PT20.S1.Q16 - brown dwarfs

AnthonyScaliaAnthonyScalia Alum Member
edited August 2017 in Logical Reasoning 330 karma

(P.S., I know this is a long and dense post, but there's an opportunity at the bottom for anyone reading this to get paid, so hopefully that's an incentive to read this :P)

Hi guys! I wanted to get some feedback from you smart people on the LR question about brown dwarf stars.

We're asked for an assumption on which the argument depends, so this is a NA and the right answer should strongly undermine if not completely discredit the argument when negated.

The claim is that any star found with no lithium is not a coolest brown dwarf (CBD.)
The support is that all stars except for CBD are hot enough to "destroy lithium completely."

The right answer, A, is that "None of the CBDs have ever been hot enough to destroy lithium."

Formal logic wise, I get CBD ---> Not(Hot enough to destroy lithium)

Negating it, you get that not none (so, some) CBDs have ever been hot enough to destroy lithium.

I don't understand how this even undermines the conclusion, let alone discredits it as we would hope for in a NA. In fact, this sounds perfectly congruent with the argument. The support says that CBDs cannot destroy lithium /completely/, completely being a deliberate word choice that does not appear in the answer. If "completely" had no bearing on the meaning of the text, it wouldn't be included.

Therefore, our negated answer, suggesting that some CBDs have ever been able to destroy lithium in some capacity, does nothing to undermine the claim that a star found without lithium cannot be a CBD.

Maybe in being able to destroy lithium in some capacity, said stars are still not hot enough to destroy lithium content in full. Even if you need to make a small assumption jump for this question, I think the most reasonable assumption is that CBDs being able to destroy lithium by some means does not really even scathe the claim that it can't finish the job. This would be a stretch for a weaken question in my opinion, let alone a necessary assumption.

My other question on the matter regards the rules for dangling modifiers in LSAT texts. In this question, a sentence reads "All stars but the CBDs are hot enough to destroy lithium completely by converting it into helium.

I'm a bit confused about how a dangling modifier would apply here in the absence of context. Does the clause following "destroy lithium" imply that the dwarfs cannot destroy lithium in full, (the process by which happens to be by converting to helium,) or that they cannot accomplish the task exclusively by converting the lithium to helium (implying that in order to complete the task in full, CBDs must destroy it in some other way than converting to helium.)

As a native English speaker, I probably wouldn't even have second thoughts and assume it was the former option, if reading or hearing that sentence. However, as an LSAT student who is actively analyzing precision of language, I find myself confused. I feel like I've seen other situations on the LSAT where failure to consider the precision of language, instead using traditional colloquial interpretations, in fact leads you to the wrong answer. Why is it different in this case? (Assuming there isn't some grammar rule I'm not aware of, which I'd love to discover!)

I realize that there is a degree of "picking the best answer" with LSAT questions, and NAs in particular. With that in mind, I still felt that B was a better, albeit still lacking answer.

Negating B leaves us with the statement that it's not the case that most stars too cool to burn hydrogen (TCBH), (which concretely includes CBDs,) are too cool to destroy lithium completely. This still leaves open a wide range of possibilities for some (but less than 51%) TCHB stars to indeed be able to destroy lithium completely. If we know that this class of star may very well have members capable of destroying lithium completely, and that CBDs are a member of that class, we at least have a hint that maybe some CBDs can destroy lithium completely, undermining his support and damaging the argument consequently.

Do I like answer choice B? Definitely not. However, it matches the specific, important diction of the prompt ("destroys completely") whereas A does not, and it gives concrete reason, when negated, to suggest something that directly contradicts and soils the argument. A, when negated, simply refrains from counting out a possibility that would damage the argument, rather than in any way suggesting that the argument is in fact damaged. Knowing that some CBDs have ever been hot enough to destroy lithium at all leaves the possibility that they can destroy it all the way, which we would want to be the case for this answer to be right since that breaks the argument. But, it just as neutrally, it allows for the possibility that they still can't go all the way, which would leave the argument in tact and reaffirm the support from which the conclusion is derived.

TL;DR, I see A suggesting an ambiguity that reads neutrally: If negated, A only weakens the argument by indicating that the opportunity for the argument to be undermined exists, not that it likely does or does not.

B, when negated suggests an ambiguity that, while still concluding nothing, leans towards something that would hurt the argument. Obviously that's a fine distinction that the question doesn't even mean for you to consider, but I don't think it can be avoided if you read the text precisely, and reasonably interpret "destroy completely" and "destroy" as meaning different things. If they meant the same thing, they would say the same thing.

Lastly, any question I ever get wrong on LR is similar to the conundrum I have with this one. I overanalyze the question, but even when I recognize that I'm overanalyzing and need to read a little more simply, I cannot for the life of me figure out when it's appropriate to make which particular little assumptions. If I redid this question 1000 times, I'd think that the logic leap in choice B is more realistic than the choice B counterpart all 1000 times.

Are there any tutors who would be particularly well suited to help me with this very specific challenge? It's frustrating because I literally have no clue what I need to change in order to get these questions write. I'm a native English speaker, born and raised in the midwest United States, and so I don't know what other factors could be causing me to be so clueless when it comes to figuring out which little logic leap/assumption is the right one. If I'm not overlooking some other caveat to this problem, how can the LSAT justify the correct answer if there isn't a concrete reason why their leap is more valid than mine.

If anyone thinks they would be able to help me remedy this conclusion, I'd be more than happy to pay for your time. I'll take help any way I can get it, so don't hesitate to take my money even if you're not a tutor and are a fellow student! Thanks guys :)

Comments

  • OlamHafuchOlamHafuch Alum Member
    2326 karma

    If any CBDs had ever been hot enough to destroy lithium, you could not conclude that any star that has lithium is not a CBD -- perhaps it is a CBD but had been hot enough at one point to destroy all lithium. So it sounds like it totally destroys the argument to me.

  • LSATcantwinLSATcantwin Alum Member Sage ⭐
    13286 karma

    So I'm going to attempt to tackle this - go easy on me because my confidence in explanations is very low.

    The stimulus says;

    "All Starts but the coolest of brown dwarfs are hot enough to completely destroy lithium."

    We are on the same page here. Every single star, that is not a cool brown dwarf, completely burns lithium.

    Then we get the conclusion;

    "Any star found that contains no lithium is not one of those coolest brown dwarfs"

    Answer choice A says;

    "None of the coolest brown dwarfs has been hot enough to destroy lithium."

    Negation: "At least one of the coolest brown dwarfs has been hot enough to destroy lithium"

    Okay so we have our arguments laid out.

    If at least one of the brown dwarfs has been hot enough to destroy lithium - then doesn't it follow that it's possible to have a cool brown dwarf with no lithium?

    If that's the case, doesn't the claim of "any star that contains no lithium is not one of the coolest brown dwarfs" fall apart?

  • LSATcantwinLSATcantwin Alum Member Sage ⭐
    edited August 2017 13286 karma

    I didn't really fully answer what you asked.

    So we see by my first comment that AC: A does have some baring on the conclusion of the argument. It's creates the possibility that a CBD has no lithium.

    Now for AC: B

    "Most stars that are too cool to burn hydrogen, are too cool to burn lithium."

    Negation: "Not most stars, (or less than 50% of stars) that are too cool to burn hydrogen, are too cool to burn lithium."

    How does this touch the conclusion at all?

    C: "Any star found that contains no lithium is not one of those coolest brown dwarfs"

    Okay, but so what? It doesn't tell us nearly enough about CBD's. It leaves the door wide open for what/if's.

    More succinctly, AC A has a direct baring on the conclusion - it address exactly what the conclusion states.

    AC B: is a board claim leaving the door wide open and does not directly address the claim about CBDs.

    Again, I'm not the best teacher. I hope this helps a bit though to see how the test writers think?

  • AnthonyScaliaAnthonyScalia Alum Member
    edited August 2017 330 karma

    @uhinberg said:
    If any CBDs had ever been hot enough to destroy lithium, you could not conclude that any star that has lithium is not a CBD -- perhaps it is a CBD but had been hot enough at one point to destroy all lithium. So it sounds like it totally destroys the argument to me.

    Thank you so much for your response!

    The conclusion you cite assumes that it is indeed possible for the CBDs to burn lithium completely, and that there aren't other forces that would keep lithium in the star (this latter point is one that can be reasonably discarded in my opinion, but the former one is legit.)

    You're right in that the prompt leaves the possibility for circumstances that would pretty much destroy the argument, but it just as plausibly leaves the possibility for circumstances that leave the argument perfectly in tact. With that in mind, i agree that A weakens the argument because it legitimizes the possibility that the argument is flawed/broken.

    However, B does the same thing, but additionally gives us reason to believe that the argument-breaking possibility is in fact likely, whereas A merely tells us it's a possibility. Any possibility hurts the argument, but a semi-concrete description of that possibility weakens the argument more than a purely neutral possibility without any indication of the actual likelihood.

    @LSATcantwin said:
    So I'm going to attempt to tackle this - go easy on me because my confidence in explanations is very low.

    The stimulus says;

    "All Starts but the coolest of brown dwarfs are hot enough to completely destroy lithium."

    We are on the same page here. Every single star, that is not a cool brown dwarf, completely burns lithium.

    Then we get the conclusion;

    "Any star found that contains no lithium is not one of those coolest brown dwarfs"

    Answer choice A says;

    "None of the coolest brown dwarfs has been hot enough to destroy lithium."

    Negation: "At least one of the coolest brown dwarfs has been hot enough to destroy lithium"

    Okay so we have our arguments laid out.

    If at least one of the brown dwarfs has been hot enough to destroy lithium - then doesn't it follow that it's possible to have a cool brown dwarf with no lithium?

    If that's the case, doesn't the claim of "any star that contains no lithium is not one of the coolest brown dwarfs" fall apart?

    I appreciate your response! I follow your logic up until the last line. Let's skip to addressing the penultimate line.

    Yes, it's possible that a cool brown dwarf can burn all of the lithium and be rid of it completely, which would break the claim like we want it to.

    But it also follows that it's possible for the (at least one) brown dwarfs that burn lithium to not /completely/ burn lithium, and I don't understand how that distinction could be ignored. If it didn't matter it wouldn't be in there, not to mention that B makes a parallel distinction (says destroyed completely) while A does not.

    So we're left with a possibility that breaks the conclusion, and one that keeps it squeaky clean. Of course, the mere existence of a possibility that breaks the argument could satisfy the test of hurting the argument, but as I mentioned in my original post, and reply to the guy above, B also leaves those two possibilities.

    With A negated, it's possible that brown dwarves can never burn all the lithium, but also possible that they can (which would break the argument.) We don't know which possibility is true.

    With B negated, it's possible that brown dwarves can never burn all the lithium, but also possible that they can (which would break the argument.) We don't know which possibility is true.

    I don't understand how you can derive a concrete and replicable way to distinguish between the two once that's established. Both weaken the argument by validating the mere possibility of a condition that would break the argument, but they don't do much else to suggest which one creates a more likely possibility. In both answers, the affirmative case breaks the argument pretty completely, so we can't really cite the degree of breaking the argument. That aspect is binary. The implications of both situations are identical outside of the probability that a possible argument-breaker is true, and we don't have information to evaluate that only distinguishing factor.

  • LSATcantwinLSATcantwin Alum Member Sage ⭐
    13286 karma

    @AJordanMD Sorry - I made a second comment about AC B.

    I think you are making decent arguments - but the important thing is to remember what the main conclusion is.

    AC:A directly talks about CBD's and gives the possibility to have a CBD without lithium.

    AC:B talks about stars in general and leaves the door way to open for interpretation.

    I hope this helps maybe a little.

  • AnthonyScaliaAnthonyScalia Alum Member
    edited August 2017 330 karma

    @LSATcantwin said:
    I didn't really fully answer what you asked.

    So we see by my first comment that AC: A does have some baring on the conclusion of the argument. It's creates the possibility that a CBD has no lithium.

    Now for AC: B

    "Most stars that are too cool to burn hydrogen, are too cool to burn lithium."

    Negation: "Not most stars, (or less than 50% of stars) that are too cool to burn hydrogen, are too cool to burn lithium."

    How does this touch the conclusion at all?

    C: "Any star found that contains no lithium is not one of those coolest brown dwarfs"

    Okay, but so what? It doesn't tell us nearly enough about CBD's. It leaves the door wide open for what/if's.

    More succinctly, AC A has a direct baring on the conclusion - it address exactly what the conclusion states.

    AC B: is a board claim leaving the door wide open and does not directly address the claim about CBDs.

    Again, I'm not the best teacher. I hope this helps a bit though to see how the test writers think?

    I see what you mean here as well. With answer choice B, you are correct in that it but indirectly addresses brown dwarves since they are only a subset of group that choice B talks about. It does leave open the possibility that those conditions apply to a different subset in the group, but it's neutral in that it doesn't suggest that the conditions likely do or likely do not apply to the specific brown dwarf subject.

    That distances answer choice 'B' from the claim, I agree.

    But 'A' also leaves a lot open. 'A' welcomes the possibility that the (1 or more) brown dwarves that burn lithium don't burn it completely, which is the distinction the claim makes. We have no clue whatsoever if the ability for 1 or more dwarf to burn lithium means that any of them can even get close to burning it all out.

    It's possible that all of those 1 or more brown dwarves burn all the lithium, just as it's possible that brown dwarves are part of the subset of those too-cool-to-burn-hydrogen stars that burn all the lithium.

    It's possible that some or none of those brown dwarves burn all the lithium, just as it's possible that brown dwarves have no representation in the subset of too-cool-to-burn-hydrogen starts that burn all the lithium.

    'A' leaves ambiguity regarding whether such dwarves can burn the lithium /completely/ or not. If they can't burn it completely, then the argument rests. If they can, the argument falls apart. We don't know which way it goes, just that both possibilities exist.

    'B' leaves ambiguity regarding whether dwarves are part of the too-cool-to-burn-hydrogen stars that do completely burn the lithium. If none of them are in that subgroup, the argument rests. No brown dwarves will be completely free of lithium. If one or more is in that subgroup, the argument falls apart. We don't know which way it goes, just that both possibilities exist.

    'A' distances itself from the claim by lacking information about whether or not certainly existing entities meet the distinction of "completely" that the argument deliberately uses. (We know for sure that both could be the case, but nothing about which is the case)

    'B' distances itself from the claim by lacking information about whether the body of certainly existing entities that would definitely meet all of the distinctions includes at least one of the brown dwarves. (We know for sure that this could be the case since brown dwarves are a subset of the group being described, but nothing on if it's actually the case.)

    They're different gaps in different ways, and neither leaps out as clearly more sinister than the other.

  • LSATcantwinLSATcantwin Alum Member Sage ⭐
    13286 karma

    @AJordanMD

    I know what you are saying but use the arguments main conclusion as a base for what you are comparing them to.

    Main conclusion:

    Any star that contains no lithium is not a CBD

    AC: A essentially says - well it's possible a CBD use to be hot enough to burn all lithium before it cooled down.

    That DIRECTLY talks to the main conclusion. It straight says look dude, it's possible what you just said about CBD's is wrong.

    AC:B you have to say, well there's this sub group, and a CBD coulda been part of that subgroup, if it was a part of the subgroup then the conclusion wouldn't follow.

    Do you see how AC:B takes more steps to weaken the conclusion? That's why it's not right.

    (On mobile, sorry if there are any errors, the app kind of sucks)

  • LSATcantwinLSATcantwin Alum Member Sage ⭐
    13286 karma

    @"Cant Get Right" can you shed some light for him. I think I just made everything less clear by trying to help

  • AnthonyScaliaAnthonyScalia Alum Member
    edited August 2017 330 karma

    I feel like you have to decide which part of the conclusion is more essential, the subject or the condition.

    Take the example "If it's a labrador then it's fur is completely brown"

    We have the options:

    A: "No labrador has ever had any amount of brown in its fur"

    and

    B: "Most dogs don't have fur that's completely brown."

    Negating A leaves the option that at least one labrador has at least a patch of brown in its fur, but tells us nothing about whether that brown is just a patch or the entire fur. That's a huge difference, and the argument only fails if it some of those labs have only brown spots.

    ********Basically, A is a sufficient assumption in my eyes, but not a necessary one. If we have A, we know for sure that the argument is true, but there's room for the argument to be true even if A is not correct.********

    Negating B leaves the option that some dogs don't have fur that's completely brown. If even one of those dogs that don't have completely brown fur is a labrador, the argument fails.

    ********On the flip side, B isn't a sufficient assumption. If we have B, the argument could still be false. There's not an ironclad certainty that the claim is true even if B is true. A concretely provides more than is but necessary, where B does not. "None" is very strong language, it's tough for such an absolute to be necessary for arguments in general.**********

    \' definitely applies to the subject but has little insight on applicability to the condition.

    'B' assures that entities that meet the condition exist, but isn't completely clear on whether or not at least one labrador is in that group of entities.

    I don't see how one can definitely conclude that the former is more or less damaging than the latter. In an argument like this, we're told that a quality belongs to this thing. If we know we're talking about the thing but don't know if we're talking about the quality, we're off a bit. If we know we're talking about the quality but not about the thing, we know we're off.

    I personally found the quality to be more important because 'A' doesn't address the quality at all while 'B' cites a group of things that includes our thing.

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