• Ben Olson

Necessary Assumptions and Destruction

Test 17 - Section 2 - Question 21

Logical Reasoning

Difficulty: 5


It's hard to stomach the premise "no minor fault in a geologically quiet region produces an earthquake more often than once in any given 100,000 years." But this is a premise, so I have to grit my teeth and take it. It sounds ridiculous, but part of LSAT mastery involves the ability to accept ridiculous premises for purposes of argumentation.

If we buy the premise, is the conclusion justified? The conclusion is, "it follows that of all potential nuclear reactor sites in such a region, those that are least likely to be struck by an earthquake are ones located near a fault that has produced an earthquake within living memory." You'll be shocked, I'm sure, to learn that I just don't buy it. My first objection here is "hold up: what if this site is near several minor faults? Can't the other minor faults still produce an earthquake, even if one local minor fault cannot?"

It's a Necessary Assumption question, so we're looking for an answer that the speaker would have to agree is true. Putting that another way, we're looking for the one that, if false, would destroy the argument.

A) Nope! The premises and the conclusion are only about earthquakes and earthquake avoidance. Overall safety is not the issue. For all we know, the speaker might be like "of course, earthquakes are the least of our concerns and we should really be focusing on building reactors away from population centers, or secure from terrorist attacks, or far from hurricanes" or any other thing. The speaker does not have to agree with A.

B) This is wrong for the same reason A is wrong. 

C) I didn't predict this, but the speaker would be forced to agree with it. If this is false, the argument will lose. What if there are some sites that are located by no faults, not even minor faults? If that's true, then the argument's insistence on looking for a fault that has recently produced an earthquake is a dumb plan. So this is the answer.

D) The speaker does not have to agree with this. As a matter of fact, the speaker might disagree with it because it seems like it's the beginning of my "hey, what if there are multiple minor faults" objection.

E) Nah, the speaker said "at most once in a given 100,000-year period." That doesn't mean it's a minimum as well as a maximum. The speaker wouldn't care if this is false. 

The answer is C because it's he only one the speaker would be forced to agree with. If C is false, the argument will lose.

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