LSAT 111 – Section 4 – Question 13

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October 2001

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J.Y.’s explanation

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Mayor: The law prohibiting pedestrians from crossing against red lights serves no useful purpose. After all, in order to serve a useful purpose, a law must deter the kind of behavior it prohibits. But pedestrians who invariably violate this law are clearly not dissuaded by it; and those who comply with the law do not need it, since they would never cross against red lights even if there were no law prohibiting pedestrians from crossing against red lights.

Summarize Argument
The mayor concludes that the law prohibiting pedestrians from crossing against red lights is useless. He supports this with three premises:

(1) To be useful, a law must prevent the behavior that it bans.

(2) Pedestrians who always break this law are not dissuaded by it.

(3) Pedestrians who always follow the law don’t need it, because they wouldn’t cross on red even without the law.

Identify and Describe Flaw
This is the cookie-cutter flaw of “false dichotomy,” where the author falsely divides the world into two binary halves. In this case, the mayor divides the world into pedestrians who always break this law and pedestrians who never break it. He doesn’t consider that there might be other people who only sometimes break this law; how might the law affect them?

A
takes for granted that most automobile drivers will obey the law that prohibits them from driving through red lights
The mayor only addresses pedestrians and the law that prohibits them from crossing against red lights. Whether drivers obey the law that prohibits them from driving through red lights is irrelevant.
B
uses the word “law” in one sense in the premises and in another sense in the conclusion
This is the cookie-cutter flaw of “equivocation.” The mayor doesn’t make this mistake because he uses the word “law” consistently throughout his argument.
C
ignores the possibility that a law might not serve a useful purpose even if it does deter the kind of behavior it prohibits
This may be true, but it isn’t a flaw in the mayor’s argument. He just claims that a law is only useful if it does deter the kind of behavior it prohibits.
D
fails to consider whether the law ever dissuades people who sometimes but not always cross against red lights
The mayor falsely divides the world into people who always cross against red lights and people who never cross against red lights. He doesn’t consider people who sometimes cross against red lights or how the law might affect them.
E
provides no evidence that crossing against red lights is more dangerous than crossing on green lights
It’s true that the major never provides evidence about the danger of crossing on red or green lights, but this isn’t a flaw because his argument is only about crossing against red lights. So (E) is irrelevant.

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LSAT PrepTest 111 Explanations

Section 1 - Logical Reasoning

Section 2 - Reading Comprehension

Section 3 - Logical Reasoning

Section 4 - Logical Reasoning

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