Objection to something in the powerscore lr bible

secretad
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Objection to something in the powerscore lr bible

Postby secretad » Mon Mar 14, 2011 4:30 pm

I am in the chapter discussing formal logic and it says that:

If A -> B

It says next that we know some a's are b's. And due to the reversing nature of some statements that some b's are a's. However, what if there are no a's in the world. I do not believe powerscore is correct in making this an inherent inference as they call it.

Am I correct in my understanding of this?

ExpectLess
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Re: Objection to something in the powerscore lr bible

Postby ExpectLess » Mon Mar 14, 2011 5:37 pm

You are correct. Going from A->B to BsomeA requires the assumption that at least one A exists. The LSAT doesn't use this very frequently, however, and they will always let you know that one or more A's actually exist if they do.

The only time they really use this is if they have something like A->B and A->C to get you to come up with BsomeC. But like I said, they will always tell you that A's exist and to my knowledge have never had a question where they omitted that piece of information if they're looking for you to make that inference. So don't get too hung up on it.

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suspicious android
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Re: Objection to something in the powerscore lr bible

Postby suspicious android » Mon Mar 14, 2011 10:56 pm

secretad wrote:I am in the chapter discussing formal logic and it says that:

If A -> B

It says next that we know some a's are b's. And due to the reversing nature of some statements that some b's are a's. However, what if there are no a's in the world. I do not believe powerscore is correct in making this an inherent inference as they call it.

Am I correct in my understanding of this?


The LR bible does make some short cuts for the sake of expediency, I guess they figure this is close enough to true for the purposes of LSAT study. I get slightly annoyed by this, since in a book you have essentially unlimited space to be as precise as you want.

This idea actually comes up occasionally. The thing is, it's been pretty obvious when they use it. I don't remember the precise wording, but I think there was an argument about successful utopian societies where getting the answer was easier if you recognized that there has never been a successful utopian society. It went something like:


P: U --> X
P: U --> Y
P: X --> ~Y

Then answer was something along the lines of ~U, which follows validly here.

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TIKITEMBO
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Re: Objection to something in the powerscore lr bible

Postby TIKITEMBO » Mon Mar 14, 2011 11:14 pm

Am I getting too meta here to be exploring the idea that because the label "A" is given then the thing exists because it's been given a label?

As long as a thing has a label, maybe it can be considered real in the LSAT world so often referenced in Powerscore study materials.

Sorry if this sounds ridiculous. It's late. Late night posting on forums can sometimes be the equivalent nuisance of drunk dialing...

ExpectLess
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Re: Objection to something in the powerscore lr bible

Postby ExpectLess » Tue Mar 15, 2011 1:53 am

TIKITEMBO wrote:Am I getting too meta here to be exploring the idea that because the label "A" is given then the thing exists because it's been given a label?

As long as a thing has a label, maybe it can be considered real in the LSAT world so often referenced in Powerscore study materials.

Sorry if this sounds ridiculous. It's late. Late night posting on forums can sometimes be the equivalent nuisance of drunk dialing...


All pink flying elephants live in America.

Therefore, some things that live in America are pink flying elephants.

The first sentence does not contradict real life. The second does. Just because it has a label, it doesn't mean it exists, nor can you simply assume that it does. Only under the assumption that at least one pink flying elephant exists can we derive the second sentence.

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Jeffort
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Re: Objection to something in the powerscore lr bible

Postby Jeffort » Tue Mar 15, 2011 2:13 am

There are several LR problems that play on the subtly of whether or not something actually exists that is talked about in the stimulus in a way that does not establish existence because of the way the premise is phrased.

There is one (don't remember specific reference off hand) where the credited answer choice simply says "XYZ exists" that has a premise about XYZ that was phased in a hypothetical way that didn't establish existence.




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