Logical Reasoning · Weaken

PrepTest 143 · 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

Weaken

The one idea it tests

A shared trait does not prove a shared job if the trait shows up anyway in your own group without that job.

The basic logic

The argument says group A has trait T because it does job J. Creature B has T, so B did J. Pop it by showing that B's own kind often has T without doing J, so T has another source.

Tiny words that decide it

“also”
“conclude”
“most”

Playground logic

Marcus's patty: basketball kids wear high-tops to protect their ankles, and Grandpa wore high-tops. His top bun: Grandpa played basketball. Pop it by showing most kids at Grandpa's old school wore high-tops even if they never played.

Step 2 · The kid story

Detective Marcus and the Old Photo

Marcus found an old photo of Grandpa as a kid, wearing tall high-top sneakers. "Case closed," Marcus said. "Basketball players wear high-tops so their ankles stay safe when they jump. Grandpa wore high-tops. So Grandpa was a basketball player!" Nadia looked at the photo. "Hmm. What if, back then, most kids at Grandpa's school wore high-tops even if they never touched a basketball?" Marcus blinked. "Like, it was just the shoe everybody had?" "Right," said Owen. "Then the high-tops don't point to basketball at all. They just point to being a kid at that school." Grandpa chuckled from his armchair. "I was in the chess club," he said. Marcus quietly took off his detective hat.

Step 3 · Try the model question

Marcus: Basketball players wear high-top sneakers because the tall sides protect their ankles when they jump. Grandpa wore high-top sneakers as a kid. So Grandpa must have played basketball.

Which fact, if added, does the most damage to Marcus's reasoning?

Step 4 · The courtroom tale

The Case of the Muddy Boots

In a fictional trial, a prosecutor argued: "Hikers on Ridge Trail wear thick boots because the trail is rocky. The defendant owns thick boots. So the defendant hiked Ridge Trail that night." Mr. Whitfield stood up and adjusted his suspenders. "Your Honor, I call the town shoe seller." The seller testified that most people in town, hikers or not, wear thick boots because the winters are snowy. Mr. Whitfield smiled. "If nearly everyone here wears these boots without hiking, then the boots prove nothing about the trail." The judge set down her pen. "The boots have lost their voice in this case."

Step 5 · Work the real question

Open PrepTest 143, Section 4, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the burger: a trait in one group is explained by a job, then a different creature shares the trait, so it must have done the job. To pop it, look for a choice showing the trait appears without the job, especially among the creature's own relatives. Reject choices that talk about other traits, that leave the trait-job link untouched, or that would actually help the argument.