Logical Reasoning · Necessary Assumption

PrepTest 120 · Section 3 · Question 13

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

If a thing lacks a quality because it was built to prevent a problem, the argument assumes that quality tends to cause that problem.

The basic logic

The conclusion says something lacks a nice quality. The reason given is that it was designed to block a certain risk. The gap is the link between the quality and the risk: the argument needs the nice quality to make the risk more likely. Pull the Plank: if that quality did not raise the risk, the reason would explain nothing.

Tiny words that decide it

“since”
“by design”
“thus”
“more likely”

Playground logic

Grandpa says the playground fence is plain and has no decorations, since it was built to keep kids from climbing over. The secret sauce: decorations would make the fence easier to climb. Pull that plank and the 'since' falls apart.

Step 2 · The kid story

Why the Fence Is So Boring

Nadia stared at the new playground fence. "It is so plain! No painted flowers, no wooden animals, nothing." Grandpa, picking up Tommy, leaned on it. "That fence is built to stop little ones from climbing over into the parking lot. A kid on the wrong side could get hurt, so it is plain on purpose." Marcus scoffed. "That makes no sense. What does plain have to do with climbing?" Priya sipped her juice box and thought. "Grandpa must be thinking that decorations would give kids something to grab and step on. If fancy fences were just as hard to climb, plain would not help at all." Lulu wrote it down: plain because safe only works if fancy means easier to climb. Biscuit sniffed the fence and approved.

Step 3 · Try the model question

The playground fence is plain, with no decorations at all, because it is designed to stop children from climbing into the parking lot. A child climbing over could be badly hurt.

The argument depends on which assumption?

Step 4 · The courtroom tale

The Case of the Dull Medicine Bottle

Judge Haverford held up a gray bottle. "The design firm says its bottle is ugly because it is meant to stop children from opening it." A rival designer objected. "Ugly? Why would ugly be safer?" Mr. Whitfield tugged his suspenders. "Your Honor, the firm's reasoning needs one hidden piece. It assumes a colorful, cheerful bottle would tempt children to open it. If bright bottles drew no extra interest, the firm's 'because' would be empty." The firm's lawyer whispered, "That is exactly our view." "Then say it out loud," said the judge. "An argument cannot lean on a plank it never names."

Step 5 · Work the real question

Open PrepTest 120, Section 3, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the conclusion: something lacks a certain quality. Find the reason: it was designed to prevent some risk. Ask what connects the quality to the risk. The needed assumption says that having the quality would make the risk more likely. Pull the Plank on each choice: negate it and see if the reason stops explaining the conclusion. Skip choices that compare importance or bring in outside groups.