Logical Reasoning · Necessary Assumption

PrepTest 105 · Section 1 · Question 21

Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.

Step 1 · Pre-Question Guidance · read this first

Question type

Necessary Assumption

The one idea it tests

A case where two causes both played a part only refutes a theory if that theory insists its favorite cause is the whole story.

The basic logic

The argument rejects a view saying every case must involve factor X by pointing to cases with X plus Y. But X is still there! The rebuttal only works if the view means 'X and nothing else.' Find the choice that reads the view that strictly. Pull the plank to confirm.

Tiny words that decide it

“must”
“any”
“both”
“however”

Playground logic

Marcus says every soccer win must be explained by Owen's goals. Frankie says every win must be explained by the goalie's saves. Priya rejects both because some wins had both. Secret sauce: Marcus's theory means Owen's goals are the only thing that counts.

Step 2 · The kid story

Who Really Wins the Games?

Marcus had a theory. "Every win our soccer team gets must be explained by Owen's goals." Frankie had a rival theory. "No, every win must be explained by the goalie's amazing saves." They argued for a week. Then Priya opened her notebook. "You are both wrong. Last Tuesday we won because Owen scored twice and the goalie stopped a penalty kick. Both mattered." Marcus scratched his head. "But Owen's goals were in there. How does that prove me wrong?" Lulu tapped her clipboard. "Good question. Priya's game only beats your theory if your theory means Owen's goals are the only thing that can explain a win. If you just meant his goals are always part of the story, Tuesday fits you perfectly." Priya sipped her juice. "Then I am assuming you meant only."

Step 3 · Try the model question

Marcus holds that every team win must be explained by Owen's goals. Frankie holds that every win must be explained by the goalie's saves. Both are mistaken, since some wins came from Owen's goals and the goalie's saves together.

Priya's argument relies on which assumption?

Step 4 · The courtroom tale

The Case of the Two Recipes

Two bakers sued each other. The first claimed every prize-winning cake in town must be explained by her secret flour. The second claimed every winner must be explained by his secret sugar. Mr. Whitfield, hired by the judges' panel, stood up. "Both claims fail. This year's winner used her flour and his sugar." The first baker's lawyer objected. "Your Honor, my client's flour was in that cake! Her claim survives." Judge Okafor turned to Whitfield. "He has a point, counsel. Your example defeats her only if she claimed her flour is the sole explanation." Whitfield nodded. "Her advertisements say 'flour alone wins.'" The judge smiled. "Then your plank holds."

Step 5 · Work the real question

Open PrepTest 105, Section 1, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write down exactly what each rejected view says and what counterexample is offered. Notice whether the counterexample still contains the factor the view requires. If it does, the argument needs the view to mean that factor is the only one. Find the choice that reads the view that strictly. Negate it to check: if the view could allow other factors, the counterexample no longer defeats it.