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

[Jon Denning]

Hi Nick,

Thanks for the questions and welcome to the Forum! I figured since I posted the original explanation here it should be me that responds, so I hope this helps clear things up for you

Your initial analysis/interpretation of the conclusion in the stimulus is close...but not quite right. Consider the conclusion again, exactly as it's given:

"If we find out whether Selena’s claim is true, we will thereby determine whether it is possible to have psychic powers."

That isn't the same as "If Selena is telling the truth, then psychic powers are possible," which is how you—and it should be noted, many others—interpreted it (and diagrammed it) originally. Why not? Because the conclusion is merely about us finding out whether she's telling the truth, not about whether she actually is.

Your follow up though seems to nail that point, so nicely done! I'm going to elaborate on it a bit for the benefit of others who might happen to read this and not see the distinction as clearly as you do

It might help to think of this conditionally.

How would we diagram the actual conclusion? Let's imagine the two scenarios that would trigger the relationship given, where we simply have to determine her truthfulness:

We find out Selena is telling the truth

We find out Selena is not telling the truth

In either case we're told that, since we found out her truthfulness, we can determine whether psychic powers are possible. It doesn't matter if she's honest or not...only that we can know which is the case. (Nevermind the flawed logic for now, just note that either situation above would initiate the conclusion)

Now compare that to the common, mistaken interpretation: "If Selena's claim is true, we can determine whether it is possible to have psychic powers."

Here there's only ONE situation that would trigger the conditional:

Selena is telling the truth

Trying to also draw conclusions from "Selena is not telling the truth" in this situation would be a mistake, specifically what we call a Mistaken Negation (the absence of the sufficient condition is used to try to prove something, which can't be done).

So it's a very subtle difference, but it's critical on a test like the LSAT that subtleties get noticed! So to your question about whether that type of nuance could really be tested, not only is the answer "Yes it could!," it's "Yes, and it's being tested right now!"

Lastly, let me touch on the flaw in the conclusion, and it starts with a point you quote from my first post:

"...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."

Only one of the two scenarios for Selena would let us know anything about psychic powers: if we find out she's telling the truth and has them. If that's the case, then they're definitely possible (she's got them, so they're legit). The alternative though, where we find out she's lying and doesn't have psychic powers, doesn't let us conclude anything! They might still be possible (even if Selena's are absent) or they might be impossible (for Selena and everyone else). We'd have no idea. That's the old "absence of evidence isn't evidence of absence" notion at work.

So the original argument has got that rather glaring hole in it, and the correct Justify answer choice predictably comes in to close it: if psychic powers are possible, then Selena has them. Meaning if we find out she's lying and doesn't have them (the contrapositive of B) then we'd know for sure they're not possible, and the absence error is resolved. That gives us: she has them so they're possible; she doesn't have them so they're not possible. Determined for sure in either case, which is the conclusion.

But what about the erroneous interpretation of the conclusion that I've described above, where it's just treated as "If her claim is true then we can determine whether it's possible to be psychic"? That lacks the flaw at the heart of this question, the one the answer choice resolves! Saying "if she has psychic powers then we'd know whether they're possible" is a factual statement and doesn't need justifying...it's already logically sound. So the mistaken reading that a lot of people make changes the very nature of the question and causes some real problems.

Be extremely careful and pay close attention to every word, because every word on the LSAT Is liable to matter!

I hope that helps!

Jon

[asalmen]

Would negating answer choice B be a good way to arrive at that answer? If it is possible to have psychic powers, then Selena DOES NOT have them, then isn't the entire conclusion destroyed?

[Adam Tyson]

A better negation here, asalmen, would be "even if it's possible to have psychic powers, Selena doesn't have to have them." That's logical opposition, saying the necessary condition is not necessary (rather than saying it does not happen). Either way, in this case, that negation does the trick! Nice work!

[T.B.Justin]

Hey,

I have this diagrammed as follows, since the "whether" caused me to think this was one or the other, and what would follow:

True Psychic

Psychic True

For (E):

Psychic True

But, since, in my opinion, we know that there is another way to determine that psychic powers are possible this doesn't justify the conclusion, not certain about this.

[lathlee]

I am confident that this is a good approach.

if powerscore staff sees this post, plz comment it asap since it will help out a lot of people including myself and Powerscore as well By listing this q as an example of necessity cause Q) in next powerscore bible

This question bothered a hell out of this person really long time cuz no explanation i found web satisfied my curiosity. when i started to think in Powerscore terms, then i got it and realized This Q can be very good example of what powerscore referred as Sufficient cause vs Necessary cause example

This question uses the word (Determine) world indicating sufficiency and necessity relationship exists as well as sufficient and necessity relationship

IF Sufficient condition that is also necessary cause (Possible to find out selena has psychic power) ---> resulted into necessity result (possible to DETERMINE psychic powers)

Then the test taker should start to think if Sufficient condition of necessity cause is not there (Not possible to find out selena has psychic power) -----> then result into result doesn't also occur (Not possible to DETERMINE psychic powers)

The correct answer B is the contrapositive of telling the tests takers of if necessity case does not exist, result cannot occur.

thereby assuming answer choice B, if a test taker assumes this setting, this argument is a full proof air tight by Establishing only Necessity cause (also sufficient condition) can cause Aka DETERMINE the result will occur.

Am I correct?

[Adam Tyson]

I don't see this one as causal at all, lathlee. I think you are missing the conditions here. It's not about whether it is possible to find out whether Selena has psychic powers or not. The element of possibility is referring to the possibility of those powers existing, in Selena or in anyone else.

If Selena has the powers, then it is a tautology, a self-proving situation - if she has the powers, then it is possible for someone to have the powers.

The problem is in the negative. If Selena does NOT have the powers, the author thinks that will determine the truth for everyone. In other words, if she doesn't have them, nobody else can either. That's what we need to find in the answer choices. Nothing causal here, just conditional.

[queenbee]

Hi
I read through all the discussion and I am still stuck.

Here is how I diagramed it:
If Claim is True --> Possible to Determine

then the contra is:
If NOT Possible to Determine --> NOT Claim is true (or claim is not true)

Answer choice B is:
If possible to have psychic powers --> Selena has them

The passage said they would be able to "determine if it's possible to have psychic powers"

it seems so important to be particular about the wording in these questions...I am struggling to understand why this is the correct answer choice. I ruled (B) out so quickly and went with E, even though E was not matching the contra perfectly....not sure how I could get around that

[Robert Carroll]

queenbee,

An issue right at the start is that your diagram is not accurate. The sufficient condition is "we find out whether the claim is true" and the necessary condition is "we will determine whether psychic powers are possible." Your sufficient condition says "if the claim is true"...but determining whether something is true can have a negative result. We could determine that her claim is false - we have thereby determined "whether her claim is true", so in the correct diagram, the sufficient condition is true in that case - we proved she's wrong, so we have determined whether her claim was true.

Moving onto the necessary condition, there is a big difference between "It is possible to determine X" and "It is determined that X is possible". The necessary condition in the stimulus is about the possibility of psychic powers, not the possibility of the determination.

Jon's post earlier in this thread does go over some of this!

Adam's post above yours explains why answer choice (B) works, so please check that out for explanation of that. You're getting caught in the diagramming weeds and not considering how answer choice (B) covers the situation where Selena's wrong, and yet that still somehow lets us determine whether psychic powers are possible.

Moving onto answer choice (E), do the Unless Equation on it:

we determine whether psychic powers are possible we find out whether Selena's claim is true

That's the Mistaken Reversal, not the contrapositive, of the claim in the stimulus, and that's true even as you diagrammed the stimulus. Thus, there's no reason to expect this answer to be correct.

Robert Carroll

[quan-tang@hotmail.com]

stimulus says 'if we find out whether Selena’s claim is true, we will thereby determine whether it is possible to have psychic powers.;,

so the stimus says we can just know the power exist OR NOT by simply obersving selena along, and we dont need to look anywhere else.

so if power exists, selena must have it, thats the only possibility that stimulus can make the above conclusion. otherwise, if selena lie and she dosnt have power, then we can conclude nothing, since someone else might has power.

Hi
I read through all the discussion and I am still stuck.

Here is how I diagramed it:
If Claim is True --> Possible to Determine

then the contra is:
If NOT Possible to Determine --> NOT Claim is true (or claim is not true)

Answer choice B is:
If possible to have psychic powers --> Selena has them

The passage said they would be able to "determine if it's possible to have psychic powers"

it seems so important to be particular about the wording in these questions...I am struggling to understand why this is the correct answer choice. I ruled (B) out so quickly and went with E, even though E was not matching the contra perfectly....not sure how I could get around that

[Adam Tyson]

Correct! Well put!