#21 - In modern deep-diving marine mammals, such as whales

[Administrator]

Complete Question Explanation

Weaken. The correct answer choice is (C)

Since the key to weakening an LSAT argument is to focus on the conclusion, it is essential to break
down the argument, which is structured as follows:

Premise: Modern deep-diving marine mammals have porous bone shells.

Premise: Porous bone shells make it easier for animals to swing back to the
surface after a deep dive.

Premise: The outer shell of the bones was also porous in the ichthyosaur, a
prehistoric marine reptile.

Conclusion: Ichthyosaurs were deep divers.

The conclusion of the argument is the final sentence, which contains the conclusion indicator “we
can conclude from this that…” Although this appears to be a causal argument (porous bones cause
the animals to swim back to the surface more easily), the relationship between the premises and the
conclusion is a conditional one:

DDM = Deep Diving Mammals
PBS = Porous Bone Shells
DD = Deep Divers


S N

Premise: DDM PBS

Premise: PBSIchthyosaur

Conclusion: DDIchthyosaur

The author’s conclusion is flawed for several reasons. First, ichthyosaurs are marine reptiles, not
mammals. Due to the skeletal or other physiological differences between the two, it is possible that
a marine reptile with porous bones uses them for a different purpose than a marine mammal does.
More importantly, the conclusion takes the form of a Mistaken Reversal:

Mistaken Reversal: PBSIchthyosaur DDIchthyosaur

Just because the ichthyosaur shares a feature common to all deep diving marine mammals does
not necessarily mean that ichthyosaurs were also deep divers. Having porous bone shells may be a
necessary condition for deep diving, but it is not a sufficient one.

To weaken this argument, look for an answer choice attacking this Mistaken Reversal. For instance,
you can show that at least some animals whose bones have a porous outer shell are not deep divers.
Alternatively, you can show that at least some non-diving animals do have porous bones. Indeed,
any answer choice attacking the necessary condition in the Mistaken Reversal would undermine the
conclusion of the argument.

Answer choice (A): This answer choice implies that having porous bones is not a necessary
precondition for deep diving in general. This fact, however, is consistent with the author’s premise
in the first sentence of the stimulus, which only addressed deep-diving marine mammals, not marine
species in general. Furthermore, the fact that having porous bones is not a necessary precondition
for deep diving does not weaken the conclusion of the argument, because we already know that
ichthyosaurs do have porous bones. The conclusion is assuming that having porous bones is a
sufficient condition for an animal to be a deep diver, not a necessary precondition for deep diving.

Answer choice (B): The fact that most modern marine reptiles do not have porous bone shells is
irrelevant, because it is unclear whether these marine reptiles are actually deep divers. Perhaps the
ichthyosaur, a marine reptile, was unlike most modern marine reptiles in that it was a deep diver
while the modern marine reptiles are not.

Answer choice (C): This is the correct answer choice. If most marine reptiles that are not deep
divers have porous bones, this would show that having such bones is not a sufficient condition
for an animal to be a deep diver. This would be consistent with the premise of the argument, but
immediately undermine the conditional relationship upon which the conclusion depends.

Answer choice (D): Just because ichthyosaurs do not share some other characteristics suited to deep
diving has no bearing on whether ichthyosaurs were themselves deep divers. There is no evidence
that the characteristics shared by whales are necessary for deep diving: they are merely suited to it,
and are shared by some marine mammals, not reptiles.

Answer choice (E): The fact that ichthyosaurs could have been deep divers even without porous bone
shells only strengthens the conclusion of the argument. Again, the author is assuming that having
porous bones is a sufficient condition for an animal to be a deep diver, not a necessary precondition
for it. Therefore, it is entirely plausible that ichthyosaurs could have been deep divers for some
reason other than their porous bone shells.

[netherlands]

Hi there PS,

I have another question for you - and when looking at this structure I think I may be starting to recognize a pattern thats used a lot in the LR portion of the test (please correct me if I'm wrong! )

I feel like I see many questions on the test that follow the pattern:

I) Object 1 has characteristic "A ".
II) Object 1 is thus "B".

I) Object 2 has characteristic "A".
II) Therefore Object 2 must also be "B"

In my head (after reading about typical flaws and misuses of evidence in LR) I always say to myself "proof of some evidence doesn't mean the claim is true" . Then I fiddle around and try and find an answer that looks good. I can often get the right answer, but my method is so imprecise and time consuming.

But, looking at your Conditional explanation makes it all so much clearer - I'm just not sure how you got there. What is it about the pattern above that should make me recognize that this is something I can easily use conditional reasoning for - since, like in this example, there aren't always obvious verbal indicators. And how did you identify which was the Suff. and which was the Necessary?


Maybe its just the mere fact that I'm assigning categories based off the presence of characteristics - which innately kind of has the conditional "if...then" relationship?

As always, thanks so much for your help in advance!

[Lucas Moreau]

The trick with this question, in that sense, is...let me diagram it the way you did.

I) Modern deep-diving marine mammals have porous bones
II) Therefore, they have these porous bones to assist their deep diving

I) Icthyosaurs had porous bones
II) Therefore, icthyosaurs have porous bones to assist their deep diving

From which the general principle could be stated:

If creatures have porous bones, then they will use them to assist their deep diving.

You Weaken it with the correct answer choice by saying "Many marine reptiles, past and present, that are not deep divers, have porous bones." It weakens the connection between porous bones -> must be for diving by showing that some animals have porous bones that do not use them for diving.

Hope this helps.

[netherlands]

Hi there,

I do understand the endpoint - but I guess I don't fully understand how we got there. Since there was no obvious language that indicated there was a conditional relationship ( and I realize that there won't always be) I'm not sure what my indicator should have been to use conditional reasoning.

After understanding that point - I'm then not sure in this case how I should have decided which was the sufficient and which was the necessary condition.

Basically - the online explanation diagrammed it as :

If Animal is DDM Animal has PBS

What I don't understand is - how did we get to that point? Off what basis did you guys decide that the DDM should be the sufficient? My first guess was that maybe there was a safe rule that the category becomes the sufficient condition, which then requires the necessary characteristic.

Afterwards, I do recognize the Mistaken Reversal:

If Ich has PBS Ich is DDM - and see that you negated it by showing that the sufficient condition didn't have to occur.

[Nikki Siclunov]

The reason why DDM is a sufficient condition lies in the implicit "all" (sufficient condition indicator) that applies to DDM. We are told that DDM's have porous bones. So, if you see a DDM, you can safely assume that it has porous bones. The relationship is absolute, because it can be reduced to a basic "if.../then..." statement. Thus, it is inherently conditional.

[netherlands]

I listened to the online Virtual Module of LSAT Scenarios last night and I believe he was saying the same thing - So anything that creates an absolute relationship between two or severals thing could basically be considered conditional, right?

Basically when I see that the presence of something, or categorizing of something, or existence of something, etc. is said to depend on something else - then this is considered conditional because there is a relationship being built there. Is that a proper understanding?

[Jon Denning]

I think you've got the idea, yes. Any time things are related to one another in a way that can be considered absolute/unwaveringly consistent, you've got conditionality. So certainly when one thing is said to depend upon, or require, something else, that's an extremely well-defined absolute relationship and the conditional connection should be recognized.

[lsatstudying11]

Hello!

To me, this argument felt causal--as in, the flaw in the stimulus is that it assumes the porous bones causes deep diving capabilities. I can see how this might actually be a conditional argument. Is thinking that this is causal totally off base of me? Thanks !

[Bob O'Halloran]

Hi Lsatstudying11,
Thank you for your question.
You are correct that this first appears to be a causal stimulus. However the major flaw that presents itself here is a conditional one-mistaken reversal. This is mapped out above. Sometimes the test makers will throw you a curve this, so you want to be aware of how they can subtly do this.
I hope this helps.
Let us know if you have any additional questions.
Bob