Logical reasoning PrepTest 134 · Section 2 · Question 16

Correct. Argument or Facts:
Argument

Valid or Flawed:
Flawed

Question Type:
Flawed Parallel Reasoning

Stimulus Summary:
Believe in aliens → Believe in UFOs
UFOs don't exist
Therefore — Aliens don't exist

Answer Anticipation:
This is a tough question. The LSAT has a few of these, where the stimulus is short and seems straightforward, but the correct answer is hard to spot because of some quirk of logic being exploited by the question.

Here, it's hard to see exactly what the flaw is. It seems like it might be a conditional logic flaw, but the nonconditional premise is related to the negation of the necessary condition, which is how you validly apply the contrapositive. It seems that there should be a Perception vs. Reality flaw because that conditional premise is related to what people believe and the nonconditional premise is about the actual reality of UFOs (they don't exist), but the conclusion is about a belief, as well.

But hold up a second—is it? Think about what the actual contrapositive of the conditional statement is, and what those negated terms actually mean:
not Believe in UFOs → not Believe in aliens
Translated back to plain English, this would read, "Anyone who doesn't believe in UFOs also doesn't believe in aliens." These statements are about what people don't believe—it's not about whether that belief is false, as the conclusion here states.

So let's circle back to our Summary—we paraphrased the conclusion as, "Aliens don't exist." That's because if a belief is false, then the opposite of that belief must be true. If a belief in aliens is false, then aliens don't exist. Just like if someone who believes the LSAT is easy is wrong, then the LSAT isn't easy.

So this argument does have a Perception vs. Reality flaw, using a conditional about belief, but then treating its contrapositive as if it were about the reality/truth of those beliefs.

Answer Explanation:
Believe in unicorns → Believe in centaurs
Centaurs don't exist
Therefore — Unicorns don't exist
This answer commits the same Perception vs. Reality flaw, in that the conditional premise is about a belief, but the argument treats the contrapositive as if it established something about whether the mythological creatures that some believe in actually exist. Same jump as the stimulus, so this is the correct answer. This answer is difficult to select because the conclusion doesn't bring up a belief, but remember that the stimulus concluded a belief is false, which is a statement about reality.

Key Takeaway:
Whenever an LSAT argument mentions what someone thinks, believes, or says, there's likely a Perception vs. Reality flaw. But be careful when it's involved in conditional logic! The negation of someone believing something is that person not believing that thing, not that thing being false. And a belief being false is a statement about reality—if a belief in bigfoot is false, that means bigfoot doesn't exist.

Incorrect. The nonconditional premise and conclusion here are about what someone believes, so it's not parallel to the stimulus since it doesn't jump from perception to reality. (This is actually a valid argument.)

Incorrect. The nonconditional premise and conclusion here are about what someone believes, so it's not parallel to the stimulus since it doesn't jump from perception to reality. Additionally, this doesn't have the same attempted application of the contrapositive of the conditional—it features an illegal negation.

Incorrect. This argument is about whether a belief is justified, not whether it's true/false. An unjustified belief can still be true! After all, people make random guesses all the time that end up to be correct.

Incorrect. Very close! However, this argument has a nonconditional premise that, ignoring the jump from perception to reality, negates the sufficient condition of the conditional premise, not the necessary condition as the stimulus does. Note, however, that this answer feels better because that conclusion brings up perception as the stimulus does.