Q22 - Anatomical bilateral symmetry

[vik]

How can the stimulus be valid reasoning. It looks like circular reasoning to me.

[theaether]

It's common --> confers advantages.
~confers advantages --> ~common.

The stimulus just used the contrapositive to back up the original argument.

(C) gets this format down.
Lack skills --> not arbitrator.
~not arbitrator --> ~lack skills

(A) City taking the matter seriously in the first part and insist that negotiations be deferred in the second part don't match up. The city could be deferring it because they take it so seriously.

(B) No one's more qualified than T. If you've seen him at work --> you'd know there was no one more qualified. No contrapositive action going on here.

(D) Varga away --> Rivers conducted negotiations. That's the first part.
Unless Varga is unavailable translates to If Varga is available, which is the same thing as ~Varga away so:
~Varga away --> ~Rivers negotiates. When we match this up with the first part, we see that this argument is just some mistaken negation going on. A-->B and then ~A-->~B. Incorrect.

(E) Wong arbs --> prompt decision. The second part is basically saying Wong is the arb, and that therefore we follow the logic chain to expect a prompt decision. Well that's like saying A-->B, and then since we have A, we will have B. A valid argument but not similar to the stimulus.

[giladedelman]

Great explanation! The argument uses some known conditional statement to draw a conclusion, and that conclusion is the contrapositive, which we know is valid. So it's not circular reasoning, but it does depend on whether the conditional statement used as the premise is true, as all arguments depend on the truth of their premises!

[tamwaiman]

Although the question stem does not show that there is a flaw, I consider this argument is flawed because of the circular reasoning.
Can you please help to clarify?
Thank you.

[demetri.blaisdell]

tamwaiman,

I can see why you think there's a flaw here but, as far as the LSAT is concerned, this is good logic!

Premise: If BS didn't confer survival advantages, it wouldn't be common + it is common

Conclusion: It confers survival advantages

It might help to see it written as conditional logic:

-Adv. ---> -Common
Common ---> Adv. (contrapositive)
Common (given)
Therefore, survival advantages

You might think this argument was circular if you heard your friend say it but in LSAT land, we assume the premise to be true. If that premise is true, there's no flaw and the argument is valid!

Take another look and let me know if you have any more questions.

Demetri

[minhee22]

I understand why C is the correct answer with the conditional diagramming, but I am confused why A cannot be a contender as a correct answer choice.

[maryadkins]

(A) is incorrect because it doesn't use the same pieces in the conclusion as there are in the premise. The premise talks about Sawyer not being available. The conclusion talks about the city taking the matter seriously. Two statements can't be contrapositives of each other if one has information in it that the other one doesn't have.

(B) isn't a statement and its contrapositive, and neither are (D) and (E).

[cwolfington]

I first chose (A), but I see why it's wrong. The first two sentences contain an assumption by the author, and that assumption is proved in the third sentence. The third sentence is the premise, which is diagrammed: ~Confer advantages->~Common; and the author's conclusion is: Common trait (anatomical bilateral symmetry)->Confer advantages.

The conclusion is not just the contrapositive of the premise (circular reasoning), because the premise does not refer to "anatomical bilateral symmetry". A.B.S.

The conclusion for (A) is: Sawyer negotiates->City takes it seriously; and the premise is: ~Sawyer->~Negotiations. This is incorrect because there is a massive gap between "city takes it seriously" and "~negotiations", and because the conclusion negates the sufficiency. In terms of logic, (A) makes absolutely no sense.

(C), however, has the premise: ~Superior skills->~arbitrator; and the conclusion.: arbitrator->superior skills. (C) proves it's conclusion by using the contrapositive, which is the same reasoning used in the stimulus. And to see that (C) does not use circular reasoning, read the first sentence last; this gives (C) the same ordering as the stimulus, and, like the stimulus, the premise proves a broad rule, which is applies to a narrower conclusion.