Logical reasoning PrepTest 111 · Section 4 · Question 13

Incorrect. The conclusion is about the law prohibiting pedestrians from crossing against red lights, so a law that applies to drivers is out of scope.

Incorrect. The argument is always talking about a specific law, so there's no shift in meaning of that word.

Incorrect. The argument establishes as a premise that a law must deter certain behavior to be useful, so it doesn't ignore this possibility—it eliminates it.

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Errors in Reasoning

Stimulus Summary:
Principle - ~Deter behavior it prohibits → ~Law serves useful purpose
Law prohibiting pedestrians crossing against the red light:
(1) Those who always break law - it won't change their behavior
(2) Those who always follow law - it won't change their behavior
Conclusion - The pedestrian crossing ~Law serves useful purpose

Answer Anticipation:
This argument establishes a principle that defines a necessary condition for a law to serve a useful purpose. Since the conclusion is that a certain law doesn't serve a useful purpose, we wrote the contrapositive out in our Summary. Doing so shows us that we can justify the conclusion if the law in question doesn't deter pedestrians from crossing against the red light.

Do the premises establish that? Well, it establishes that it won't influence the behavior of those who always break the law or never break the law. Is that everyone? Nope—there are almost certainly people out there who sometimes follow the law but sometimes don't (maybe they cross when they're in a rush, or when there are no cars in sight). If the law influences their behavior, then it could serve a useful purpose.

In ignoring a key group here and analyzing just the extremes, the Mayor's argument is flawed, so let's find an answer stating that.

Answer Explanation:
The premises address those who always break the law and those who always follow the law. That ignores those who sometimes do and sometimes don't follow the law. If this group's behavior is influenced by the law, then it could serve a useful purpose. This answer highlights the flaw in the reasoning by pointing out a situation where the law does serve a useful purpose.

Key Takeaway:
This argument highlights a common and sometimes difficult concept on the LSAT—binary characteristics, and spectrum characteristics.

Binary characteristics only exist in two forms—it's either present, or it isn't. For instance, a light can either be on or off (even with a dimmer switch, the light is still on—but the dimmer switch does introduce the spectrum characteristic of brightness).

Spectrum characteristics exist at different levels—it can be present to varying degrees, such as height, or flavor, or, as we just saw, brightness.

Many LSAT arguments will treat spectrum characteristics as binary—as we saw here. Whenever the LSAT is talking about polar opposites—those that always/never follow the law—consider whether there's a middle ground.

Incorrect. Danger is out of scope, as the argument never establishes it as the reason for the law. It could be that the law is meant to improve the flow of traffic!