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# PT68.S3.Q23 - art historian: this painting

Alum Member
edited January 2017 51 karma
Hi everyone! For this question, I understand how answer choice B wrecks the argument, making it a solid necessary assumption. It was my original answer choice, but I thought I was being tricked at the back end of the test because it seemed like a sufficient assumption. Is it both? Can it be both? If it's not a SA, what am I missing? I hate NA vs SA.

Answer choice B provides the following as a necessary assumption ...
CW-->PBSEKW
The contrapositive of the above conditional plus the author's conclusion make this ...
not(PBSEKW)-->not(CW)
not(PBSEKW)
------------------------------------------------
not(CW)

Seriously, how is this not a SA? It is enough to make the conclusion valid. Please help!

https://7sage.com/lsat_explanations/lsat-68-section-3-question-23/

• Alum Member
12637 karma
Bumping so more people see. Also edited title to better fit with our formatting.
• Yearly + Live Member Sage 7Sage Tutor
edited January 2017 10801 karma
So actually you are asking a very good question:

There are two types of NA answers: bridging and blocking.
The bridging answer choices act more like SA answers because they are connecting a very necessary gap in the stimulus. So if there is a gap, it needs to be true for the answer choice to work.
For example: All apples are Red.
Therefore: Some fruits are red.
We have a gap here that our premise is about apple but our conclusion is about fruits.
What needs to be true is that "some fruits are apple". This is both an SA and NA answer choice. Because some fruits are apple and all apple are red, guarantees our conclusion 100% that some fruits are red. But this is also necessary at the same time. If none of the fruits are apples then we cannot jump from our premise that all apples are red to some fruits are red.

So in a NA quesetion we can have a gap between two premise or between a premise and a conclusion. A right answer can bridge that gap and its both a sufficient assumption and necessary assumption. It's an NA answer because needs to be true, without that connection between apple and fruit our conclusion would fall apart, and its also sufficient to make our conclusion true because if some fruit are apple then its sufficient to conclude some fruits are red.
So an NA answer can do both.

In the above example, we have 1 premise: the brush style is not found in any other painting of C
Conclusion: painting is not genuine C's work

So our answer choice "B" says: C's other works are not painted using a brush style not exhibited in any of his other works can act as a NA answer because this needs to be true as it wrecks our argument but can act as a bridging too. I don't see this answer choice as a SA answer choice because it doesn't guarantee our conclusion 100%. It could be that this painting was an exception that Mr. C was experimenting with. So although this needs to be true that none of his other paintings had a similar brush stroke, just because they didn't doesn't guarantee our conclusion that this proves 100% that this painting is not genuine. An SA answer choice would say something like, No artist uses a brush stroke not exhibited by any of his other paintings. This would seal the argument and guarantee it and make it a SA answer choice.

But your intuition that an SA answer choice could also be NA wasn't wrong. I don't think it works in this particular example but it does work in other LSAT questions. Really depends on the gap that we are bridging in our stimulus and what the answer choices say.