#20 - Selena claims to have psychic powers. So if we

[Administrator]

Complete Question Explanation

Justify the Conclusion. The correct answer choice is (B).

Answer choice (A):

Answer choice (B): This is the correct answer choice.

Answer choice (C):

Answer choice (D):

Answer choice (E):

This explanation is still in progress. Please post any questions below!

[Jon Denning]

A student inquiry came in today regarding one of the more troublesome questions on the December 2015 LSAT, and since I know a lot of students out there struggled with this one I thought I'd share the inquiry and my response here.

Here's the question we received:

"To whom it may concern,

I am having a hard time figuring out how to come to the correct answer choice in section 4 of PT 77 question 20 dealing with Selena's psychic powers... Ive tried to diagram, I've tried to get to the correct answer choice by eliminating answer choices that don't contain what's mentioned in the conclusion.....Please help as this question is haunting my dreams as I prepare for feb LSAT!"

And my reply:

Haha hey XXX – sorry to hear you’re having trouble (haunted dreams, even) with this one, but hopefully I can help clear things up!

This is slightly tricky Justify the Conclusion question, where we need to prove that the conclusion is true based on the premises and correct answer choice. So let’s start by finding the conclusion. In this case it’s the second sentence: if we find out whether Selena’s claim to have psychic powers is true or not, we will know whether it’s possible to have psychic powers.

On the face of it this really only makes sense in the affirmative. That is, if we find out that Selena does indeed have psychic powers, then clearly we’d establish the possibility of having psychic powers. If we can conclusively demonstrate that Selena’s claim is false (she definitely doesn’t have psychic powers), then we haven’t ruled out anything but psychic powers in this one instance; it could still be possible to have them, even if Selena does not. (Note: inconclusive results are irrelevant here, as the conclusion is only about finding out for sure one way or the other)

So we need an answer choice then that helps to address the “negative” (absence) side of the issue. Simply put, if we could show that Selena is perfectly representative of the potential to have psychic powers—if they’re possible then she would have them, and if she doesn’t then they’re impossible (no one else could either)—we’d be able to use her as a test case for psychic powers in others, and the conclusion would be true.

I personally wouldn’t diagram this question, but since you asked I’ll quickly point out that this idea could be diagrammed as:

Psychic Possible Selena Psychic (meaning if anyone could have psychic powers, then Selena
will be among that group)

Which answer does that?

Answer choice (B)! It gives us exactly what we need by stating that Selena will definitely have psychic powers if the possibility of having them exists. In other words, if we can show that she doesn’t have them, then we would know the very possibility of having them must not exist (otherwise she’d be psychic). With (B) we’ve addressed the absence issue I described above, and thus it allows for the conclusion here—“Selena is psychic then the power exists (and are clearly possible); Selena not psychic then being psychic isn’t possible”—to be drawn with certainty.

Again, diagrammatically (B) looks like our prephrase above:

Psychic Possible Selena Psychic, with the contrapositive Selena Not Psychic Psychic Not Possible

Not the easiest question in the world to be sure, but not too bad either once you spot the missing piece and can see which answer choice provides it. I hope that helps!

Jon

[maximbasu]

Hello Jon Denning,
Thanks for your reply.

Why is E wrong? I'd simplify it as "S has psychic powers only if we find out whether she's telling the truth."

That seems to make sense.

Is E wrong because it's very absolute + the stimulus is a conditional "if-then" statement?

Thank you,

MB (Official initials of The Maxim. All Rights Reserved.)

[Shannon Parker]

Hi maxim,

Answer choice "E" is basically taking the stimulus and adding unless to it. So instead of, "If we find out whether Selena's claim is true, we will thereby determine whether it is possible to have psychic powers", it becomes "If and only if we find out whether..."

This does nothing to resolve the issue that John mentioned in the previous post, namely that Selena not having psychics powers does not prove that they are not possible, unless it is a given that she would have them if they were possible.

I hope this clears it up some.

~Shannon

[JSLSAT]

Is C a necessary assumption?

[Emily Haney-Caron]

Hi JSLSAT,

C is sort of implied, but isn't necessary; it could be not possible to determine whether Selena has psychic powers and still true that if it were possible, we would know whether it is possible to have psychic powers. Hope that helps!

[jlam061695]

I understand why B is correct now, but my diagram is the complete reversal of Jon's:

SPP (Selena claims to have psychic powers)
___________________________________________
SPPT (if S claim about having psychic powers is true/false) XPP (then we can determine whether it is possible to have psychic powers/or not)


So if S's claim about having psychic powers is true (one option of the sufficient condition) then it is possible to have psychic powers (one option of the necessary condition). So then S would have psychic powers, since it's possible to have them.

[calftemo]

Hi,

I'm a bit confused by this question.

When I was solving this question during the test, my prephrase ("If it's possible to have psychic powers, Selena's got it.") matched (B) almost exactly. That is, Selena having them was a necessary condition of being able to determine if it's possible to have these powers. But then I tried plugging in B back into the stimulus, to see how it would fit in between the support and conclusion, and I thought that B represented a mistaken reversal of what we actually needed. To elaborate:

(1) If possible --> Selena has powers. (Answer choice (B))
(2) Selena claims to have powers.
(3) If her claim is true --> Selena has powers. (satisfies NEC condition of (1).)
C: If her claim is true, we can determine if it's possible to have powers.

After reading Jon's explanation above, I don't really understand why the gap in this argument rests on the case where Selena's claim is false. It never even occurred to me to examine the side of the argument where her claim is false. What specifically in this stimulus indicated that we should examine that case? I feel like clearly I missed something if I didn't even interpret the stimulus in that way.

In the end, I picked (A) because I eliminated B (for the above reasons); C, D, and E were all obviously wrong in filling the gap, and I thought that A could be used to imply that Selena was necessary to make the determination in the conclusion. That is, if no one else has these powers, and Selena's claim is true (ie: Selena has them), then we can determine if it's possible.

In addition, can you help identify what kind/category of flaw this is?

Please help me figure out where I went wrong.

[Adam Tyson]

Hey there Pretender, thanks for the question. First things first - this is a Justify the Conclusion question, not a Flaw question, so don't get caught up trying to identify the type of error. There is one - the author left out some crucial evidence - but that's not what we need to focus on.

The goal here is to prove that the conclusion is true - we need to prove that finding out whether Selena is telling the truth or not will establish, absolutely and for everyone everywhere, whether psychic powers are even possible.

If we find out she has them, then the conclusion is self-evident - it has to be possible for someone to have the powers, because at least one person (Selena) has them. But what happens if we find out that she doesn't have them? Our author wants us to believe that finding that out will also establish everything we need to know about the possibility. How would negative evidence (Selena doesn't have the power) prove that those powers are not possible for anyone, everywhere, ever?

To get there, we need to add the idea that if she doesn't have them, nobody can. The contrapositive of that claim is that if someone can have them (if they are possible), then Selena must have them. That's answer B.

Answer B isn't a Mistaken Negation, because it isn't a conclusion. Mistaken Negations and Mistaken Reversals are bad conclusions based on conditional premises. Rather, answer B is a new conditional claim that, if true, establishes the truth of our conclusion. If she has them, they are possible, and if she doesn't have them, they are not possible - that is what we need in order to prove that once we know whether she has them or not, we will know if they are possible.

This is a tough one, as evidenced by all the questions! Spend a little more time on this explanation and those that came before in this thread and see if this makes sense now. Good luck!

[nickmoroni1985]

Hello,

Re: Jon Denning's explanation, I'm really struggling with the explanation of the formal logic in the conclusion. I follow the explanation up until the point in bold.

"On the face of it this really only makes sense in the affirmative. That is, if we find out that Selena does indeed have psychic powers, then clearly we’d establish the possibility of having psychic powers. If we can conclusively demonstrate that Selena’s claim is false (she definitely doesn’t have psychic powers), then we haven’t ruled out anything but psychic powers in this one instance; it could still be possible to have them, even if Selena does not."

The conclusion states: "If we find out whether Selena's claim is true, we will thereby determine whether it is possible to have psychic powers."

I interpreted that as being very simple formal logic. If Selena is truthful re Psych Powers --> Possible to have psychic powers, the contra of which would be If NOT possible to have psychic powers --> Selena NOT truthful re Psychic Powers.

I don't understand how, in your explanation, Selena being "false," or NOT truthful, was moved into the sufficient. This seems like a mistaken reversal to me. (If Selena NOT truthful --> NOT possible to have psychic powers.) Also, I don't understand how, in your explanation, it is diagrammed as "Psychic Possible Selena Psychic (meaning if anyone could have psychic powers, then Selena will be among that group)"

My best guess at interpreting all of the analyses I've read is that "whether" affirmative and negative, so if one is to diagram, one could write the conclusion as

1. If Selena truthful re Psychic Powers --> Possible to have psychic powers
If NOT possible to have psychic powers --> Selena NOT truthful re: Psychic powers
or
2. If Selena NOT truthful --> NOT possible to have psychic powers
If possible to have psychic powers --> Selena truthful

Is this something that could come up on the LSAT with the use of "whether?"