Normal full-term babies are all born with certain instinctive reflexes that disappear by the age of two months. Becau...

Mehran January 29, 2017

@varcity64 (B) is a valid positive argument.

The stimulus is a valid contrapositive argument, which is exactly what we encounter in (D).

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Tony October 1, 2018

I thought ''instinctive reflexes" would be the sufficient condition for this question since its introduced by "all"

Mariana January 16, 2019

I am having issues in general with logic problems, and this question highlights one particular struggle. How do you determine which pieces of information need to be pulled out to even start solving logic problems? I wrote down NFT (Normal full-term) --> IR (Instinctive reflexes), but obviously somehow you need to incorporate the information about age (3 mos.) Is there a good rule for how to figure out what to pull out, and what is extraneous information?

Ravi January 16, 2019

@melissakaijukags,

"All" is not referring to instinctive reflexes here. "All" refers to
normal full-term babies, as its describing a characteristic that all
of them have. Certain instinctive reflexes that disappear by the age
of two months is the necessary condition; it's what they all
necessarily have.

@Mariana,

Your diagram is off to a great start, but you forgot to include some
key information in your necessary condition. The necessary condition
isn't just IR, it's IR that disappear by the age of two months.

NFTB - ->IRDby2months

We're then told that a three-month-old baby exhibits these reflexes.
This information fails the necessary condition

/IRDby2months

Since the necessary condition is failed, the sufficient is also failed

/NFTB

This is exactly what the stimulus concludes.

The structure of the argument in the stimulus is

A - ->B

/B

therefore,

/A

Answer D perfectly matches this structure

Answer D diagrams like this:

Opossums - ->Abdominal Pouch

/Abdominal Pouch

/Opossum

This structure is identical to the structure found in the stimulus,
and it's our correct answer.

There is no clear 'rule' for figuring out how to properly diagram
conditional statements, but with continued practice, you'll develop a
greater intuitive feel for knowing how to do it.

The biggest recommendation I can give us to make sure you're fully
aware of what is contained within both the sufficient and necessary
conditions of what you're diagramming.

It sounds like you got tripped up on this question because you failed
to note that the necessary condition in the stimulus included
information about the instinctive reflexes disappearing by the age of
two months. Having this info in mind allows us to see how a
three-month-old baby exhibiting these reflexes fails the necessary
condition, which thereby triggers the failure of the sufficient
condition.

Does this make sense? Let us know if you have any more questions!

Jamie-Porto July 1, 2020

I am still confused as why B is not correct. I, like the original poster took
talk-> not ape

therefore the contrapositive would be
ape-> not talk

this follows the same logic that the stimulus did?

shunhe July 3, 2020

Hi @Jamie-Porto,

Thanks for the question! So the problem with (B) is that while it’s pretty similar, and would be correct if there were no closer answer choices, there is a closer answer choice in this problem. Let’s take a look at the stimulus:

Full-term babies —> Reflexes disappear
~Reflexes disappear
Conclusion: ~Full-term baby

In other words

A—>B
~B
Conclusion: ~A

And we see that logically speaking, if we take the contrapositive of the first statement, then (B) exhibits the same logic. BUT (D) is the same and also doesn’t make you take the contrapositive. So (D) is closer and therefore right, since we’re always looking for the best answer on the LSAT.

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emilymorganized October 10, 2020

Thank you for all the attention to this tricky question! Is the big takeaway here to default to the original rules?

ie, If we remember that "All A's are B's" can be diagrammed
A-> notB
OR
B ->notA

should that have been a red flag? I understand that D is the BETTER choice as it is a valid contrapositive argument and B is a valid contrapositive argument OR a positive argument (depending on how you diagram it). But I imagine if I were taking this test live, I would mark B and move on for time's sake!