Assumptions - Sufficient/Necessary

Darmody
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Assumptions - Sufficient/Necessary

Postby Darmody » Fri Aug 02, 2013 2:56 am

To my understanding, sufficient assumptions are able to prove the conclusion on their own and can prove it more than the bare minimum of logic needed to justify it. Necessary assumption cannot prove the conclusion on their own but are needed to partially hold the conclusion to be valid.

Now I understand the Assumption LSAT's question stems asks to either find the necessary or sufficient assumption, although in different terms, but how come a sufficient assumption answer can't answer a question asking for a necessary assumption? If the conclusion can be proven wholly by a sufficient assumption, why would this answer choice be wrong? I am seeing this like the all>most>some. If the necessary is some, then sufficient assumptions should be all and include everything. So if the question asks which answer choice helps hold the conclusion together, and one of the options holds it the conclusion to a 100%, then should it be still correct since it is still "holding" it?

Lastly, what are ways of quickly identifying whether the stem is asking for a necessary or sufficient assumption?

Required, hold, etc. = necessary
fully prove, logically complete, etc. = sufficient

What are some more indicators?

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the_pakalypse
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Re: Assumptions - Sufficient/Necessary

Postby the_pakalypse » Fri Aug 02, 2013 3:14 am

Because a sufficient assumption is NOT needed. It's too strong. It's too big. Necessary requires bare minimum type thinking, whereas sufficient questions are huge chunks.

You need $2 dollars to buy a Kitkat at a store.

Now suppose I said you had one million dollars. Is that sufficient? YES! But it's so much more than what's needed.

If you had ONE dollar, then is it sufficient? NOPE. But is it necessary? YES! Because if you don't even have one dollar, there's no way you could have two (which is the bare minimum required).

Now what if you had exactly $2?

Here is where it's interesting: it is both necessary and sufficient. You will ALWAYS need $2 (don't get confused, even if you have a million, you still have the $2)[necessary], AND having $2 is ENOUGH to guarantee you the outcome [sufficient].

bp shinners
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Re: Assumptions - Sufficient/Necessary

Postby bp shinners » Fri Aug 02, 2013 12:05 pm

Darmody wrote:To my understanding, sufficient assumptions are able to prove the conclusion on their own and can prove it more than the bare minimum of logic needed to justify it.


As long as you mean "on their own with the stimulus information added in", then correct.

Necessary assumption cannot prove the conclusion on their own but are needed to partially hold the conclusion to be valid.


Incorrect. Sometimes, a necessary assumption will also be sufficient. You're looking for something without which the argument as presented can't work. In other words, if you take it away, the premises have no chance of supporting the conclusion. The conclusion could be true, but you'd need to approach it from a completely different line of argumentation.

Now I understand the Assumption LSAT's question stems asks to either find the necessary or sufficient assumption, although in different terms, but how come a sufficient assumption answer can't answer a question asking for a necessary assumption? If the conclusion can be proven wholly by a sufficient assumption, why would this answer choice be wrong?


Sometimes it can, but not as a rule. This is because, as you said above, it can be stronger than needed. If a sufficient assumption is stronger than needed, it can't be necessary (by definition - if you don't need it, it's not necessary).

Lastly, what are ways of quickly identifying whether the stem is asking for a necessary or sufficient assumption?


Assumption tells you it's an assumption question.

"If" or language that sounds like a Must be True question means it's Sufficient Assumption.

A word that means the same as "necessary" or a prompt that says, "The argument makes the following assumption:" is Necessary Assumption.

TylerJonesMPLS
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Re: Assumptions - Sufficient/Necessary

Postby TylerJonesMPLS » Fri Aug 02, 2013 6:36 pm

That's an interesting question. It goes to the heart of what a conditional really is.

Well, first, Conditional Logic is different from the All/Most/Some logic. So don't confuse the two kinds of logic! Whenever you see a conditional, use Conditional Logic.

P --> W is a conditional. The "P" is the sufficient condition. "W" is the necessary condition. But these are still just conditions. You can't conclude anything from P ---> W as it stands. To be able to conclude anything, you need the assertion that P is true, or the assertion that ~W is true. Like this:
P ---> W
P
Therefore, W
So, you are right: The assertion that the sufficient condition, P, is true, allows you to totally conclude that W is true. And the truth of P is all that it takes to conclude that W is true. Or, in other words, P is sufficient to conclude W. That's why P is called the sufficient condition.

Now, let's take the necessary condition:
P ---> W
~W
Therefore, ~P
If W is not true, then there is no way that P can be true. What is why W is called the necessary condition. If W isn't true, then P necessarily cannot be true, and if P cannot possibly be true, then ~P must be true. That's how you conclude from the fact that ~W is true, that ~P is also true.

So you are right that if the stimulous gives you P ---> W, and one of the Answer Choices gives you P, then you can pick that Answer Choice and absolutely conclude W. But this may not be what the Question Stem is asking you for!!! So you have to be careful.

If the Question Stem asks you for a necessary condition, not a sufficient condition, and you choose an Answer Choice that names the sufficient condition, then you got that Question wrong.

Here's an example:
If some person is pregnant, then that person is a woman. If the Question Stem asks you which of the Answer Choices is a necessary condition, and one of the Answer Choices says "being a woman," then that is the correct Answer Choice. That's because if you aren't a woman, then there is no way that you can be pregnant. But if you pick an answer choice that has the sufficient condition instead, then you are saying that a necessary condition of being pregnant is being pregnant. So the conditional would be P ---> P, *but* the conditional that you were given is P ---> W. So that's the conditional you have to use. If you choose an Answer Choice that only works with the conditional P ---> P, but doesn't work with P ---> W, you picked an incorrect Answer Choice.

You asked for indicator words. As you said, a Question Stem that asks for a necessary condition will always have an indicator word that refer to some sort of necessity. It's easy to spot them because the LSAT uses the same indicator words for necessary conditions over and over again. The most common are: depends, requires and relies. I made a list of some examples and pasted it below.

By the way, since others have mentioned that a necessary condition can also be a sufficient condition, I'll add my 2 cents to that discussion. It is very rare to see a necessary condition that is also a sufficient condition on the LSAT, but it does happen sometimes. This kind of conditional is called a biconditional. The "bi" part of the word means "two," so a biconditional is two conditionals in one. An example is, If you are a batchelor, then you are an unmarried man, AND if you are an unmarried man, then you are a batchelor. The sufficient condition in the first part of the biconditional is the necessary condition in the second part. There's a special symbol for biconditionals in logic, but don't have that symbol on my keyboard. So I'll write it this way.
B <---->UM
All that the biconditional means is: B ---> UM *and* UM ---> B


Sorry if this explanation sounds like what a professor would say. The problem is that I am a professor, and I taught logic and critical reasoning for 25 years, so I am bound to sound like a professor.

Anyway, I hope this helps.


Necessary Assumption Indicator Words:
(By the way, I've left out the particular things mentioned in the Questions, so I'm not actually quoting. What I've written is just the general form of the questions to give examples of the common indicator words.)

Which one of the following is an assumption on which the argument depends? 46.3.20, 38.4.22, 44.2.18, 44.2.09, 44.2.05, 43.2.16, 43.2.10, 37.4.19,
Which of the following is an assumption the argument depends on? 46.2.10
Which one of the following assumptions does the argument depend on? 38.4.20
The argument depends on the presupposition that 37.4.23
The argument depends on assuming which one of the following 33.3.16
The argument depends on the assumption that 33.3.11
Which one of the following is an assumption on which the argument depends? 33.1.19
The argument depends on which one of the following assumptions? 33.1.13


Which one of the following is an assumption required by the argument? 46.3.17, 44.4.16, 44.4.07, 44.2.23, 42.2.14, 38.1.12, 37.4.15, 37.2.15
The argument requires the assumption of which one of the following? 38.4.06
The argument requires assuming which one of the following? 38.1.14


Which is an assumption on which the argument relies? 46.3.15, 37.2.19.
The argument relies on which one of the following assumption? 33.3.25, 30.4.19

Darmody
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Re: Assumptions - Sufficient/Necessary

Postby Darmody » Fri Aug 02, 2013 8:16 pm

Thanks for the replies guys! Really in depth and helpful. I really appreciate it

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RobertGolddust
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Re: Assumptions - Sufficient/Necessary

Postby RobertGolddust » Fri Aug 02, 2013 10:51 pm

Imagine that the right answer choice for a justify stem is like a guardian angel descending from the sky, offering your argument infallible protection.

TylerJonesMPLS
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Re: Assumptions - Sufficient/Necessary

Postby TylerJonesMPLS » Sat Aug 03, 2013 1:03 am

I forgot indicator words for sufficient conditions. Here are some:


The conclusion follows logically if which of the following is assumed? 30.4.01, 32.1.05, 32.4.04, 34.2.10, 35.1.20, 35.1.22, 35.4.14, 35.4.19, 36.1.26, 36.1.18, 36.3.12, 37.2.05, 37.4.09,
Which one of the following, if added as a premise to the argument, most helps to justify its conclusion? 30.4.15
The conclusion follows logically if which one of the following is added to the premises? 28.1.24,
Which of the following, if assumed, helps most to justify the conclusion?
The conclusion can be properly inferred if which one of the following is assumed? 36.1.22
Which one of the following, if assumed, enables the conclusion of the argument to be properly drawn? 37.4.20

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Jeffort
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Re: Assumptions - Sufficient/Necessary

Postby Jeffort » Sat Aug 03, 2013 6:51 pm

TylerJonesMPLS wrote:I forgot indicator words for sufficient conditions. Here are some:


The conclusion follows logically if which of the following is assumed? 30.4.01, 32.1.05, 32.4.04, 34.2.10, 35.1.20, 35.1.22, 35.4.14, 35.4.19, 36.1.26, 36.1.18, 36.3.12, 37.2.05, 37.4.09,
Which one of the following, if added as a premise to the argument, most helps to justify its conclusion? 30.4.15
The conclusion follows logically if which one of the following is added to the premises? 28.1.24,
Which of the following, if assumed, helps most to justify the conclusion?
The conclusion can be properly inferred if which one of the following is assumed? 36.1.22
Which one of the following, if assumed, enables the conclusion of the argument to be properly drawn? 37.4.20


You are incorrect.

The phrases you bolded are not the specific words that make those sufficient assumption questions instead of necessary assumption questions. They are an important part of the phrasing that makes some of them SA questions but are not the definitive sufficient indicator words.

The phrases 'properly drawn', 'logically follows', 'properly inferred', etc. do not by themselves distinguish between sufficient and necessary assumption question types since they are not sufficient condition indicator words. They are also used in some necessary assumption question stems. The phrases are merely ways to describe logically valid conclusions and do not specify sufficient or necessary. It is their presence ALONG WITH a sufficient condition indicator word or phrase that makes a stem a sufficient assumption question type. A sufficient condition indicator such as 'if' along with a word or phrase describing a logically valid (must be true) conclusion such as 'logically correct' is what determines that a stem is asking for a SA.

For example:

The conclusion can be properly inferred if which one of the following is assumed?

vs.

The conclusion can be properly inferred only if which one of the following is assumed?

The second example is a necessary assumption question type, not a sufficient assumption question stem. The combination of 'properly inferred' and the sufficient condition indicator word 'if' is what determines that the first stem is asking for a sufficient assumption, while use of the phrase 'only if' (a necessary condition indicator phrase) in the second example determines that the stem is asking for a necessary assumption.

Same thing with The conclusion follows logically if which of the following is assumed?. Replace 'if' with 'only if' and it is a necessary assumption question instead of SA. The principle holds true on the other example SA stems you provided that use the word 'if'.

Some of the stems you provided are not even SA questions:

Which one of the following, if added as a premise to the argument, most helps to justify its conclusion?
This is not a sufficient assumption question stem, it is just a normal strengthen the argument question. If the stem said 'justifies' instead of 'most helps to justify', then it would be a sufficient assumption question.

Same thing with:
Which of the following, if assumed, helps most to justify the conclusion?
This is just a strengthen question, not SA. If you change the bolded phrase to 'justifies the conclusion', then it would be a SA question since it also contains the sufficient condition indicator 'if'.

These two stems do not require the correct answer choice to perfect the argument, they only ask for something that provides additional support for the conclusion ('helps to justify' vs. 'justifies').

It's nice that you are trying to help others (you mentioned that you are a teacher), but you are wrong about this stuff and your advice could damage some students. It is common for students to have trouble distinguishing SA questions from NA questions on the test and end up selecting incorrect answer choices due to mis-identification because many forms of the stems are very similar with only one or two words of difference (suff indicator words vs. necc indicator words). What you say are sufficient indicator words are actually not, those phrases are valid conclusion indicators that have no preference between sufficient or necessary by themselves.

lalalany
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Re: Assumptions - Sufficient/Necessary

Postby lalalany » Sun Aug 04, 2013 10:38 pm

.
Last edited by lalalany on Sat Oct 19, 2013 9:58 am, edited 1 time in total.

TylerJonesMPLS
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Re: Assumptions - Sufficient/Necessary

Postby TylerJonesMPLS » Sat Aug 10, 2013 9:16 am

Hi Jeffort,

It was nice of you to take the time and trouble to correct what you thought was wrong in my post. I appreciate it, and I’m sure everyone else on the board appreciates your efforts. Please take my reply in the same spirit.

I checked over my post, and the only thing I could find that was incorrect was that I had forgotten to delete a "most justifies" in my list of sufficient assumption indicators.

However, you are seriously mistaken about how to tell the difference between necessary assumption questions and sufficient assumption questions.

You say:
The conclusion can be properly inferred if which one of the following is assumed?
vs.
The conclusion can be properly inferred only if which one of the following is assumed?
The second example is a necessary assumption question type, not a sufficient assumption question stem. The combination of 'properly inferred' and the sufficient condition indicator word 'if' is what determines that the first stem is asking for a sufficient assumption, while use of the phrase 'only if' (a necessary condition indicator phrase) in the second example determines that the stem is asking for a necessary assumption.

This is incorrect.
There are three ways that show that you are mistaken.

First, no question (stem) asking for a necessary assumption on any LSAT contains the phrase “only if.” You can easily check this. Get out your copies of LSATs, and try to find just one example. There aren’t any.

Here is the second way to see that you are mistaken. Get out your copy of the LSAT Super Prep. In “A Guide to Logical Reasoning Questions,” look for the heading “Assumptions” and under that look for “Necessary Assumptions.” Start reading there. It’s page 29 in my edition. Here is a quote of what it says:
Typical wordings of questions that ask you to identify necessary assumptions include the following:
The argument relies on assuming which one of the following?
The argument depends on the assumption that...
Which one of the following is an assumption required by the argument?

Notice that none of the LSAC's own examples of questions asking for necessary assumptions contains the phrase "only if" or any other indicator of a necessary condition. So, according to the LSAC itself, you are mistaken about your example of a necessary assumption question type.

Now, look at my examples of questions asking for necessary assumptions. You will find that it is almost the same as LSAC's. The only difference is that I have provide a lot of examples and citations of the necessary assumption wording/indicators. There is nothing incorrect about my list.

Keep reading the section headed "Necessary Assumptions." The section ends on page 31 in my edition. Notice that there is no discussion at all of necessary conditions. Why? Because necessary conditions are entirely different from necessary assumptions. The LSAC knows that there is no reason at all to discussion necessary conditions under the section addressing Necessary Assumptions, so it doesn't mention necessary conditions at all.

On page 24 there is a discussion of necessary and sufficient conditions, and in that section there is no mention of necessary and sufficient assumptions.

OK, this is the third way you can see that you are mistaken. As you know, Super Prep contains three LSATs and has explanations for all the questions on all three LSATs. "The Guide to Logical Reasoning Questions" has already given you the typical wording/indicators of questions that ask for a necessary assumption. On Super Prep tests A, B and C this language appears in the following questions: A.1.5, A.4.8, A.4.19, B.1.8, B.1.18, B.1.21, B.1.23, B.4.4, B.4.14, C.2.5, C.2.15, C.2.20, C.2.23, C.3.1, C.3.11. If you look up the explanations for these questions, you will see that there is no mention of necessary conditions at all.

My point is: a necessary assumption is entirely different from a necessary condition, and a sufficient assumption is entirely different from a sufficient condition. You have confused assumptions in arguments with conditions in conditional statements.

You say:
Same thing with The conclusion follows logically if which of the following is assumed? Replace 'if' with 'only if' and it is a necessary assumption question instead of SA. The principle holds true on the other example SA stems you provided that use the word 'if'.

I hope you can now see that what you say just above is incorrect. There are no questions asking for a necessary assumption that contain the phrase "follows logically."

Now, let's turn to sufficient assumptions. I made a mistake when I didn't bother to proofread the paragraph listing sufficient assumptions indicators that I added at the last moment. I obviously didn't intend to include as typical wording for a sufficient assumption question, "Which one of the following, if added as a premise to the argument, most helps to justify its conclusion?" The mistake happened because I was going to write an additional paragraph about sufficient assumption questions that appear under other question type headings, decided it would take too long, deleted the paragraph, and forgot that I had "parked" one example of a "most justifies" question, 30.4.15, in the sufficient assumption indicators paragraph.

But, since we're on the subject, you say:
This is not a sufficient assumption question stem, it is just a normal strengthen the argument question. If the stem said 'justifies' instead of 'most helps to justify', then it would be a sufficient assumption question.

This is not correct. If something completely justifies the conclusion, then it helps to justify the conclusion. If you look at 30.4.15, you will find that the correct answer-choice completely justifies the conclusion. In effect, 30.4.15 is a sufficient assumption question, and the fact that it asks which one of the following be added as a premise, suggests this very thing. This is because sufficient assumptions are unstated assumptions that must be added to the premises to close a gap between the conclusion and the other premises as they stand.

If you are an LSAT tutor, you really should work on getting straight on the difference between conditions as opposed to assumptions. This confusion that could lose points for your students on test day.

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Jeffort
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Re: Assumptions - Sufficient/Necessary

Postby Jeffort » Sat Aug 10, 2013 7:34 pm

TylerJonesMPLS wrote:Hi Jeffort,

It was nice of you to take the time and trouble to correct what you thought was wrong in my post. I appreciate it, and I’m sure everyone else on the board appreciates your efforts. Please take my reply in the same spirit.

I checked over my post, and the only thing I could find that was incorrect was that I had forgotten to delete a "most justifies" in my list of sufficient assumption indicators.

However, you are seriously mistaken about how to tell the difference between necessary assumption questions and sufficient assumption questions.

You say:
The conclusion can be properly inferred if which one of the following is assumed?
vs.
The conclusion can be properly inferred only if which one of the following is assumed?
The second example is a necessary assumption question type, not a sufficient assumption question stem. The combination of 'properly inferred' and the sufficient condition indicator word 'if' is what determines that the first stem is asking for a sufficient assumption, while use of the phrase 'only if' (a necessary condition indicator phrase) in the second example determines that the stem is asking for a necessary assumption.

This is incorrect.
There are three ways that show that you are mistaken.

First, no question (stem) asking for a necessary assumption on any LSAT contains the phrase “only if.” You can easily check this. Get out your copies of LSATs, and try to find just one example. There aren’t any.

Here is the second way to see that you are mistaken. Get out your copy of the LSAT Super Prep. In “A Guide to Logical Reasoning Questions,” look for the heading “Assumptions” and under that look for “Necessary Assumptions.” Start reading there. It’s page 29 in my edition. Here is a quote of what it says:
Typical wordings of questions that ask you to identify necessary assumptions include the following:
The argument relies on assuming which one of the following?
The argument depends on the assumption that...
Which one of the following is an assumption required by the argument?

Notice that none of the LSAC's own examples of questions asking for necessary assumptions contains the phrase "only if" or any other indicator of a necessary condition. So, according to the LSAC itself, you are mistaken about your example of a necessary assumption question type.

Now, look at my examples of questions asking for necessary assumptions. You will find that it is almost the same as LSAC's. The only difference is that I have provide a lot of examples and citations of the necessary assumption wording/indicators. There is nothing incorrect about my list.

Keep reading the section headed "Necessary Assumptions." The section ends on page 31 in my edition. Notice that there is no discussion at all of necessary conditions. Why? Because necessary conditions are entirely different from necessary assumptions. The LSAC knows that there is no reason at all to discussion necessary conditions under the section addressing Necessary Assumptions, so it doesn't mention necessary conditions at all.

On page 24 there is a discussion of necessary and sufficient conditions, and in that section there is no mention of necessary and sufficient assumptions.

OK, this is the third way you can see that you are mistaken. As you know, Super Prep contains three LSATs and has explanations for all the questions on all three LSATs. "The Guide to Logical Reasoning Questions" has already given you the typical wording/indicators of questions that ask for a necessary assumption. On Super Prep tests A, B and C this language appears in the following questions: A.1.5, A.4.8, A.4.19, B.1.8, B.1.18, B.1.21, B.1.23, B.4.4, B.4.14, C.2.5, C.2.15, C.2.20, C.2.23, C.3.1, C.3.11. If you look up the explanations for these questions, you will see that there is no mention of necessary conditions at all.

My point is: a necessary assumption is entirely different from a necessary condition, and a sufficient assumption is entirely different from a sufficient condition. You have confused assumptions in arguments with conditions in conditional statements.

You say:
Same thing with The conclusion follows logically if which of the following is assumed? Replace 'if' with 'only if' and it is a necessary assumption question instead of SA. The principle holds true on the other example SA stems you provided that use the word 'if'.

I hope you can now see that what you say just above is incorrect. There are no questions asking for a necessary assumption that contain the phrase "follows logically."

Now, let's turn to sufficient assumptions. I made a mistake when I didn't bother to proofread the paragraph listing sufficient assumptions indicators that I added at the last moment. I obviously didn't intend to include as typical wording for a sufficient assumption question, "Which one of the following, if added as a premise to the argument, most helps to justify its conclusion?" The mistake happened because I was going to write an additional paragraph about sufficient assumption questions that appear under other question type headings, decided it would take too long, deleted the paragraph, and forgot that I had "parked" one example of a "most justifies" question, 30.4.15, in the sufficient assumption indicators paragraph.

But, since we're on the subject, you say:
This is not a sufficient assumption question stem, it is just a normal strengthen the argument question. If the stem said 'justifies' instead of 'most helps to justify', then it would be a sufficient assumption question.

This is not correct. If something completely justifies the conclusion, then it helps to justify the conclusion. If you look at 30.4.15, you will find that the correct answer-choice completely justifies the conclusion. In effect, 30.4.15 is a sufficient assumption question, and the fact that it asks which one of the following be added as a premise, suggests this very thing. This is because sufficient assumptions are unstated assumptions that must be added to the premises to close a gap between the conclusion and the other premises as they stand.

If you are an LSAT tutor, you really should work on getting straight on the difference between conditions as opposed to assumptions. This confusion that could lose points for your students on test day.


Nice try but you are still incorrect.

The reasons you provided to say that I am incorrect about the factors that determine whether a stem is asking for a sufficient assumption or necessary assumption do not support your position.

Perhaps you misunderstood the main point of my post. I said you are wrong to say that the words/phrases you put in bold are what determines those stems to be SA question stems. By themselves 'logically follows' and other variations of the phrase are insufficient to determine question type, you need to also look at other words in the stem to differentiate SA from NA question types. Just because a stem includes the phrase 'logically follows' does not automatically mean it is for sure a SA question, there is more to it than just presence of such a phrase. That is my point that I explained in detail.

The list of example question stems given in the SuperPrep book is not an exhaustive list of all the different forms of stems that have been used for sufficient assumption questions or necessary assumption questions on the LSAT. Using the fact that LSAC didn't include any stems with 'only if' as examples to argue that they don't exist is short sighted and clearly a flawed way to argue that I am wrong. You should be able to identify the flawed method of reasoning you engaged in with that line of argument.

Just because LSAC didn't include any in the examples in the SuperPrep study guide doesn't mean that they don't exist, you just didn't look hard enough and/or assumed that LSAC tells people everything there is to know about the LSAT in the short study guide in the SuperPrep (they do not!). I mean really, you do see the flawed assumption you made here right?

Your positions against what I said are destroyed by examples:

There have been necessary assumption questions that use the phrase 'only if' in the stem, and simply removing the word 'only' from those stems would convert them into a sufficient assumption question stem for the reasons I stated in my post above!

Here is an example of a stem that illustrates my point and shows that your position is flawed:

Oct 94, LR1 #13 "...only if which one of the following is assumed?"

Remove the word 'only' from this stem and instead of asking for a necessary assumption, it would be asking for a sufficient assumption. The reason is because the phrase 'only if' refers to something that is necessary whereas the word 'if' in isolation refers to something that is sufficient, thus being the critical factor that distinguishes whether you are being asked to find a necessary assumption or a sufficient one.

Your line of reasoning about necessary conditions being different from necessary assumptions is completely misplaced and really doesn't make much sense in the context of how to distinguish NA question stems from SA question stems. Maybe I don't understand your point here. How does that even matter when talking about determining whether a stem is asking for a NA or a SA? The phrase "only if" refers to something necessary whether you are talking about assumptions or conditions, just like the word 'if' when used in isolation (no only) always refers to something sufficient. The sufficient/necessary indicator keywords in the stems ARE part of what determines whether a stem is asking for a SA vs. NA! How can you disagree with that? Your list of necessary assumption example stems was fine, and if you look at them, almost every one has a word or phrase that refers to something that is necessary while all the SA stems include a sufficient indicator word/phrase. Do you see the connection to my main point? Almost all NA question stems contain necessary indicator words/phrases such as depends, relies upon, based on, etc, the main exception being variations of the short "The argument assumes..." stem that contains no necc or suff indicator words/phrases, all SA stems contain a suff indicator word/phrase. Does that help highlight the key point here?

This is pretty basic conceptual stuff when it comes to talking about conditional reasoning and words/phrases used to refer to sufficient vs. necessary conditions/assumptions/factors/relationships/whatever. I'm surprised you are even trying to argue about it, especially with a seriously flawed argument partly based basically on 'LSAC doesn't say so in the SuperPrep book!'. Shame on you as a professor for making such a poor argument! Seriously, you are wrong and your argument to make your point is logically flawed. I mean really, you are a professor that has taught logic before, what don't you get about this and don't you see the serious flaws in your argument? Certain words and phrases indisputably refer to necessary things while others always refer to sufficient things.

You also said that there are no NA stems that contain the phrase "follows logically". Again you are incorrect. Keep in mind that there are several synonymous phrases used on the LSAT such as 'logically follows' & 'logically correct' that are used interchangeably.

June 91, LR1 #19 "...argument to be logically correct, it must make which one of the following assumptions?"

This is a necessary assumption question stem because it is asking for an assumption that is NECESSARY to the argument, not one that is sufficient. The word 'must' in the stem refers to something that is necessary, not something that is sufficient, thus being the critical word to determine the question type.

You seem to have gotten a bit miffed by my criticism of your post and overreacted to the point of kinda turning this into a pissing match, which is not what I wanted and especially didn't expect from somebody that has taught logic and claims to have expertise. My post above was mainly meant to elaborate on yours with additional information that is important to the topic.

My critique of your post was simply that the phrases you bolded in the stems (logically correct, follows logically, etc.) are not BY THEMSELVES definitive of whether a stem is asking for a SA or NA. While it is true that those phrases rarely appear in NA stems, and frequently do appear in SA stems, you cannot rely solely on the presence of the phrase to determine SA vs. NA. You also need to look for suff/necc indicator words/phrases in the stem because there are stems that use those phrases that are NOT SA questions!!! There are NA questions and also MBT question stems that also use variations of the phrase 'logically follows'. This means that phrases such as 'follows logically' are not by themselves sufficient assumption indicator phrases as you said in your post I criticized. I guess one way to say it is that the phrase 'logically follows' BY ITSELF is NOT sufficient to determine that a question stem is asking for a SA, but that phrase ALONG WITH a sufficient condition indicator word or phrase such as 'if' rather than having a necc indicator word/phrase is sufficient to conclude it is a SA question. Get it? Does that make sense? (It's my main point!! ;) )

It's really basic logical stuff to understand that certain words and phrases refer to sufficient things and others refer to necessary things, and that it's important to understand which words and phrases refer to which when you need to distinguish sufficient from necessary in a particular context. I'm not sure why you take issue with this and say that I'm wrong when I say that 'only if' refers to necessary while 'if' by itself refers to sufficient, and that in the context of asking an assumption question, the necc and/or suff indicator keywords in the stem are important for determining which type of assumption the stem is asking for. Why do you disagree with this? I'd love to hear an argument based on logic for your position.

Other than your really lame 'it didn't say so in the SuperPrep' argument that failed, and your false claim about nonexistence of certain question stem forms, do you have a valid argument against using conditional indicator words to help differentiate sufficient from necessary assumption questions?

Your insult at the end of your post was unwarranted, especially given that you are still incorrect. It's nice that you have taught logic before and think you have a better grasp of it than myself or others, but your flawed reasoning and incorrect facts you are working from show that you are far from being an expert on the LSAT and the various ways it tests peoples understanding of certain logical concepts. Being familiar with basic logic and reading the SuperPrep study guides doesn't make you an LSAT expert. Be a student and not a teacher for now and you might learn something useful about the LSAT from some of us here that may help you when you take the test. Please refresh your memory about good logic too, your argument to supposedly prove me wrong is just plain silly and based on several really obvious flawed methods of reasoning that you should know better than to use in an argument as a professor of logic. I mean really dude, seriously, cuz LSAC didn't say so in the book and didn't mention nec conditions in the part about nec assumptions so you're right and I'm wrong? Seriously? lol

PS: I'm not going to dig out a bunch of references, you can look through the tests and find them on your own if you care to waste a bunch of time, but there are other examples of NA question stems that use variations of the phrase 'logically follows' and/or 'only if'. They are not very common but they do exist and pop up every now and then. Since they are used less commonly for NA questions than typical NA stems that use words such as depends, requires, relies upon, etc. students typically get them wrong because they mistakenly treat them like SA questions due to falsely thinking the phrase 'follows logically' is what determines a stem to be asking for a SA. These less common NA stems are exactly the reason I responded to your post. Misinformed students mistakenly believe that every stem including the phrase 'follows logically' is a SA question, but that is not the case!!!!!!!!!!!!!!!!!!

Anyway, a lot of text about an issue that is not a very big deal in the big scheme of things since the chances of getting a NA stem in a form that is easily mistaken as a SA stem or vice versa without close inspection on any given test-form is pretty small.

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tl;dr version:

First, no question (stem) asking for a necessary assumption on any LSAT contains the phrase “only if.” You can easily check this. Get out your copies of LSATs, and try to find just one example. There aren’t any.

Wrong! See example listed above. There are also other examples if you look through all available test questions. Please become familiar with stuff before you make up/invent factual claims and assert them as true, it took me less than a minute to find an example for reference in the first LSAT book I grabbed from a stack.

There are no questions asking for a necessary assumption that contain the phrase "follows logically."

Incorrect! See example listed above. There are also other examples if you look through all available test questions. Strike two, you made up this false fact just like your claim about 'only if'. Please stop making stuff up and saying its true when you don't know what you are talking about, it kills your credibility instantly and shows that you are prone to just making $hit up to try to be right in an argument. Sometimes people that do such things are called liars too!

My point is: a necessary assumption is entirely different from a necessary condition, and a sufficient assumption is entirely different from a sufficient condition. You have confused assumptions in arguments with conditions in conditional statements.

Ok, so conditions are different than assumptions. So what? What does that have to do with using suff/necc indicator words to distinguish NA question stems from SA question stems? I don't see how this point of yours is relevant to determining question type. Please explain if you can. That stuff you said about what LSAC did and didn't discuss in certain parts of the SuperPrep study guides doesn't seem relevant to determining question types from stems.

I didn't confuse assumptions in arguments with conditions in conditional statements, you are confused. We are not even talking about that difference since it's unimportant for determining question type, I don't even know what you are talking about with that statement in the context of this discussion. Whether you are talking about sufficient and necessary assumptions or s/n conditions, the same sets of indicator words and phrases are used to distinguish/refer to suff stuff vs necc stuff, hence why using 'only if' instead of 'if' changes the meaning of what is being described.

I think you missed that whole point. This conversation has nothing to do with conditions vs assumptions, it's all about determining question type from the stem, either SA or NA, that's it! I think you got confused about the topic of discussion, or at least what my earlier post in the thread is about vs what it is not about. Some of the stuff in your earlier post in the thread about conditions vs assumptions and the examples you provided was pretty confusing and didn't seem to fit in with or be on point with what others were talking about in the thread and certainly has nothing to do with the issues I'm talking about. Perhaps you got confused about which topic I was addressing or which aspect of suff vs necc was under discussion, IDK.

LSAC simply did not include a section in the guide that specifically and fully addresses all the factors that determine whether a particular stem is asking for a SA vs. a NA. They could have, there is stuff to talk about (like what I wrote!), but they just didn't go into depth about the topic.

That stuff about the strengthen question 30.4.15 is irrelevant and completely misses the point of this entire discussion about properly identifying question types from the stem. So what if the correct answer choice on some strengthen questions actually provides enough information to qualify as a correct answer had the stem asked for a SA? This discussion is about the logical criteria for the correct answer choice that is established by the stem, not about situations where the correct answer provides more than the stem requires.

If you are an LSAT tutor, you really should work on getting straight on the difference between conditions as opposed to assumptions. This confusion that could lose points for your students on test day.


You are the one that is confused and misunderstanding things. I am offended by your statement, especially given the position of authority you are trying to approach the discussion from with mention of being a professor of logic for 25 years while also literally making up false facts about the LSAT and claiming them to be true as proof that I am wrong.

Shame on you again professor, whatever school you taught logic for would be ashamed of your argument here, especially the false claims of fact you made up as the primary support for your position. Seriously, did you just make those claims up and hope you were right (or that I wouldn't know enough to know those claims to be false) in order to try to sound superior and/or like an expert to win the argument because you didn't like that I said you were incorrect? It's a pretty pathetic method of argument, especially from a self identified professor of logic making an argument about facts and logic about a frigging test of logic! I mean wow, what a paradox man!!! Whew, what a doozey! How'd you manage to work all those logical fallacies into one big post in an argument about facts and logic of the LSAT? It's pretty impressive!

As I already pointed out, nothing in my post was about assumptions vs. conditions. You missed the point of discussion: differentiating SA question stems from NA question stems and the importance of suff vs necc indicator words and phrases. Whether you are talking about suff/necc assumptions vs suff/necc conditions, the vocabulary of suff vs necc indicator words and phrases is the same in both. Get it now? Nobody is talking about conditions vs assumptions but you.

Please re-learn logic and learn more about the LSAT before you try again to sound professorial since you failed pretty miserably this time. I wouldn't have been harsh and would have been understanding of your incorrect position if you were just an unsure student going off hearsay rather than a professor that is willing to make up facts to try to be right in an argument. It's pretty pathetic of you to have done that and it bothers me much more than the fact that your position is incorrect and could mislead students.

Oh well, this version didn't turn out to be shorter than the first above one, but I do feel better for chastising you about your shady reasoning tactics!




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