) gives us neither of these two cases: it’s a case in which deforestation continued, and the koala became extinct. In other words, (A) is consistent with the politician’s claim, because it’s completely possible that in the world where deforestation continues and the koala becomes extinct, if deforestation had hypothetically stopped, then the koala hypothetically would have been saved.

Hope this helps! Feel free to ask any other questions that you might have.

Thomas-Montgomery September 28, 2020

In the politician's statement, why isn't "save koala" the sufficient? It is introduced as the S condition by the introductory "all". So why "stop deforestation --> save koala" Thx