Logical Reasoning · Necessary Assumption

PrepTest 147 · Section 4 · 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

An analogy needs the key feature of the first case to exist in the second case too.

The basic logic

Find what makes the first case work (a whole judged by whether it does its job). The analogy only holds if the second case has a matching 'job' to be judged by. Pull the Plank: if the second case had no such job, does the comparison collapse?

Tiny words that decide it

“only”
“similarly”
“cannot”

Playground logic

Priya's burger compares cakes to gardens: only a recipe author can judge a cake by whether it does what it is meant to, and similarly only a garden designer can judge a garden that way. The secret sauce: gardens have something like a cake's purpose. Pull that plank and the 'similarly' has nothing to stand on.

Step 2 · The kid story

Priya's Cake and Garden Lecture

At Aunt Rosa's house, Priya gave a speech nobody asked for. 'Only the person who wrote a recipe can judge a cake by whether it turned out the way it was meant to. The grocer knows the flour and eggs, but the grocer can't tell you what the cake is supposed to be for.' Kevin nodded, eating frosting. 'Similarly,' Priya went on, 'only a garden designer can judge a garden by whether each flower bed does its job in the whole design. The soil tester knows dirt, but not the plan.' Lulu raised her hand. 'Wait. A cake is meant to be a birthday treat. Is a garden meant to be anything?' Priya paused. 'Good catch. My comparison only works if a garden has a purpose too, the way a cake does.'

Step 3 · Try the model question

Priya: Only a recipe author can judge a cake by whether it serves its intended purpose; a grocer knows the ingredients but not the purpose. Similarly, only a garden designer can judge a garden by how each bed serves the whole design; a soil tester cannot.

Priya's comparison depends on which one of the following?

Step 4 · The courtroom tale

The Case of the Two Experts

Mr. Whitfield faced the jury. 'Only a ship captain can judge a ship by whether it does what a ship is for. The welder knows the steel, not the voyage. Similarly, only a coach can judge a relay team by whether each runner serves the whole race.' The opposing lawyer stood. 'Your Honor, that comparison rests on something unstated. A ship has a purpose: getting somewhere. Counsel must be assuming the relay team has a purpose of its own, a goal the whole team works toward. If it had none, his comparison falls apart.' Judge Okafor smiled. 'Fair point. Every analogy borrows a feature. Make sure the second case actually has it.'

Step 5 · Work the real question

Open PrepTest 147, Section 4, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Identify the two cases joined by the word marking the analogy. Ask what feature lets the expert in the first case do the judging. Then ask whether the argument quietly needs that same feature in the second case. Apply Pull the Plank to each choice: negate it and see whether the comparison collapses. Avoid choices that compare the cases too strongly or that add claims the analogy never uses.