Logical Reasoning · Necessary Assumption

PrepTest 102 · Section 2 · Question 6

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 reaction is taken to prove one specific cause, the argument needs that nothing else produces the reaction.

The basic logic

Evidence: having met X makes you react to X's signal. Conclusion: anyone who reacts must have met X. That treats the reaction as a fingerprint. The plank: the signal's power to trigger the reaction belongs to X alone. Pull that plank and the conclusion falls.

Tiny words that decide it

“anyone who”
“has been”
“only”

Playground logic

Mr. Whiskers hisses at a sock carrying Rex's smell, because cats that have met Rex remember it (patty). So Tommy says any cat that hisses has met Rex (top bun). The secret sauce: nothing but Rex's smell sets off that hiss.

Step 2 · The kid story

The Hissing Sock Test

Tommy invented a test. 'Rex, the dog next door, chases cats. Once a cat meets Rex, it remembers his smell and hisses at it. So I rubbed this sock on Rex. Any cat that hisses at the sock has met Rex.' He held the sock out. Mr. Whiskers hissed. 'Proof!' Lulu tapped her clipboard. 'Wait. Does only Rex's smell cause the hiss? What if the sock smells like the bully dog across the street too, or like Biscuit, or just like old gym socks?' Priya nodded. 'If other smells on that sock can trigger a hiss, then hissing does not prove a cat met Rex.' Tommy sniffed the sock and made a face. 'It does smell like gym class.'

Step 3 · Try the model question

Any cat that has met Rex remembers his smell and hisses at it. When a sock rubbed on Rex is held near a cat that has met him, the cat hisses. So Tommy concludes that any cat that hisses at the sock has met Rex.

Tommy's reasoning requires which assumption?

Step 4 · The courtroom tale

The Paint Splatter Case

A mural was splashed with a rare green paint. The prosecutor said, 'Anyone who handled this paint gets a green tint under the fingernails. The defendant has that tint. So he handled the paint.' Mr. Whitfield rose slowly. 'Your Honor, my client works at a lime juice stand. Could lime peel tint fingernails green?' The expert witness blinked. 'Actually, yes.' Mr. Whitfield smiled. 'Then the tint is not a fingerprint for the paint. The prosecution assumed only the paint makes that tint, and that assumption just fell off the stage.' Case dismissed.

Step 5 · Work the real question

Open PrepTest 102, Section 2, Question 6 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Write the argument as: having met the cause produces a reaction, so a reaction proves the cause was met. Spot the gap: other things might produce the same reaction. Then test each choice with the Pull the Plank test: negate it and see whether the conclusion collapses. The right choice says the trigger is unique to the one cause. Skip choices that are broader than needed, that describe symptoms, or that are about side effects of the test.