Logical Reasoning · Necessary Assumption

PrepTest 158 · 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

Necessary Assumption

The one idea it tests

When an argument rules out a 'natural' explanation and jumps to a 'someone helped' explanation, it must assume the thing couldn't have survived without help.

The basic logic

The patty shows that no natural supply was around (proved by the absence of others who would have come for it). The top bun says a person must have been providing. The secret sauce: without a natural supply, the creature would not likely be there unless someone was taking care of it. Pull the Plank: if the creature could do fine on its own without any supply, the argument collapses.

Tiny words that decide it

“thus”
“any”
“no other”
“unless”

Playground logic

Nadia's top bun is that someone was feeding the hamster in the garage. Her patty is that there was no loose food there, proven because no mice showed up. Her secret sauce is that a hamster wouldn't stay healthy all week without a food source unless someone was feeding it.

Step 2 · The kid story

The Garage Hamster Mystery

Over spring break, Owen's hamster, Nugget, got loose in the garage. A week later, Nugget turned up plump and happy. "How did he eat?" asked Owen. Marcus put on his detective hat. "He found crumbs!" Nadia shook her head. "No way. If there were any loose food in that garage, mice would have moved in. Mice always find food. But we found zero mice, zero droppings, nothing." She pointed at Tommy. "So somebody was feeding Nugget." Tommy turned red. "Okay! I was hiding candy out there and I gave him sunflower seeds too." Priya nodded slowly. "Nadia's argument needs one thing to be true. A hamster can't stay healthy for a week with no food source unless somebody feeds him. If hamsters could live on air, her whole case falls apart."

Step 3 · Try the model question

Nugget the hamster stayed healthy for a week in the garage. If there had been any loose food in the garage, mice would have come, and no mice came. So someone must have been feeding Nugget.

Nadia's argument requires which assumption?

Step 4 · The courtroom tale

The Well-Watered Fern

A landlord accused a tenant of sneaking into a locked apartment. Mr. Whitfield represented the landlord. "Your Honor, the fern in that apartment was green after a month. The sprinklers were off. If any pipe had leaked, the floor would show stains, and the floor is spotless. So someone came in and watered it." The judge raised an eyebrow. "Counsel, your case rests on something unsaid. You assume a fern would not stay green for a month without water unless someone gave it some. What if it is a plastic fern?" Mr. Whitfield checked his notes, then turned pink. "Your Honor, may I have a short recess?"

Step 5 · Work the real question

Open PrepTest 158, Section 2, Question 3 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Map the burger. The patty rules out a natural source by pointing to something that would have shown up if that source existed, but didn't. The top bun says the thing must have been cared for by people. Ask: what must be true for 'no natural source' to point to 'people helped'? The bridge is that the thing couldn't likely be there without a source unless people supported it. Test each choice by negating it. The right one breaks the argument when flipped.