This reasoning is most similar to.. (contrapositives) Forum

[Saltqjibo]

I've never read the LR bible or taken a course, but I usually do okay on LR. My question is: for those "this reasoning is similar to.." questions, does the the presence of a contrapositive or the formulation of the same logical stucture but in negative form negate the answer choice

i.e. is the reasoning similar in

All Bs are A, C is not A, therefore C is not B

and

All Bs are A, C is A, therefore C could be B

seeing as how they are both structurally consistent with eachother?

[EarlCat]

I've never read the LR bible or taken a course, but I usually do okay on LR. My question is: for those "this reasoning is similar to.." questions, does the the presence of a contrapositive or the formulation of the same logical stucture but in negative form negate the answer choice

i.e. is the reasoning similar in

All Bs are A, C is not A, therefore C is not B

and

All Bs are A, C is A, therefore C could be B

seeing as how they are both structurally consistent with eachother?

Generally the correct answer will be more similar than what you have presented. Obviously if the other four are completely inconsistent with the stimulus, then you know what you've got to go with.

There was one parallel question where the argument was essentially A-->B and B-->C therefore A-->C. One answer choice was X-->Y and Y-->Z, therefore ~Z-->~X. Even though it's logically consistent, it was incorrect. The correct answer concluded X-->Z and was thus more similar.

[r6_philly]

Hey EarlCat what are you doing over here!

[matt@atlaslsat]

Really good question!

I think the way I understand the question you initially pose is different then the way I understand your two hypothetically similar arguments. Let me take them both on as I understand them.

First, is the presence of a contrapositive grounds for dismissal? Let's look at PT45, S1, Q25. This one highlights your question very well I think. If you don't have access to this question, you should be able to follow below regardless.

Stimulus Key: VP (visitors to the park), EPH (engage in practices that harm animals), KH (know the practices cause harm)

VP some EPH
KH --> ~EPH
----------
VP some ~KH

the two answer choices I think are useful to look at are (B) and (C)

(B) Key: SP (signed the petition), MS (mayor's supporters), DM (denounced by the mayor)

SP some MS
SP --> DM
----------
DM some MS

(C) Key: P (polled), LOC (live outside of the city), V (vote)

P some LOC
V --> ~LOC
----------
P some ~V

Both answer choices (B) and (C) are valid arguments. The stimulus relies on a contrapositive, so since answer choice (C) does and (B) doesn't, answer choice (C) is more correct. However, there are other examples where the stimulus does not rely on a contrapositive but the correct answer does. This is because there were no other better options. I think this is major weakness of the LSAT. Correctness should not be about being better than the other options, it should either parallel or not parallel. The idea that you can dismiss answer choice (B) here because it does not rely on a contrapositive but would be the correct answer if answer choice (C) was not present is bothersome.

The question you pose in your notational structure, however, definitely does not parallel. The reason is that the strength of the conclusion is paramount. In the first case you have a definite state of being, "C is not B." In the second case you a potential state of being, "C could be B." Just because the terms appear in similar order does not imply that the reasoning is similar.

I hope this helps!

[Saltqjibo]

I'm pretty sure I get what you're saying. Could anybody clear up the notation for me though, what does ~ mean?

[Philo38]

I parallel reasoning questions I find the first step should always be to compare conclusions alone. What type of conclusion is in the stimulus? Chances are you will be able to knock out 2 of the answer options on these grounds alone.

Obviously, this will not likely get you to an answer alone, but the trouble with parallel reasoning questions is the time sink of considering the logic of five seperate arguments. By comparing the conclusions first, you can easily narrow your choices and save loads of time.

[Saltqjibo]

In what sense do you compare the conclusions? Sorry I'm just not clear. I'm usually fairly decent at these questions but they seem to take up alot of time and on a bunch I feel like I'm just going with intuition rather than clearly understanding my choice

[Atlas LSAT Teacher]

I'm pretty sure I get what you're saying. Could anybody clear up the notation for me though, what does ~ mean?

~ means "not"

There are other ways of showing it. Some people simply put a negative sign.

[Bodega]

To compare conclusions, you can look at the difference in logical force or scope, i.e. words like "all" or "never"

[Philo38]

In what sense do you compare the conclusions? Sorry I'm just not clear. I'm usually fairly decent at these questions but they seem to take up alot of time and on a bunch I feel like I'm just going with intuition rather than clearly understanding my choice

Really the only way to explain this is with examples and I don't have any with me right now, sorry. What I would suggest is get a parrallel reasoning question and pick out the conclusion of each argument, which (hopefully) you should be able to do rather quickly as this is a valuable skill for many LR question types.

Read the conclusions out loud side by side, and I think you will see what I mean very quickly and clearly. For example, if I have a stimulus that has a prediction as its conlusion--therefore, x will happen--than any answer choice which does not have a prediction as its conclusion may be immedietly discarded, regardless of the premises.

If you need more clarification I will try to find an example to work from.

[Philo38]

To compare conclusions, you can look at the difference in logical force or scope, i.e. words like "all" or "never"

Exactly. If you have a stimulus with the conclusion type:

therefore all A are B

Don't even bother with the answer choices that have conclusions that say, therefore some A are B

[Saltqjibo]

Ah, I see. Thanks guys. Great answers