assumption required?

jmjm
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assumption required?

Postby jmjm » Fri Aug 09, 2013 11:41 am

The necessary assumption questions that ask for the phrase 'assumption required' seem to have intended the term 'required' to be interpreted in a nontraditional manner.

For an argument with premises A-> B -> C concluding A -> D, an answer with the missing link C -> D is a necessary assumption. But if B->D were true then C->D is not really "required" in the english language sense in that conclusion can be true even without C->D assumption being true.
The term "required" makes sense only if one assumes that lsat writers intended it to apply to the most direct line of concluding the argument.

I did not find it explicitly stated. Is it explicitly stated anyplace in the parameters of the test?

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patfeeney
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Re: assumption required?

Postby patfeeney » Fri Aug 09, 2013 2:32 pm

jmjm wrote:The necessary assumption questions that ask for the phrase 'assumption required' seem to have intended the term 'required' to be interpreted in a nontraditional manner.

For an argument with premises A-> B -> C concluding A -> D, an answer with the missing link C -> D is a necessary assumption. But if B->D were true then C->D is not really "required" in the english language sense in that conclusion can be true even without C->D assumption being true.
The term "required" makes sense only if one assumes that lsat writers intended it to apply to the most direct line of concluding the argument.

I did not find it explicitly stated. Is it explicitly stated anyplace in the parameters of the test?


What do you mean by "explicitly stated"? As in this form of logic being explicitly detailed in the test?

In terms of your concern, the phrase "assumption required" must be interpreted in a traditional manner to figure out this logic chain.

If B->D is true, then A->D COULD be true. HOWEVER, B->D does not prove C->D. If C->D is NOT true, or C-> -(D), then it is impossible for B->D to be true, since B->C. Therefore, C-> D is necessary. B->D is necessary too, but C->D cover it. B->D does not prove C->D, in contrast. We NEED to know C->D in order for the rest to be correct.

bilbaosan
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Re: assumption required?

Postby bilbaosan » Fri Aug 09, 2013 5:14 pm

jmjm wrote:The necessary assumption questions that ask for the phrase 'assumption required' seem to have intended the term 'required' to be interpreted in a nontraditional manner.

For an argument with premises A-> B -> C concluding A -> D, an answer with the missing link C -> D is a necessary assumption. But if B->D were true then C->D is not really "required" in the english language sense in that conclusion can be true even without C->D assumption being true.


Are you sure you're not confusing necessary vs sufficient assumption? "Missing link" is typically a sufficient assumption. For example if we take "Today is Tuesday, hence Helen must be at work". Here you've got "DAY -> TUE, hence HW" and looking for missing link "TUE->HW". But you're looking for a *sufficient* assumption like "Helen always goes to work every Tuesday". Note that we can't conclude from this information what happens if today is NOT Tuesday. At the same time if it is Tuesday, Helen must be at work.

I don't see how a necessary assumption can be a missing link. For example, "Helen usually goes to work on Tuesdays. Helen is at work today, therefore today is Tuesday". Here we have "T -> HW, therefore HW -> T". Now what is required for this conclusion to be true? A *necessary* assumption like "Helen never goes to work unless it is Tuesday". If it is false, i.e. if Helen might go to work on Wednesdays, the conclusion cannot be properly drawn. Comparing to above, we can conclude what happens if today is NOT Tuesday - Helen is not at work. At the same time we cannot conclude what happens if today IS Tuesday - Helen may be at work, but she may be not - the assumption doesn't require her to be at work.

Now what if we try to use the necessary assumption as a missing link in an example above? "Today is Tuesday, hence Helen must be at work" combined with "Helen never goes to work unless it is Tuesday"? It doesn't work, because it doesn't require Helen to actually go to work at all. So it may be Tuesday and she might stay at home; this won't break the assumption but this will break the conclusion.

TylerJonesMPLS
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Re: assumption required?

Postby TylerJonesMPLS » Sat Aug 10, 2013 11:08 am

bibaosan is right.
You are confusing a sufficient assumption with a necessary one.

A ---> B ---> C therefore A ---> D
There is obviously a gap in the argument.
The premise C ---> D fills the gap.
Since C ---> D is an unstated premise, it is a sufficient assumption.
The premises and conclusion you are given say nothing about B ---> D. So, while B ---> D may be true, or may not be true, you cannot deduce anything from it because it is not in the set of premises you are given.

You should check out “Necessary Conditions and Sufficient Conditions” in “A Guide to Logical Reasoning Questions,” in Super Prep.
If you don’t have a copy of Super Prep (published by LSAC), then get a copy. It is indispensable.

Also, the definitive discussion of sufficient and necessary conditions is in Super Prep, in “A Guide to Logical Reasoning Questions”, under the heading “Assumptions.”

There is a lot of confusion about necessary and sufficient assumptions on the Board.

There was a recent discussion of the confusion in: Assumptions - Sufficient/Necessary.

TylerJonesMPLS
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Re: assumption required?

Postby TylerJonesMPLS » Sat Aug 10, 2013 11:22 am


jmjm
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Re: assumption required?

Postby jmjm » Mon Aug 12, 2013 3:53 pm

The issue in OP is related to pt-35, s1, q14 in which necessary assumption is the missing link. Quite likely this issue is not the difference between necessary and sufficient assumption based on ps lrb and manhattan guide. Also discussed on page 273 PS LRB under 'assumptions and conditionality.'

peke
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Re: assumption required?

Postby peke » Fri Aug 16, 2013 2:34 pm

OP, I had the exact same thought and made a similar post about it on another forum. I feel by far the best way to think of a necessary assumption is to think of its contrapositive. If the assumption was negated, will it destroy the argument.

However, trying to write the wff won't differentiate the two:

___ (and) (B->C) -> (A->C)

The assumption (A->B), which may be either necessary or sufficient, will satisfy that wff.

jmjm
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Re: assumption required?

Postby jmjm » Tue Aug 20, 2013 10:51 pm

Yes, an assumption can be both necessary and sufficient, and agree trying to write wff wouldn't help with distinction.

From op, if there's an argument with premises A-> B -> C which concludes A -> D, some lsat questions (and books, ref powerscore bible page 273) have C->D as the 'necessary' assumption. But, even if C->D were false and B->D (which is a weaker condition than C->D) were true, the argument stays valid. Thus, negation of C->D is not enough to destroy the argument and so it's not a necessary/required assumption.

As an example consider q 35.1.14
The conclusion is, novelist great -> ~acad
Premises are,
intuitive grasp -> (immersion everyday life)
immersion everyday life -> ~acad

The necessary assumption and the credited answer is,
novelist great -> intuitive grasp

But it's a necessary assumption only when argument is considered narrowly within the confines of most direct line to reach the conclusion of the argument. Else, one didn't need to have the (novelist great -> intuitive grasp) be true so long as (novelist great -> immersion everyday life) was true and still have the valid argument.

So (novelist great -> intuitive grasp) is not really a 'necessary' assumption in the general sense. It's a 'necessary assumption' only with certain qualifications as discussed above. my concern is, lsac question paper never specifies these qualifications. Anyone else also notice this?




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