Logical Reasoning · Necessary Assumption

PrepTest 149 · Section 1 · 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

If you date an activity by an object you found, you must assume the object was really there at that time and did not arrive later.

The basic logic

The evidence is an object of a certain kind found in a place linked to a certain date. The conclusion is that the activity happened at that date. The argument quietly needs the object to belong to that date. Use the Pull the Plank test: negate the answer. If the object showed up later, the argument falls apart.

Tiny words that decide it

“likely”
“therefore”
“depends on”

Playground logic

Nadia finds an art club sticker inside the time capsule sealed in 2020 and concludes the art club existed in 2020. The secret sauce: the sticker was already in the box back then. If someone slipped it in last week, the whole burger slides off the plate.

Step 2 · The kid story

The Time Capsule Sticker

The class opened the school time capsule, sealed in 2020. Inside were old drawings, a yo-yo, and a shiny sparkly star sticker. "That's the kind of sticker the art club uses!" said Nadia. "So the art club must have existed back in 2020." Owen nodded. Marcus nodded harder. Then Lulu raised her clipboard. "Wait. When was the sticker put in the box?" Nadia blinked. "In 2020, obviously." "Is it obvious?" said Lulu. "The lid was loose. Tommy hides candy in everything. What if somebody stuck it in there last month?" Tommy looked at the ceiling. Priya sipped her juice box. "Nadia's idea only works if the sticker was in the box when it was sealed. Pull that plank and the argument falls through the floor." Tommy quietly removed a lollipop from the capsule.

Step 3 · Try the model question

The school time capsule was sealed in 2020. Inside it, Nadia found a sparkly star sticker of the kind the art club uses. So the art club probably existed in 2020.

Nadia's reasoning requires assuming that the sticker

Step 4 · The courtroom tale

The Case of the Buried Bowling Pin

Mr. Whitfield held up a dusty bowling pin. "Your Honor, this pin was found under the foundation of the old town hall, poured in 1950. It is the style used by the town bowling league. So the league existed in 1950." Judge Pemberton raised an eyebrow. "Counselor, the town hall was repaired in 1990. Workers dug under that foundation." Mr. Whitfield paused. "Then my case needs the pin having been there since 1950, not tossed in during the repairs." "Can you show that?" "I can show the soil layer was never disturbed." The judge nodded. "Then the plank holds. Proceed."

Step 5 · Work the real question

Open PrepTest 149, Section 1, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the date in the conclusion and the object in the evidence. Ask what must be true for that object to tell us anything about that date. Negate each answer choice and see if the argument collapses. The needed assumption is often modest. Reject answers that are too strong (things like only, all, never), since the argument can survive without them, and reject answers about features of the object that do not touch the timing.