December 2016 LSAT Section 1 Question 13

If the purpose of laws is to contribute to people's happiness, we have a basis for criticizing existing laws as well ...

smilde11 on November 7, 2018

PT 80, S1, Q13

Can you explain why the answer is A? To me it seems that argument takes a necessary condition to be sufficient, which I didn't see, so I chose C. If Purpose of laws is to Contribute to Happiness, Then Basis for criticizing existing laws. PLCH --> BCEL not BCEL --> not PLCH Then the argument makes a subsidiary conclusion (I think) If not PLCH (the necessary of the contrapositive), then not BCEL (The sufficient from the contrapositive). Is that not taking necessary as sufficient? Thanks for your help!

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Mehran on November 10, 2018

Hi @smilde11, thanks for your post. Your reasoning is right, and actually only supports the correct answer, (A).

Premise: If the purpose of laws is to contribute to people's happiness (PLCH), we have a basis for criticizing existing laws (BCEL) as well as proposing new laws (PNL).

Diagrammed: PLCH ==> BCEL and PNL
Contrapositive: not BCEL OR not PNL ==> not PLCH

Subsidiary conclusion: Hence, if that is not the purpose (not PLCH), then we have no basis for the evaluation of existing laws.

Notice that here, "not PLCH" is set as sufficient. But in the premise given (in the contrapositive formulation), "not PLCH" is necessary.

The only answer choice that describes a sufficient & necessary reasoning flaw is (A), which is the correct answer.

Answer choice (B) can be eliminated because the flaw in this stimulus is not mistaking a correlation (two things happening at or near the same time) for a cause and effect relationship (one thing must have caused the second).

Answer choice (C) can be eliminated because there is no "term" that takes on a different meaning in the premise vs. in the conclusion. All the terms in this stimulus are static.

Answer choice (D) can be eliminated because the flaw in this stimulus is not mistaking what "is" with what "ought to be." There is nothing prescriptive about the information presented in the stimulus ("we should do X").

Answer choice (E) can be eliminated because the flaw in this stimulus is not a "part to whole" flaw, which is what is describe in (E). For example: just because a set of things has a certain property (basketball teams tend to be taller than average people) does not mean that each member of that set has the property (not every single member of every single basketball team will necessarily be taller than the average person).

Hope that helps. Understanding the meaning of each answer choice on a flawed method of reasoning question can help you aggressively and correctly employ process of elimination to select the correct answer, even if you are otherwise unsure.

Meredith on October 23, 2019

I completely understand your explanation and I too noticed that the N was used as a S condition; however, I didn't choose that answer because the conclusion makes no sense with the premises provided. The jump to legitimacy threw me so I picked D since I felt A wasn't strong enough to explain the huge jump to the very last part of the sentence where it says we must conclude... Can someone please explain how you know to go with A even though it seems to not be the only flaw in the argument? Thanks