Critical Reasoning

Hi all,

such a great question! This is an official LSAT question that tests formal logic in a way that the GMAT never would.

The second-to-last sentence:

"All stars but the coolest of brown dwarfs are hot enough to destroy lithium completely by converting it to helium"

yields the following conditional statement:

IF a star has lithium, THEN it is c.b.d. (coolest of brown dwarfs). This conditional statement is the author's evidence.

The last sentence:

"Accordingly, any star found that contains no lithium is not one coolest brown dwarfs"

yields the following conditional statement:

IF there isn't lithium in the star, THEN it is not a c.b.d. This conditional statement is the author's conclusion.

If an arguer's evidence is a sole conditional statement, then the only legitimate conclusion the author can draw is that statement's contrapositive. The contrapositive negates and reverses the elements (If A, then B becomes IF not B, then not A).

Here, in the conclusion, the author negates the terms but fails to reverse them.

The author's evidence establiahes that c.b.d.s are the only kind of stars that may contain lithium. He mistakenly infers from this that all c.b.d.s have lithium. Thus, th unstated necessary piece of evidence is that all c.b.d.s are too cool to destroy lithium (choice A).


______



We can also understand this argument less technically by analogizing:

"The only kind of dogs that imitate their owner's behavior is the English Bull Terrier. Therefore, if a dog isn't imitating an owner's behavior, it is not an English Bull Terrier"

is an analagous argument.

But, of course, just because English Bull Terriers are the only kind of breed to imitate, doesn't mean EVERY English Bull Terrier will imitate.

Likewise, just because c.b.d.s are the only kind of stars that can contain lithium (because they may not be hot enough to destroy it by converting it to helium), doesn't mean that EVERY c.b.d will have lithium in it. So, the author is not necessarily correct when he concludes that if there isn't lithium it's not a c.b.d.--because it may well be a c.b.d.

As gmatmachoman points out, we can also use the denial test to prove that the author's reasoning depends on choice A. Also, fibo's tip about noticing the difference in the degree of confidence (any vs most) is a great one--we can use it here to eliminate a wrong answer. Of course, any one who knows me on this site knows that my position is that predicting and matching is a far superior procedure to that of process of elimination!

But, I will reiterate that a question like this would never come in GMAT CR.

@TestLuv,

I was expecting you to pitch in for this one..Since it was a Pure LSAT one, I know u would love to share your Gyan on that. & as usual u commente d on the applicability of this formal logic in GMAT!! So only I opened up the very old post of mine where at one point of time i was a novice in this CR!

Nice Info!

Thx once again!!

Guys can anyone give a decent reason why A is not right.