Logical Reasoning · Necessary Assumption

PrepTest 154 · Section 2 · Question 19

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

If a fix worked, the arguer assumes it worked for the reason they think, not some other reason.

The basic logic

The argument guesses a cause, applies a fix aimed at that cause, sees improvement, and concludes the guessed cause was real. Pull the Plank on any other way the improvement could have happened.

Tiny words that decide it

“therefore”
“in fact”
“decided”

Playground logic

Lulu's patty: the cat stopped eating, she moved the bowl away from the noisy dryer, and the cat ate again. Her top bun: the noise was the problem. The secret sauce is that the cat did not start eating for some other reason that came along with the move.

Step 2 · The kid story

Mr. Whiskers Goes on Strike

Mr. Whiskers refused his dinner three nights in a row. Lulu checked her clipboard. His bowl sat right next to the clothes dryer, which rumbles like a dragon. "Loud noise can make a cat nervous, and a nervous cat may skip meals," she said. So she moved the bowl to the quiet hallway. That night, Mr. Whiskers ate every bite. "Case closed," Lulu said. "The dryer noise was why he stopped eating." Tommy raised a hand. "Um, when you moved the bowl, I also opened the brand new bag of fancy salmon food." Lulu froze. Priya, sipping her juice box, smiled. "Your proof only works if the salmon food did not do the trick," she said. "You are quietly assuming the fix worked for your reason."

Step 3 · Try the model question

Mr. Whiskers stopped eating. His bowl was beside the noisy dryer, and noise can make a cat too nervous to eat. Lulu moved the bowl to a quiet hallway, and the cat ate his whole dinner. So the dryer noise was why he had stopped eating.

Lulu's reasoning depends on assuming which one of the following?

Step 4 · The courtroom tale

The People v. the Squeaky Swing

Witness Dana Plum testifies: "The park swing squeaked. I guessed the chain was dry, so I oiled it, and the squeak stopped. So a dry chain caused it." Mr. Whitfield rises. "Ms. Plum, did anything else change that day?" She blinks. "Well, the park crew also tightened the seat bolts." Gasps fill the room. "Your Honor," says Mr. Whitfield, "her conclusion only stands if the squeak stopped because of the oil, and not because of the bolts. That is the hidden assumption." The judge nods. "A fix that worked does not prove your guess, unless nothing else could have done the fixing."

Step 5 · Work the real question

Open PrepTest 154, Section 2, Question 19 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Map the burger: a symptom, a guessed cause, a treatment aimed at that cause, then improvement, then the conclusion that the guess was right. Ask what else could explain the improvement. The needed assumption rules out another route to the same result. Use the Pull the Plank test on each choice: negate it, and if the conclusion falls apart, keep it. Toss choices that are too strong, like ones claiming something is the only or best way.