Logical Reasoning · Necessary Assumption

PrepTest 125 · Section 2 · Question 2

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

When the conclusion mentions an idea that the premises never connect to, the argument needs a bridge linking the new evidence term to the conclusion term.

The basic logic

Line up the premise terms and the conclusion terms. Find the one link that is never stated: the premises say people do a certain thing, and the conclusion says that thing causes a certain harm. Pull the Plank: if that link is false, the whole argument falls.

Tiny words that decide it

“many”
“therefore”
“in fact”
“usually”

Playground logic

Nadia says the kids who hate library noise are the very ones adding to it, because they all wear squeaky sneakers. The patty is about sneakers, the top bun is about noise. The secret sauce is that squeaky sneakers actually make noise in the library.

Step 2 · The kid story

The Squeak Patrol

Ms. Rivera started a Quiet Library Club. The members were the kids who complained loudest about noise in the library. Lulu had a clipboard full of shush rules. Nadia noticed something. "Almost every club member got the new Zoom-Squeak sneakers for their birthday," she said. "So the Quiet Club is actually adding to the library noise!" Marcus gasped. "Betrayal!" Priya calmly sipped her juice. "Wait. Your evidence is about sneakers. Your conclusion is about noise. You are assuming those sneakers really squeak on the library floor." Kevin piped up. "The library has carpet. Carpet is like a quiet dinosaur nest." Everyone paused. If the sneakers do not squeak on carpet, Nadia's whole case falls apart. Her argument needed that missing link to be true.

Step 3 · Try the model question

Nadia: Many of the students most opposed to library noise are members of the Quiet Club. Nearly all club members wear Zoom-Squeak sneakers. Therefore, many of the students most opposed to library noise actually add to it.

Nadia's argument relies on which one of the following assumptions?

Step 4 · The courtroom tale

The Case of the Glitter Guardians

The Clean Streets Society sued its own members for littering. Their lawyer, Ms. Patel, declared, "Your Honor, nearly every member decorates birthday cards with loose glitter. So these members spread litter!" Mr. Whitfield rose and tugged his suspenders. "Your Honor, my opponent proved that members use glitter. She never proved that glitter on a birthday card ends up as litter on the street. Cards are sealed in envelopes. They live on refrigerators." The judge nodded slowly. "Counsel, your evidence talks about glitter. Your accusation talks about litter. Without the link between them, I cannot rule for you." Ms. Patel looked at her notes. There was glitter on them. "Fair point," she muttered.

Step 5 · Work the real question

Open PrepTest 125, Section 2, Question 2 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the conclusion in short form and circle any term in it that the premises never mention. Then find the premise term that is supposed to connect to it. The needed assumption is the bridge saying the premise item really produces the conclusion item. Test each choice with Pull the Plank: negate it and see if the argument breaks. Ignore choices that are too strong or talk about people outside the argument.