Logical Reasoning · Necessary Assumption

PrepTest 158 · 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

To say you MUST work on outside causes, the argument needs that working on inside causes alone won't do the job.

The basic logic

The patty says a problem has more than one kind of cause. The top bun says the helper must address one particular cause. The secret sauce: fixing only the other cause is not enough. Pull the Plank: if fixing the other cause alone works, the 'must' collapses.

Tiny words that decide it

“not purely”
“in part”
“must”

Playground logic

Tommy says Biscuit's barking comes partly from the cat next door, so to calm Biscuit he must change things around Biscuit. The secret sauce is that training Biscuit's own habits alone can't calm him.

Step 2 · The kid story

Why Biscuit Barks

Biscuit had been barking all week. Tommy sat on the porch with Priya. "I figured out the problem," said Tommy. "Biscuit's barking isn't all about Biscuit. It's partly because Mr. Whiskers keeps sitting on the fence and staring at him." Priya nodded. "So?" "So to calm Biscuit down, I must focus on changing what goes on around him. Like moving Mr. Whiskers's favorite fence spot." Priya tapped her juice box. "Wait. What if we just trained Biscuit to ignore staring cats? Treats, practice, the works. If that alone calmed him, you wouldn't need to change his surroundings at all." Tommy frowned. "Then my 'must' falls apart." "Right," said Priya. "Your argument quietly needs this: working only on Biscuit himself won't be enough."

Step 3 · Try the model question

Tommy: Biscuit's barking does not come only from Biscuit himself; it comes in part from things around him, like the cat on the fence. So to calm Biscuit, I must focus on changing what goes on around him.

Tommy's argument depends on which assumption?

Step 4 · The courtroom tale

The Late Bread Trucks

In an invented contract dispute, a bakery owner claimed his manager had to reroute the delivery trucks. His lawyer said, "Your Honor, late deliveries don't come only from slow baking. They come in part from traffic. So the manager must focus on changing the routes." Mr. Whitfield rose. "Counselor, what if speeding up the ovens alone would get every loaf there on time? Then rerouting wouldn't be required at all." The lawyer paused. "Then I suppose my client's argument needs that fixing the ovens alone can't solve it." "Exactly," said Mr. Whitfield. "That is the hidden plank."

Step 5 · Work the real question

Open PrepTest 158, Section 3, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Identify the premise: a problem has causes of more than one kind. Identify the conclusion: a helper must address one specific kind. Ask what gap lets the 'must' through. Use Pull the Plank on each choice: negate it and see whether the conclusion collapses. The right answer rules out succeeding by working on the other cause alone. Watch for choices that guarantee success, which are sufficient, not necessary.