Logical Reasoning · Flaw

PrepTest 116 · Section 3 · Question 6

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

Someone planned to bring a certain thing; that thing showed up; so the arguer says that person must have come. But others might bring the same thing.

The basic logic

The argument treats a clue as pointing to only one source. Ask: is this clue unique to that person? If other people could produce the same clue, the conclusion jumps too far. It is a Backwards Arrow cousin: if X came, the thing appears; the thing appeared, so X came.

Tiny words that decide it

“must”
“since”
“only”
“could”

Playground logic

Marcus's patty: Kevin planned to build a dinosaur sandcastle, and one appeared on the beach. His top bun: Kevin must have been at the beach. The missing piece: only Kevin would build a dinosaur sandcastle, which Marcus never shows.

Step 2 · The kid story

Detective Marcus and the Sand Dinosaur

Kevin said he was skipping the class beach trip. Then, on the sand, everyone found a huge sandcastle shaped like a stegosaurus. Marcus pulled out his detective magnifier. "Kevin told me last week he planned to build a stegosaurus sandcastle. And here one is! So Kevin came to the beach after all. Case closed!" Lulu looked at her clipboard. "Marcus, Kevin planned to build one. But are we sure nobody else would?" Nadia raised her hand. "Um. My little brother loves stegosauruses too. He was here all morning." Marcus slowly lowered his magnifier. Priya sipped her juice box. "A clue only points at one person if only that person could leave it. You showed Kevin could. You didn't show Kevin is the only one who would."

Step 3 · Try the model question

Kevin said he would skip the beach trip. But a stegosaurus sandcastle appeared on the beach, and Kevin had planned to build one. So Kevin must have come to the beach after all.

The reasoning above is flawed because it

Step 4 · The courtroom tale

The Purple Kite Accusation

A neighbor accused Mr. Dale of trespassing in her orchard. Her proof: a purple dragon kite was found stuck in her apple tree, and Mr. Dale had told the hardware clerk he planned to fly a purple dragon kite that weekend. Mr. Whitfield, hooking his thumbs in his suspenders, called one witness: the toy shop owner. "How many purple dragon kites did you sell last month?" "Forty-two," she said. The courtroom gasped. Mr. Whitfield turned to the judge. "Your Honor, my client could have left that kite. So could forty-one other people." Case dismissed. The lesson: a clue that fits a suspect proves nothing unless it fits only that suspect.

Step 5 · Work the real question

Open PrepTest 116, Section 3, Question 6 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the conclusion: a certain person or group must have been present. Find the evidence: something appeared that this person had planned to provide. Ask whether the evidence could have come from anyone else. The flaw is treating a trait of some people as if it belonged only to them. Choose the answer describing that gap, and ignore answers that describe moves the argument never makes.