LSAT Logical Reasoning, Oct 2001, Section 1, Q23: My take

Some statisticians claim that the surest way to increase the overall correctness of the total set of one’s beliefs is: never change that set, except by rejecting a belief when given adequate evidence against it. However, if this were the only rule one followed, then whenever one were presented with any kind of evidence, one would have to either reject some of one’s beliefs or else leave one’s beliefs unchanged. But then, over time, one could only have fewer and fewer beliefs. Since we need many beliefs in order to survive, the statisticians’ claim must be mistaken. 

The argument is most vulnerable to criticism on the grounds that it 

(A) presumes, without providing any justification, that the surest way of increasing the overall correctness of the total set of one’s beliefs must not hinder one’s ability to survive.

(B) neglects the possibility that even while following the statisticians’ rule, one might also accept new beliefs when presented with some kinds of evidence. 

(C) overlooks the possibility that some large sets of beliefs are more correct overall than are some small sets of beliefs. 

(D) takes for granted that one should accept some beliefs related to survival even when given adequate evidence against them. 

(E) takes for granted that the beliefs we need in order to have many beliefs must all be correct beliefs.

Answer: (A)

Again, it is fun to deduce this. First, let us eliminate the wrong answer choices.

(B) is wrong because you can't change the total set of beliefs by adding to it (the surest way criticized says so!)

(C) is irrelevant

(D) is wrong because it holds the statisticians' claim against a person who rejects the statisticians' claim

(E) is irrelevant twaddle

Let's work (A) out...

According to some statisticians:

Surest way to increase correctness of the total set of beliefs --> Reduction in beliefs (1)

According to the person who rejects the statisticians' claim, survival requires maintaining some beliefs, or, ~(reduction in beliefs).

Survival --> ~(Reduction in beliefs)

(or)

Reduction in beliefs --> ~Survival (2)

Taking an assumptive leap by combining (1) and (2),

Surest way to increase correctness of the total set of beliefs --> ~ Survival

In other words, the person who rejects the statisticians' claim is assuming that the surest way proposed by them hinders survival. This clearly implies that this person is assuming that any surest way of increasing the total set of beliefs must not hinder survival, which is what (A) says.

LSAT Logical Reasoning, Feb 1996, Section 4, Q25: My take

Louis: People's intentions cannot be, on the whole, more bad than good. Were we to believe otherwise, we would inevitably cease to trust each other, and no society can survive without mutual trust among its members.

The argument is most vulnerable to which one of the following criticisms?

(A) It fails to rule out the possibility that a true belief can have deleterious consequences.

(B) It mistakenly assumes that if two claims cannot at the same time both be true, then they cannot at the same time both be false.

(C) It challenges the truth of a claim merely by calling into question the motives of those who profess that they believe it to be true.

(D) It assumes without any warrant that in any situation with two possible outcomes, the most negative one will inevitably occur.

(E) It provides no reason to believe that a statement that is true of a given group of individuals is also true of any other group of individuals. 

Answer: (A). 

It is very easy to arrive at (A) by elimination because the others are poor contenders.

However, why (A) is correct is worth analyzing.

The conclusion here is that people's intentions cannot be, on the whole, more bad than good. The conclusion is obviously to be held as a true belief, so the "were we to believe otherwise" bit turns it into a false belief. 

Holding the false belief leads to negative things that finally end in extinction of society.

False belief -> Negative things that end in extinction of society

does not rule out

~ (False belief) -> Negative things that end in extinction of society

(or) 

does not rule out 

True belief (conclusion) -> Negative things that end in extinction of society (deleterious consequences), which is what (A) says.

The possibility that the true belief of the conclusion still leads to the negative things that end in the extinction of society renders the argument unsound.

ASIDE: As an analogy, consider the statement that those who believe in God (G) are saved (S).

G -> S only renders the contrapositive ~S -> ~G true always

It does not rule out ~G -> S 

because there may be some who do not believe in God (~G) but are still saved (S). Those who are saved (S) definitely include all those who believe in God (G) and may include at least some of those who do not believe in God (~G). What is true for sure is that if one is not saved (~S), then one definitely does not believe in God (~G).