LSAT Explanation PT 30, S4, Q14: Joseph: My encyclopedia says that the

LSAT Question Stem

Which one of the following most accurately describes a reasoning error in Laura's argument? 

Logical Reasoning Question Type

This is a Flaw question. 

Correct Answer

The correct answer to this question is C. 

LSAT Question Complete Explanation

Let's first analyze the passage and break down the argument. Joseph's argument is as follows:

Premise: Fermat died without leaving behind any written proof for his theorem, and no one else has been able to prove it.

Conclusion: Fermat was probably lying or mistaken when he claimed to have proved the theorem.

Laura's counter-argument is:

Premise: Recently, someone has proved Fermat's theorem.

Conclusion: Joseph's claim that Fermat was lying or mistaken is wrong.

Now let's understand the question type, which is a Flaw question. We are asked to identify the reasoning error in Laura's argument.

To simplify the argument, imagine a student claiming to have completed their homework but not providing any evidence. If no one else in the class can complete the homework, we might think the student was lying or mistaken. However, if another student later completes the homework, it doesn't necessarily mean the first student was telling the truth. They could still be lying or mistaken, even though the homework was eventually completed by someone else.

An "Evaluate" question for this argument would be: "Did Fermat actually prove the theorem himself?"

Now let's discuss each answer choice:

a) This answer choice is incorrect because Laura's argument does not establish its conclusion by making a claim that would contradict the conclusion.

b) This answer choice is incorrect because Laura's argument does not assume the quality of Fermat's character guarantees the accuracy of his claims.

c) This is the correct answer. Laura's argument mistakes the fact that the theorem is provable (necessary condition) for something that ensures Fermat was not lying or mistaken (sufficient condition). Just because the theorem is provable does not guarantee that Fermat was not lying or mistaken when he claimed to have proved it.

d) This answer choice is incorrect because the term "provable" is not used ambiguously or without definition in Laura's argument.

e) This answer choice is incorrect because Laura's argument does not involve confusion between true claims believed to be false and false claims believed to be true.

In conclusion, the correct answer is (C). Laura's argument mistakes a necessary condition (the theorem being provable) for a sufficient condition (ensuring Fermat was not lying or mistaken).