Logical Reasoning · Flaw

PrepTest 140 · Section 2 · Question 3

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

Step 1 · Pre-Question Guidance · read this first

Question type

Flaw

The one idea it tests

Showing that one thing gets you what you need does not show it is the only thing that can.

The basic logic

The argument proves that X brings the needed ingredient, then concludes that only X works. It quietly assumes nothing else can deliver that ingredient. That is a Backwards Arrow: 'X leads to success' becomes 'success needs X'.

Tiny words that decide it

“only”
“must”
“if”

Playground logic

Marcus's patty: tall kids can reach the hoop, and good shooting needs reaching. His top bun: only tall kids are good shooters. The missing sauce he takes for granted is that nothing but height lets you reach, which ignores ladders, jumping, and Priya.

Step 2 · The kid story

Only Tall Kids Can Shoot

At recess, Marcus made an announcement. 'Only tall kids are good basketball shooters. Here's why. If you're tall, you can see over defenders, and you can get close enough to the rim to score. And obviously, to shoot well, you have to get close enough to score.' Lulu, who is quite short, frowned at her clipboard. Priya, who is also short, took a bounce pass, jumped, and sank the ball. 'I got close enough by jumping,' she said. 'You showed being tall is one way. You never showed it's the only way.' Marcus looked at the hoop, then at Priya, then at the sky. 'Jumping is cheating,' he said. Kevin added that a long-necked dinosaur would be the best shooter of all.

Step 3 · Try the model question

Only tall kids are good shooters. If you are tall, you can see over defenders and get close enough to the rim to score. And to shoot well, you must get close enough to score.

Marcus's reasoning is open to criticism because it

Step 4 · The courtroom tale

The Bakery Oven Dispute

A bakery sued its rival. 'Our special oven makes bread rise high and stay soft,' said their lawyer. 'Any good loaf must rise high and stay soft. So only our oven can make good bread, and theirs must be copied!' Mr. Whitfield tugged his suspenders. 'Your Honor, they proved their oven is one road to soft bread. They never proved it is the only road. My client uses a slow-rise dough in an ordinary oven.' Judge Okafor tapped her pen. 'One door being open does not mean every other door is locked. Case dismissed.' The rival baker handed out rolls.

Step 5 · Work the real question

Open PrepTest 140, Section 2, Question 3 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Break the argument into its pieces: a thing that produces a needed result, a requirement for success, and a conclusion that only that thing succeeds. Ask what the author never proved: that nothing else can produce the needed result. The right flaw answer names that hidden 'only'. Be wary of answers that invent a word-shift or a mix-up the argument does not actually make.