(QUESTION # 3055)

[jenny.alcaide]

Hi, I don't seem to understand the assumption from this question:

P: Washingtonia filifera is one of many pine tree species, but it is the only one that is indigenous to California.
C: Some pine trees should not be cultured in California.

The right assumption is: A plant should be cultured in a state if and only if that plant is indigenous to that state.

But I chose: A plant should be cultured in a state if that plant is indigenous to that state.

How would you diagram this? I guess I don't understand why you need to use the "if and only if."

Thanks!

[noah]

Before we get into diagramming, if you use the assumption you chose, then why would we conclude that certain tree shouldn't be cultured in California? Your answer would allow us to conclude that we should culture the Washingtonia filifera, but not that we should exclude the other pine trees.

We need to assume that if a tree is not indigenous to a region, we should not culture it there. (~ I --> ~ C), and the contrapositive of this is C --> I (or, if you culture a tree, it must be indigenous to that region).

So, in the end, the correct answer is a bit of overkill - you don't need "plant it if it is indigenous" you just need "don't plant it if it's not indigenous." But, overkill is OK unless you've been asked for a necessary assumption specifically. Sufficient assumptions can be overkill.

Make sense?