LR Question 20 Explanation, by LSATHacks
QUESTION TEXT: Selena claims to have psychic powers. So if we…
QUESTION TYPE: Sufficient Assumption
CONCLUSION: If we find out whether Selena is correct, we’ll find out if psychic powers are possible.
REASONING: Selena says she has psychic powers.
ANALYSIS: The conclusion is about what happens if we find out whether Selena is correct. Selena’s claim is either true or not true: she has psychic powers, or she doesn’t.
Note that “find out whether Selena’s claim is true” covers both finding out the claim is true and finding out that it’s false. It’s best to think about what happens in either case:
- Claim true ➞ Selena psychic ➞ psychic possible
- Claim false ➞ Selena not psychic ➞ ?????
So we have an asymmetry. If we find out Selena’s claim is true, then we do know psychic powers are possible. But if the claim is false, we can’t make a general conclusion. Someone else could be psychic even if Selena isn’t. So to solve this question we have to somehow show that if Selena is not psychic then no one is psychic.
Note that I haven’t followed a traditional diagramming structure on this question. That’s because LSAC wrote this question in such a way that a traditional diagramming structure isn’t helpful. This question is more of a decision tree.
___________
- So? Selena still might or might not have powers. Or someone else could, even if Selena doesn’t.
- CORRECT. This covers the gap from the analysis above. Now if Selena’s claim is false we will also know psychic powers are impossible.
This answer’s contrapositive is clearer:
Selena powers➞possible to have powers - This is a necessary condition for the conclusion, but it doesn’t prove the conclusion correct. This is a sufficient assumption question.
- This destroys the argument! We’re trying to prove the argument correct.
- This is just a mistaken reversal of the conclusion. It doesn’t help prove the conclusion.
More Resources for Sufficient Assumption Questions
- Conditional Reasoning Article: Learn about conditional statements.
- LR Diagrams Guide: Learn how to draw LR diagrams.
- Intro to Conditional Reasoning: Learn conditional reasoning basics.
- Intro Course lesson: This intro course lesson covers Sufficient Assumption questions.
- Mastery Seminar lesson: This LR Mastery seminar lesson covers sufficient assumption questions.
Small note on choice (A) (because it was extremely tempting to me):
It is actually a necessary assumption, not a sufficient one.
If someone else had been found to have psychic powers, then we would not determine whether it is possible to have psychic powers or not by verifying Selena’s claim, simply because it would have already been determined.
Good distinction, hadn’t noticed this! This is like the rattlesnake question.
Note: This is an old comment but I wanted to address the point.
Hey guys,
I think a critical part has been left out and that adding it in can bring about a much clearer view of what is going on in this question and others that use the mechanism I discuss below. But Please, I’d like some feed back from contributors on the position I take.
Okay, The argument begins by giving us the premise that Selena claims to have psychic powers, but the conclusion is where we need to focus to get this question right, it then concludes that if we can determine WHETHER Selena’s claim is true, then we can determine WHETHER it is possible to have psychic powers. Notice that I put both uses of the word whether in all caps. Understanding this question, in my view, hinges on understanding what exactly the word WHETHER does. When a speaker or author uses the word whether, they are essentially setting up two “worlds”–one in which something is true and one in which that thing is false.
In this question, the conclusion states that, “if we can determine WHETHER Selena’s claim is true…” :this translates to the two “worlds” below:
Selena has psychic powers
Selena DOES NOT have psychic powers
“…then we can determine WHETHER it is possible to have psychic powers.”: this translates to the two “worlds” below:
It is possible to have psychic powers
It is NOT possible to have psychic powers
Now, If Selena HAS psychic powers, then it is indeed possible to have psychic powers (Selena having psychic powers but it being impossible to have psychic powers CANNOT happen together). Therefore,
Selena has psychic powers ———-> possible to have psychic powers.
At this point, many LSAT preppers understand whats going on. However, it seems to require some invisible premise even before we add in a sufficient assumption. How do we COMPLETELY FIX this argument?
Again,THE FIX, in my view, hinges on understanding what using the word WHETHER does (and remember its used twice).
We’ve established that “WHETHER” implies both instances of something is true and of it being false, and as it pertains to this question: the sufficient condition given in the conclusion… “if we can determine WHETHER Selena has psychic powers”, is translated
Selena has psychic powers
Selena DOES NOT have psychic powers
(But PLEASE understand that BOTH statements are implied by the sufficient condition)
And the necessary condition given in the conclusion…”then we can determine WHETHER it is possible to have psychic powers”, is translated
It is possible to have psychic powers
It is NOT possible to have psychic powers
(But PLEASE understand that BOTH statements are implied by the necessary condition)
Okay, remember that if Selena HAS psychic powers then it is possible to have psychic powers. Now, in order to FIX the argument by ensuring the conclusion, we need to connect the part of the sufficient condition that we haven’t yet addressed and the part of the necessary condition that we haven’t addressed, e.g.,
Selena DOES NOT have psychic powers —————> It is NOT possible to have psychic powers
(The test writers chose to proffer the contrapositive of this statement in the correct answer choice.
“If it IS possible to have psychic powers, then Selena HAS THEM”.)
Essentially, the question requires us to assume a biconditional relationship between Selena having psychic powers, and it being possible to have psychic powers.
TAKEAWAYS
WHETHER DOES EQUAL IF, AND IF NOT (simultaneously)
WHETHER DOES NOT EQUAL THAT
WHETHER DOES NOT EQUAL IF
Please let me know WHETHER this hits the mark.
–Thanks
This seems to be saying the same thing as the explanation. Didn’t catch this at the time, but wanted to chime in for anyone reading this comment now.
your contrapositive in your explanation doesn’t appear to be correct
stimulus:
selena powers -> powers possible
contrapositive
powers not possible -> Selena no powers
your “
Selena powers➞possible to have powers” is a negation without reversalThe analysis for (B) is showing the diagram of the contrapositive of the answer choice and not the stimulus.
The answer choice says “If it is possible to have psychic powers, then Selena has them.”
The contrapositive of this answer choice (as the explanation notes) is: ~(Selena powers) –> ~(possible to have powers)
Thank you for this, very helpful! I agree the contrapositive makes the answer much clearer.
Hi Graeme,
This was SUPER helpful!! I noticed you said this was “more of a decision tree” question rather than one that relies on traditional diagramming. Could you please elaborate a little on that?
The traditional method of finding the answer to a sufficient assumption question that uses conditional reasoning is to draw out each of the conditional statements in the stimulus, and come up with the missing conditional statement that will link the premises to the conclusion.
This question requires a bit more of a creative approach. In very basic terms, a decision tree is just a (tree-shaped) graph or model that is used to show all or some of the possible outcomes of a series of possible decisions. In the analysis for this question, Graeme used branches of a decision-tree to show the potential outcomes of investigating Selena’s claim to being a psychic. Her claim can only be true or false, so those are the two branches.
Branch 1: Her claim is true, she’s a psychic, and it’s possible to be a psychic.
Branch 2: Her claim is false, she’s not a psychic, and from there we have no idea what’s the case.
This decision-tree just provides a clear representation the gap in the argument, more so than just traditional conditional diagramming would. And determining the gap between the premises and conclusion is one of the most important factors in coming up with the correct answer to a sufficient assumption question.
First, let me thank you on your explanations. I’ve found them to be one of the biggest blessings in my LSAT prep thus far and am incredibly grateful for the service you provide.
I was hoping you could elaborate on the asymmetry part of the analysis below:
“Note that “find out whether Selena’s claim is true” covers both finding out the claim is true and finding out that it’s false. It’s best to think about what happens in either case:
Claim true ➞ Selena psychic ➞ psychic possible
Claim false ➞ Selena not psychic ➞ ????? ”
I’m confused to how the wording of the stimulus sets up this asymmetry. What in the wording indicates the true or false. Is it the word “whether”? I feel that this question is a departure from the norms of most logical reasoning questions and am worried I won’t be able to identify a similar reasoning structure in another question.
It’s the “if we find out whether Selena’s claim is true”. The whether means it might be true, and it might not be. And then the stimulus says “we will thereby determine whether it is possible”, which means that if the conclusion is true we will find out either way.