Q24 - In the Riverview building

[ohthatpatrick]

Question Type:
Match the Flaw

Stimulus Breakdown:
Conclusion: None of the 1-bed apts has a fireplace.
Evidence: The apts with balconies all have fireplaces, and none of the 1-bed apts have balconies.

Answer Anticipation:
The flaw is a classic Conditional Logic flaw. If you have a 1-bed apt, we know from the 2nd sentence that you DON'T have a balcony. The author performs an illegal negation on the first sentence, and thinks to herself, "If you DON'T have a balcony, then you DON'T have a fireplace". And that's how she arrives at her conclusion that "if you have a 1-bed apt, you don't have a fireplace".

Symbolically, we could say
p1: A --> B
p2: A --> ~C.
conc: C --> ~B
where A = have balcony, B = has a fireplace, C = 1-bed apt

We can think to ourselves about some of the salient qualities we're looking for:
there should be two premises, both conditional, both starting with the same trigger. The conclusion should bring together the right side of one with the OPPOSITE right side of the other.

Correct Answer:
C

Answer Choice Analysis:
(A) Promising.
Cat --> Have fur
Cat --> ~Fish.
We would want a conclusion to say "If you are a Fish, you don't have fur". Instead, this says "if you are a fish, you DO have fur".

(B) Eliminate as soon as we see the conclusion says "Some". The original conclusion was "all/none".

(C) Promising.
Cat --> have fur
Cat --> ~Dog.
We would a conclusion to say "if you are a Dog, you don't have fur". That's what we get!

(D) Not as promising. We'd have to rearrange premises, but we could still get two conditionals with matching triggers.
Dog --> not a cat
Dog --> not a fish.
We would want "If you're a fish, you're a cat" or "if you're a cat, you're a fish". We do get that. Ultimately, despite performing the same logical flaw, (D) just doesn't match up as nicely as (C), which mimics the original "All, None" premises and "None" conclusion. (D) gives us "None, None" premises and "All" conclusion.

(E) Eliminate as soon as we see we can't get the two premises to have the same conditional trigger.

Takeaway/Pattern: This is an unusual example of Match the Flaw because (C) and (D) both exhibit the same flaw. However, (C) does so with a closer structural match, which makes it a superior answer in terms of which of the two can MORE effectively demonstrate the flawed argument.

#officialexplanation

[samuelfbaron]

This was an overwhelming question to be met with at the end of the section.

Here, we have a parallel flaw question, so we are looking for an answer choice that mimics the flaw in the stimulus.

I'm not sure if I've mapped the argument correctly:

Every A has B
No A is a C
------------
No C has B.

Proceeding to the answer choices you'll notice the conclusion in some of the answers is the first sentence which made this VERY confusing to me.

I really can't diagram the incorrect responses. I worked by just jumping around and diagramming, I did find (C) to be correct.

Every A has B ( every cat has fur)
No A is a C (No cat is a dog)
-----------
No C has B (No dog has fur)

[noah]

This is a killer question so late in the section!

Here's how I approached this match the flaw question:

The stimulus gives us:

balcony --> fireplace
balcony --> NOT 1-bdrm
THUS: 1-bdrm --> NOT fireplace

And I would diagram this one--it's a lot of conditionals, and I'm going to have to match it.

Clearly, this is no good--and not only because LSAC tells us that it's flawed! The only think we could conclude about 1-bdrm apartments is that they have no balconies. We don't know anything about non-balcony apartments; they might also have fireplaces (perhaps EVERY apartment has a fireplace!).

A quick scan of the answers puts (B) on the probably not list since it has a "some" which doesn't match anything in the original. Looks like we have to dig deeper into the answer choices.

(A) looks tempting:

cat --> fur
cat --> NOT fish

But then we would want to see fish --> NOT fur and instead we see fish --> fur

(C) gives us:

cat --> fur
cat --> NOT dog
THUS: dog --> NOT fur

It's a match! At this point, unless I had time to kill, I'd pull the trigger and move on. But, for the geek in us all:

(D) cat --> NOT dog
dog --> fish
THUS: cat --> fish

We can use the contrapositive of the first premise and reorder to try to make this look closer to the original:

dog --> fish
dog --> NOT cat
THUS: cat --> fish

But we'd want to see cat --> NOT fish

(E) dog --> mammal
mammal --> NOT fish
THUS: fish --> NOT dog

We can see there's a premise mismatch.

[contropositive]

the same flaw was actually committed on section 2 question 25 as well. The flaw is that the author is combining two of the premises to make a conclusion out of them.
Therefore, I did a quick scan of the answers eliminated B, D, and E. Left with A and C

Original
A -- B
A -- /C
---------
C -- /B

Answer A
A -- /B
A --/C
--------
C --- B


Answer C
A -- B
A -- /C
--------
C --- /B
Perfect match!

I am not sure if I did anything wrong but I followed Patrick's explanation for section 2 and it helped me here.

[satyammerja]

I am not sure why answer choice D in question 68 section 3 of PT 68 is incorrect. Here is an explanation for why I think D can be considered the correct answer.

I made 3 assumptions about parallel reasoning questions which I would like to explain with aid of the following (flawed) reference argument:
Premises: 1. A --> B
2. A --> C
Conclusion: B -->C


Assumption 1) Contrapositives are permitted and do not change the structure of parallel reasoning question. For example, the following argument would be considered parallel to the reference argument:
Premises: 1. not B --> not A (this is a contrapositive of A --> B)
2. A --> C
Conclusion: not C --> not B (this is a contrapositive of B --> C)

Assumption 2) Swapping a given variable with its negation at every single occurrence of the said variable does not change the structure of the argument. This is because any variable can be redefined to be the negated version of its original definition. For instance, instead of defining D = dog, we can define D = not dog and then swap every single occurrence of D and not D in the argument without affecting its structure.
For example, the following would be considered parallel to the argument stated above (in assumption #1) and thus, by extension, also to the reference argument:
Premises: 1. not B --> A
2. not A --> not C
Conclusion: C --> not B
Note that I swapped A and not A as well as C and not C from the example in assumption #1.

3) The positioning of the premises is irrelevant to the structure of the argument and thus swapping them is permissible. For instance, the following argument would be considered parallel to the reference argument:
Premises: 1. A --> C
2. A --> B
Conclusion: B --> C
I just swapped premises 1 and 2 from the reference argument.

Now, onto the question. Here is how I traced out the argument in the stimulus:
Premises: 1. balcony --> fireplace
2. balcony --> no 1-bedroom
Conclusion: 1-bedroom ---> no fireplace

Replacing words with symbols in the above argument:
Premises: 1. A --> B
2. A --> no C
Conclusion: C ---> no B

Swap C and no C (assumption #2):
Premises: 1. A --> B
2. A --> C
Conclusion: no C ---> no B

Take contrapositive of conclusion (assumption #1):
Premises: 1. A --> B
2. A --> C
Conclusion: B --> C (contrapositive of no C ---> no B)

Analysis of flaw: one necessary condition leads to another necessary condition. I am going to look for parallel of this in the answer choice.

Now, onto answer choice D. Here is how I traced out the argument in the answer choice:
Premises: 1. cat --> no dog
2. dog --> no fish
Conclusion: cat ---> fish

Take contrapositive of premise #1 and conclusion (assumption #1):
Premises: 1. dog --> no cat (contrapositive of cat --> no dog)
2. dog --> no fish
Conclusion: no fish ---> no cat (contrapositive of cat --> fish)

Swap premises #1 and #2 (assumption #3):
Premises: 1. dog --> no fish
2. dog --> no cat
Conclusion: no fish ---> no cat


Change to symbols:
Premises: 1. A --> no B
2. A --> no C
Conclusion: no B ---> no C

Swap B with no B and C with no C (assumption #2):
Premises: 1. A --> B
2. A --> C
Conclusion: B ---> C

Eureka!!! Exact same structure as the stimulus.

[ohthatpatrick]

You're right that (D) is also just doing a smush-job with necessary conditions.

Dog -> not cat
Dog -> not fish

Conclusion:
creates some arbitrary relationship between cat and fish

But (C) better mirrors the original. If you actually have two answers that both work essentially the same, then start getting extra picky and care about the way the ideas were presented or even the order in which they were combined.

The question stem doesn't actually ask "Which of these has identical logical structure"; it asks, "the flawed nature of the original can most effectively be demonstrated by" which answer?

If you were trying to show someone why the original argument was wrong, it would be most effective to keep the premises in the same order/form as the original. It would be more of a challenge to demonstrate the flaw if, to do so, you needed to write several paragraphs and define several assumptions.

Good job with your conditional logic. The test writers apparently cared about more than just the conditional logic when writing this one.

[satyammerja]

Thanks ohthatpatrick for the clarification.

[egonza14]

Quick question re: my process on this question.

I was able to answer this one pretty quickly, and I want to make sure that I'm not being too careless in my approach on questions like these.

So I very very quickly eliminated A, B, and D in the same glance. I saw that each of their conclusions didn't have the negative "none" phrasing that the original conclusion had, and that left me to evaluate the conditional logic in C and E, saving me buckets of much needed time at the end of the section. I ended up choosing C.

In my review, I tried to poke holes into a method like this and thought: "it's not uncommon for the LSAT to sneak the correct form of the answer in as the contrapositive."

SO, Should I still stick with my current mode of attack for this question type or should I play it safer in the future? It's awesome to get an answer correct, but I know process will win the long game on this test.

[ohthatpatrick]

You're going to be okay doing that the majority of the time, but there are some examples on Matching questions where they trade a positive form for a negative form.

not all X's are Y = some X's are ~Y
few X's are Y = most X's are ~Y
no X's are Y = all X's are ~Y

So you could DEFINITELY kill (B) because "some" is not the same strength as all/none.

But you would technically still have to consider (A) and (D), if (C) or (E) hadn't worked.

I think it's a smart way to aggressively funnel your way towards the correct answer. If you trust your ability to match up a correct answer with the original, then finding (C) and judging it to be equivalent is enough.