LR logic problem confusing me badly - cause and effect? Forum

[ScottRiqui]

This concept is from PT63, S1, Q20, but I'm paraphrasing it here to make my point clearer.

Let's say I give you three rules:

1) If you lose weight, you'll look better
2) If you look better, you'll get a girlfriend
3) However, you'll never get a girlfriend unless you stop smoking.


Logically, combining the first two rules allows you to say "If you lose weight, you'll get a girlfriend" . But that doesn't address the smoking issue at all.

Since losing weight and looking better leads inevitably to getting a girlfriend, did losing weight or looking better somehow force you to stop smoking as well? It must have, because how else would you have gotten a girlfriend? But the "cause and effect" connection doesn't seem to be there; it seems like the combination of rules 1 and 2 form an ironclad chain from losing weight to getting a girlfriend, making it irrelevant whether or not you stopped smoking.

[Silvermanlsat]

Some conditional statements to consider here. We are told that:

The profits of downtown businesses will increase if more consumers live in the downtown area. Stated otherwise, if more consumers live in the downtown area, then the profits of downtown businesses will increase. Increased consumers is a sufficient (but not a necessary) condition for increased profits.

We are also told that a decrease in the cost of living will increase consumers, and because increased consumers was a sufficient condition for increasing profits, we can conclude that decrease in the cost of living will lead to increased profits.

Further, we are told that the profits will not increase unless congestion decreases. Another way of stating this is that only if congestion decreases will profits increase. Here, congestion decreasing is a necessary condition for profits increasing. If we were to learn that congestion did not decrease, then we can conclude that profit did not increase.

That last statement was merely a distractor here. Our answer comes from the fact that increased consumers leads to increased profits, and decreased cost of living leads to increased consumers.

And therefore, decreased cost of living leads to increased profits (choice B)

A->B and C->A, so C->B

[ScottRiqui]

Thanks. I understand why the correct answer is correct. I guess what bothers me is that the while the rules are logically valid, they don't "make sense" to me.

In short, I'm confused when you have a case where 'A' directly implies 'B', but there's another rule that says 'B' can't happen unless 'C' happens. It seems like there's a contradiction there; on the one hand, you have a rule saying "If you want 'B', you need 'C' to happen", but on the other hand, you have another rule saying "screw you - 'A' is all I need".

I know that the solution to my discomfort is to just realize that 'A' must automatically give you 'C' as well. But that leads to nonsense like Q20 where 'A' giving you 'C' translates to "Lowering the cost of living reduces traffic congestion".

[Silvermanlsat]

Let's consider that:

A directly implies B, but B can't happen unless C.

For example, if the bank is open after 5pm, then I will deposit my check. In addition, I will not deposit my check unless I get out of work prior to 3pm.

A=if the bank is open after 5pm
B=I will deposit my check
C=I get out of work prior to 3pm.

We know that the bank being open after 5pm leads to the conclusion that the check is being deposited. The contrapositive here is that if the check is not deposited, then the bank is not open after 5pm. But we also know that if the check is deposited, then I get out of work prior to 3pm. The contrapositive here is that if I don't get out of work prior to 3pm, then the check is not deposited. i.

So, we have the following: If I don't get out of work prior to 3pm, then the check is not deposited, and if the check is not deposited, then the bank is not open after 5pm. We can conclude here that if I don't get out of work prior to 3pm, then the bank is not open after 5pm.

[esquirelhomme]

Here is how I diagrammed the stim:

More consumers living downtown -> Profits increase

Decrease in cost of living -> More consumers living downtown

So, we can connect these two to make: Decrease in cost of living -> More consumers living downtown -> Profits increase

Last sentence is:

Profits increase -> Congestion decreases

Or

~Congestion decreases -> ~Profits increase

So we can connect this last sentence to the logic chain above to make:

Decrease in cost of living -> More consumers living downtown -> Profits Increase -> Congestion decreases

You didn’t really need to understand how the last link was relevant in selecting the correct answer, since the answer was an inference made through the first sentence. But, understanding it really helps to eliminate a couple of the wrong answers.

Hope this helps!

[The LSAT Trainer]

Hi Scott -- I think I understand what is causing your confusion and I have a couple of suggestions -- sorry if I'm off the mark.

1) Make sure not to confuse conditional logic and causation. The question you are asking about involves a series of conditional relationships -- you don't want your causal instincts to bias you.

2) Typically speaking, other than for biconditionals, conditional statements give us certain valid inferences, but they do not typically exclude other possibilities --

For example, if I have the phrase "If I get hurt, then I will yell," it does not exclude other ways to guarantee that I will yell (such as "If I get mad, then I will yell.") So, in terms of instincts and mindset, you typically want to think about conditional statements in terms of the inferences they allow, rather than exclude, if that makes sense.

Imagine that the following statements were true (I realize that in real life they are not):

1) In order to practice law, one must pass the bar.
2) In order to pass the bar, one must attend law school.
3) You can't attend law school unless you take the LSAT.

No problem having all three of those things be true, right? But look at the conditional statements that result:

Practice -> Pass Bar
Pass Bar -> Attend Law School
Attend Law School -> took LSAT

Same structure as what you are asking about -- and there is no issue with all these things being true at once. I bet you can imagine adding other conditional statements to that as well -- a situation can have tons of guarantees (for example, there could be many different types of clues that guarantee that one attended law school) and these guarantees don't exclude the possibility of other guarantees.

For example, we can also add the phrase: "The only people who own more than three gray suits are those who went to law school"

Own 3+ Gray Suits -> Attend Law School

And this wouldn't discredit the other conditional statements we already have in any way.

The last point I'll make (and I don't think this was your issue but just in case) is that the correct answer to an inference question does not have to tie together all parts of the stimulus, nor does it have to be the "main" inference that all the components are meant to link up to create. The right answer is simply something that is valid based on some combination of the pieces given in the stimulus. Again, I don't think this was what was caused you trouble, but just wanted to mention it just in case.

I realize that if I'm off the mark about what is bothering you, the above might make no sense at all (!) -- if that's the case, sorry -- but if not, I hope you found it helpful - MK

[ScottRiqui]

Let's consider that:

A directly implies B, but B can't happen unless C.

For example, if the bank is open after 5pm, then I will deposit my check. In addition, I will not deposit my check unless I get out of work prior to 3pm.

A=if the bank is open after 5pm
B=I will deposit my check
C=I get out of work prior to 3pm.

We know that the bank being open after 5pm leads to the conclusion that the check is being deposited. The contrapositive here is that if the check is not deposited, then the bank is not open after 5pm. But we also know that if the check is deposited, then I get out of work prior to 3pm. The contrapositive here is that if I don't get out of work prior to 3pm, then the check is not deposited. i.

So, we have the following: If I don't get out of work prior to 3pm, then the check is not deposited, and if the check is not deposited, then the bank is not open after 5pm. We can conclude here that if I don't get out of work prior to 3pm, then the bank is not open after 5pm.

Exactly. It's utter nonsense, even though it's logically valid. The individual rules themselves don't seem silly; but when you start taking contrapositives and/or combining them, the resulting relationships don't make sense.

In your example, you staying at work past 3pm somehow "forces" the bank to close before 5pm, even though one has no bearing on the other. That's the part that bothers me. I can just work through it on the test, I guess.

[Silvermanlsat]

Absolutely Scott, but keep in mind that it only seems like nonsense because we are reading words, and we know how strange it is for one of those things to make the other happen. But when considered in the abstract (for example, with symbols) it logically follows, and that is what is important for this test.

[ScottRiqui]

Hi Scott -- I think I understand what is causing your confusion and I have a couple of suggestions -- sorry if I'm off the mark.

1) Make sure not to confuse conditional logic and causation. The question you are asking about involves a series of conditional relationships -- you don't want your causal instincts to bias you.

Thanks - I think this it probably what's tripping me up. So many logical statements are phrased causally, such as "If you're good, I'll give you ice cream". In that case, getting the ice cream was a direct result of being good. It's hard not to read that kind of relationship into a logical statement.

[bp shinners]

In your example, you staying at work past 3pm somehow "forces" the bank to close before 5pm, even though one has no bearing on the other. That's the part that bothers me. I can just work through it on the test, I guess.

Nope, it's not forcing the bank to close before 5PM. Those two things just both happen together. It's conditional, not causal, as Mike pointed out. Huge difference.

For instance, I could say, "If there's smoke, there's fire." Doesn't mean that smoke is causing the fire.