PrepTest 145, Section 4, Question 20
Selena claims to have psychic powers.(22 more words not shown)
The conclusion drawn above follows(9 more words not shown)
- ANo one else(8 more words not shown)
- BIf it is(9 more words not shown)
- CIt is possible(7 more words not shown)
- DIf Selena's claim(17 more words not shown)
- EWe will not(21 more words not shown)
Reading the argument
Finding that Selena lacks psychic powers wouldn't show that everyone lacks them. She could be wrong about herself while somebody else really has those powers.
The positive result would be easier: if she actually has psychic powers, then they are possible. One real example settles that. It's the negative result that leaves the question open, so the argument hasn't yet shown that investigating Selena will settle the broader issue either way.
What we’re looking for
One way to make the conclusion work would be to establish that Selena really does have the powers. Then discovering the truth about her would establish that they are possible. But the question asks for any assumption that makes the conclusion follow, so we shouldn't insist on that particular route. We need something that makes the result about Selena decisive for the larger question.
Working the answers
- A
Nobody else having been found doesn't establish that nobody else could have psychic powers. They might exist without having been discovered. If Selena's claim is false, the possibility of another genuine psychic remains open.
- BCorrect
This gives us a different way to make Selena's case decisive: if psychic powers are possible, Selena has them. Now test both results. If she has them, we know they are possible because she is an example. If she doesn't have them, they cannot be possible under B's rule; that rule says she would have them if they were possible.
So discovering the truth about Selena settles the question either way. B doesn't say she is the only person who could have psychic powers. Other people could have them too. It says that a world in which they are possible must include Selena having them, which makes a negative result about her decisive as well.
- C
Being able to investigate Selena doesn't establish what a negative finding would mean. Suppose we can determine with certainty that she lacks psychic powers. We'd still have no answer about whether somebody else could have them. C makes the investigation possible but doesn't fix the inference from its result.
- D
This states the problem with the original argument. If a false claim leaves the general possibility unresolved, then finding out about Selena would not settle the question in every case. D undermines the promised conclusion rather than establishing it.
- E
This makes finding out about Selena a requirement for answering the larger question. It doesn't say that satisfying that requirement is enough. We might need to investigate her and still need further evidence afterward. The conclusion requires that learning about her will settle the matter, which B supplies.
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