Unplugged Practice Test: Logical Reasoning Set 2, Question 7 (Necessary Assumption)
Taking the test? Finish it first. This page shows the answer.
- Necessary Assumption questions ask for the unstated premise the argument depends on.
- Try the negation test. If negating a choice wrecks the argument, that choice is the necessary assumption.
- Every question on this test is original, written to the same logical skeletons as real questions, and Steve checked every one himself before release.
The question
Collections of regional folklore frequently describe certain early settlers of the Halvard valley as having been able to foretell eruptions of the volcano above their farms. A prediction that a volcano will erupt is accurate only if the volcano then erupts. The collections themselves, however, record eruptions that these settlers announced were coming and that never took place. So these collections undercut the very ability they attribute to the settlers.
7. The argument requires assuming which one of the following?
- (A)No person is capable of predicting volcanic eruptions accurately time after time.
- (B)A settler was able to foretell eruptions only if every eruption that settler announced actually took place.
- (C)The folklore collections usually leave unstated whether an announced eruption ever occurred.
- (D)In collections that credit settlers with foretelling eruptions, those announcements are usually central to the stories being told.
- (E)Not one folklore collection has ever described a settler who genuinely could foretell eruptions.
BWhy (B) is correct
The argument concludes the collections undercut the settlers' foretelling because some eruptions they announced never happened. Since a prediction counts as accurate only if the eruption follows, an announced eruption that never came means at least one inaccurate call. Getting from that single miss to the collections undercutting the ability needs the standard in (B): a settler could genuinely foretell eruptions only if every eruption that settler announced actually took place. Without that all-or-nothing standard, one bad prediction wouldn't touch a settler's real ability.
Why the other answers are wrong
AWhy (A) is wrong
(A) reaches too far, because the argument targets these particular settlers, not whether anyone anywhere could predict eruptions with perfect consistency.
CWhy (C) is wrong
(C) works against the argument instead of propping it up, since the case depends on some announced eruptions clearly not happening.
DWhy (D) is wrong
(D) is beside the point: the argument turns on accuracy, not on how central the announcements are to the stories that record them.
EWhy (E) is wrong
(E) overshoots badly: the conclusion only needs these settlers' record to fail, not a sweeping claim that no folklore collection anywhere describes a genuine talent.
Common questions
What type of question is this?
Necessary Assumption. Necessary Assumption questions ask for the unstated premise the argument depends on.
How do I get better at Necessary Assumption questions?
Try the negation test. If negating a choice wrecks the argument, that choice is the necessary assumption. The free LSAT cheat sheet lists the wrong-answer patterns to watch for on every type.
