Just saw this CR on BTG,which seems weird.Experts help

[rishab1988]

Every political philosopher of the early twentieth century who was either a socialist or a communist was influenced by Rosa Luxemburg. No one who was influenced by Rosa Luxemburg advocated a totalitarian state.
If the statements above are true, which one of the following must on the basis of them also is true?
(A) No early-twentieth-century socialist political philosopher advocated a totalitarian state.
(B) Every early-twentieth-century political philosopher who did not advocate a totalitarian state was influenced by Rosa Luxemburg.
(C) Rosa Luxemburg was the only person to influence every early-twentieth-century political philosopher who was either socialist or communist.
(D) Every early-twentieth-century political philosopher who was influenced by Rosa Luxemburg and was not a socialist was a communist.
(E) Every early-twentieth-century political philosopher who did not advocate a totalitarian state was either socialist or communist.

I think the answer should be D. Any explanation on why the answer is A?

This guy posted the question.

I don't think any answer in this is correct.Even A is not correct for who acts as restrictive modifier.You can't infer that these were the only type of people.

A simply says only those people who were either socialist or communist were influenced by this guy.What about other type of guys,who might have rues in the early-twentieth century?

Only if who were set off by commas,A will be correct.

Because :

Every political philosopher of the early twentieth century who was either a socialist or a communist was influenced by Rosa Luxemburg.

means that Every political philosopher of the early-twentieth century was influenced by Rosa Luxemburg,and btw these philosophers were either socialist or communist.

B is incorrect for there might have been people,who ,even though were not influenced by this guy,were not advocates of totalitarian.

C is incorrect for there could have other people too who could have influenced political philosophers.

D -This again is an outrageous claim.We have no info whether communists and socialists were the only types of people in that time

E-Similar reason,there could have been other people too!

Since this question is must be true/infer question,you can't leave any gap in the answer choice.The answer choice should be air tight.Hence I believe all answers are incorrect.

BTW this question was posted in a 300 GMAT CR question text file.

[zachthegnome]

A seems to follow logically enough

If socialist early 20th century political philosopher --> influenced by the lady
If communist early 20th century political philosopher --> influenced by the lady
influenced by the lady --> did not advocate

If socialist early 20th century political philosopher --> influenced by the lady --> did not advocate

Negated: No socialist early 20th century political philosopher advocated.

I may be underthinking this one or you may be over-thinking it. In a pinch I'd end up going with A. D can't be correct because it is outside of the scope of all information presented so far (if they aren't early 20th century political philosophers who are communist or early 20th century political philosophers who are socialist we can't infer a thing).

[rishab1988]

I doubt your logic.

The who here is actiing like a that and not like a which.What you are are deriving can't be infered unless who is set off by commas.Just check MGMAT SC strategy guide on this and you will know what I am talking about.

You are doing what is called manipulating your own logic to get an answer.

If socialist early 20th century political philosopher --> influenced by the lady

Your very first statement is flawed. Nowhere in the premise it is stated that every political philosopher who was born in 20th century was influenced by this lady.

See my usage of "who" in the above sentence.If I used your logic,my sentence would mean "No philosopher was ever influenced by this lady.Whether that philosopher was born in 10th century ,20th century ,or 2st century." But My sentence really says this "Of those philosophers that were born in 20th century,none were influenced by this lady".This means that a 21st century philosopher could have been influenced by this lady.But I make no claim about this.

Another example:

My uncle,who is in US,is a billionaire.

The above sentence means that I have only one uncle and he is a billionaire. This uncle just happens to be in US.This is the meaning that answer choice A uses.

My uncle who is in US is a billionaire.

The above sentence means that I could have possibly many other uncles,but the uncle that I am talking about is the one in US and this is the only uncle who is a billionaire.This is what the premise says..

[GMATGuruNY]

Every political philosopher of the early twentieth century who was either a socialist or a communist was influenced by Rosa Luxemburg. No one who was influenced by Rosa Luxemburg advocated a totalitarian state.
If the statements above are true, which one of the following must on the basis of them also is true?
(A) No early-twentieth-century socialist political philosopher advocated a totalitarian state.
(B) Every early-twentieth-century political philosopher who did not advocate a totalitarian state was influenced by Rosa Luxemburg.
(C) Rosa Luxemburg was the only person to influence every early-twentieth-century political philosopher who was either socialist or communist.
(D) Every early-twentieth-century political philosopher who was influenced by Rosa Luxemburg and was not a socialist was a communist.
(E) Every early-twentieth-century political philosopher who did not advocate a totalitarian state was either socialist or communist.

I received a PM asking me to comment. Zach's assessment is correct. The trick is to translate the information given in the passage into if...then statements.

Every political philosopher of the early twentieth century who was either a socialist or a communist was influenced by Rosa Luxemburg.

Every A is B means, If A, then B. Thus:

If a political philosopher of the early 20th century was a socialist --> then he was influenced by Rosa Luxemburg.
If a political philosopher of the early 20th century was a communist --> then he was influenced by Rosa Luxemburg.

No one who was influenced by Rosa Luxemburg advocated a totalitarian state.

No A is B means If A, then not B. Thus:

If a person was influenced by Rosa Luxemburg --> then he did not advocate a totalitarian state.

Connecting the conditional statements above, we get:
If a political philosopher of the early 20th century was a socialist --> then he was influenced by Rosa Luxemburg --> then he did not advocate a totalitarian state.

The connected conditionals support answer choice A:

No early-twentieth-century socialist political philosopher advocated a totalitarian state.
Translating into an if...then statement, we get:
If a person was a 20th-century socialist political philosopher, then he did not advocate a totalitarian state.

Please note that this sort of question is common on the LSAT but does not typically appear on the GMAT. GMAT arguments do not require the linking of conditional statements such as those above.

Hope this helps!

[rishab1988]

Got it.

Thanks.

Missed the word socialist.Solving a LSAT like question at 4 in the morning,I guess, can take a toll.

[David@VeritasPrep]

I also received a PM asking me to reply...guess I was not the only one!

This is a long posting but please read all the way as I think it is very helpful.

OA is A.

Great explanation by "zachthegnome." Exactly right in your application of formal logic.

Rishab1988 - We do not want commas here in this premise. The premise is most decidedly not supposed to mean that Rosa Luxemburg influenced all early 20th Century political philosophers, but only the subgroup of those philosophers that are communist or socialist. I think the commas that you talk about would make it such that all early 20th Century political philosophers were influenced by Rosa L. and that all of them were socialist or communist. This is not the meaning of the premise. The intended meaning is if you are 1) an early 20th century political philosopher and 2) socialist or communist then you where influenced by Rosa L.

One other thing - it is always good to know the source. As you see below this is actually an LSAT question that appeared on the exam. In the case of an official LSAT or GMAT question you can bet that the answer is correct and the question is very well edited - as close to perfect as possible!

First of all I am not sure who has claimed this question, but it is an LSAT question. This question is from the December 1994 LSAT test from the first Logical Reasoning section, it is question 10. (Fun fact...this was my LSAT test, as in I took this test Dec 1994 test on the way to Law School).

Anyway, this question is taught as a perfect example of FORMAL LOGIC which means that it is very valuable to the LSAT, but may be a little tricky for GMAT studying.

With that said, I think that we can approach this one with our Must be True tools in hand.

HOW TO APPROACH AN INFERENCE/ CONCLUSION QUESTION

Remember that for a Must Be True question you are looking to eliminate the four answer choices that Could Be False. Could be True means nothing - anything could be true. I could actually be a computer that was designed to answer BTG questions and not a person at all. So do not think about Could Be True only eliminate those choices that COULD BE FALSE.

There are four ways to recognize those choices that Could Be False:

1) Out of Scope (a good way to find these is to ask yourself "what does the stimulus say about ___?")

2) Predictions (those things that are in the future are not Must Be True)

3) Must Be False (Some things are actually counter to the facts already stated)

4) Commands (Telling someone what they "should" do is usually not must be true).

Let's look at this question. The first thing we want to do is understand the scope of the stimulus - what do we know from this stimulus? We know a couple of things we know something about "early-twentieth century socialists and communists" being influenced by Rosa Luxemburg. And we also know about people who do not advocate totalitarianism (those influenced by Rosa Luxemburg).

Things we do not know about? We do not know if anyone actually advocates totalitarianism. We do not know about those who are not early-twentieth century philosophers or those who are not socialists or communists. So if someone was from a different time period or was an advocate of democracy then we cannot say anything about that person - it would be out of scope.

For an inference/conclusion question I do not spend a lot of time with the stimulus - just really understand what the scope is and is not and then move to the answer choices looking to eliminate the 4 that could be false. On your first time through the answer choices it is better to eliminate those that are out of scope and/or predictions first. These are easier to see. If a choice is possibly a command or a must be false come back to those later for intense analysis.

Answer Choice A) This is specific to socialist political philosophers of the early 20th century. (I will give further discussion of this answer choice at the end - for now we would move on and not eliminate this choice because it is not out of scope and not a prediction).

B) ELIMINATE - could be false - out of scope. Remember, we do not know about every early 20th Century philosopher only the socialists, communists and those influenced by Rosa Luxemburg.

C) ELIMINATE - could be false - out of scope. We do not know whether other persons influenced every early 20th Century socialist and communist. It could be that - for example - Karl Marx also influenced each of these people.

D) ELIMINATE - could be false - out of scope. Just because we know that Rosa Luxemburg influenced all of the socialists and communists does not mean that those are the only people she influenced. She could have influenced those who favor democracy or anarchy or some other system.

E) ELIMINATE - could be false - out of scope. This answer choice states that if a political philosopher was not totalitarian that he or she was either a communist or a socialist. This is certainly beyond the scope. It is possible that some people in favor or democracy are not totalitarians.

Returning to Choice A: Here is where the formal logic comes in: if a person is an early 20th Century socialist political philosopher that person is influenced by Rosa Luxemburg and that person is then not an advocate of totalitarianism. This is like the transitive property in math. If A = B and B= C then A = C. This is an example of formal logic and would not be tested in this way on the GMAT. However that does not mean you cannot use some formal logic to help you on the GMAT!

Yet as you can see, even on a question like this, formal logic is not needed as we can eliminate the incorrect and get down to just one answer.

[rishab1988]

David,that was super helpful.

The knowledge of the type of answer choices that can be wrong was also very helpful.

Geez.You guys are geniuses out there.Can't imagine what BTG would be without you guys.

Is it true that all guys who give LSAT are smart? I know I'm making a false conclusion.

I think the,only error in logic,that I made was that I missed the word socialists in the answer choice and instead completely concentrated on the word NO!. Lol

Even though it is an LSAT question,once I found that the missed the word socialist the answer choice A totally made sense.

But I definitely believe if answer choice A did not mention socialist,it would definitely be incorrect.

To infer that " NO POLITICAL ACTIVIST INFLUENCED BY THIS WOMAN WAS TOTALITARIAN" we would need to have a comma before who and a comma after communist.In this way the modifier would become non-restrictive and we can infer what I said in CAPS.

Am I right?

[KrazyKarl]

THanks for all the really thorough explanations, Mitch and David!