Logical Reasoning · Necessary Assumption

PrepTest 133 · Section 2 · Question 2

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 the premises prove 'X will happen' but the conclusion says 'Y is impossible,' the argument needs a bridge saying X always leads to Y.

The basic logic

Premises: every system has rules, and some people will always use the rules to get ahead of others. Conclusion: a system where everyone is treated fairly is impossible. The gap is between 'some get ahead' and 'someone is treated unfairly.' Find the answer that links them, then Pull the Plank.

Tiny words that decide it

“must”
“any”
“if...then”
“always”

Playground logic

Marcus's patty: every recess game has rules, and some kid always figures out how to use them to win more than everyone else. His top bun: a game where every kid has a fair time can't exist. The secret sauce: if anyone wins more than the others, somebody isn't getting a fair time.

Step 2 · The kid story

Marcus and the Perfect Game

Nadia wanted to invent a recess game where every single kid gets a fair time. Marcus crossed his arms like a detective. "Impossible," he said. "Every game needs rules. And some kid always figures out a sneaky way to use the rules to win more than everybody else. Remember Owen and the 'tag doesn't count on the slide' rule?" Owen grinned. Priya set down her juice box. "Wait. You showed somebody will win more. You didn't show anybody gets an unfair time." Marcus blinked. "Well, obviously, if one kid wins more, somebody else is getting cheated." "That," said Priya, "is the part you never said out loud. Without it, your 'impossible' falls down." Lulu wrote on her clipboard: Hidden plank: winning more means someone loses fairness.

Step 3 · Try the model question

Marcus: A recess game where every kid gets a fair time is impossible. Every game needs rules, and in every game some kid will learn to use the rules to win more often than the others.

Marcus's argument depends on which assumption?

Step 4 · The courtroom tale

The Perfectly Fair Pie Contest

Judge Okafor heard the complaint. A baker claimed the town could never run a pie contest where every baker is treated fairly. "Every contest has rules," he said, "and some clever baker always finds a way to use them to win more ribbons." Mr. Whitfield stood up. "Your Honor, he has proven that one baker may win more often. He has not proven that anyone is treated unfairly. Winning more is not the same as cheating someone." The baker frowned. "But if one baker wins more, someone else is wronged!" "Then say so," said Mr. Whitfield, "because your whole claim rests on that unspoken rule." The judge nodded. The hidden bridge was the case.

Step 5 · Work the real question

Open PrepTest 133, Section 2, Question 2 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Mark the conclusion (something can't be achieved) and the premises about what will always happen. Notice that the conclusion uses an idea the premises never mention. Look for the answer that links the premises' idea to the conclusion's idea, as a conditional. Pull the Plank: negate it, and if the argument breaks, keep it. Be careful about strength: the argument needs only enough to show the goal fails for someone, not a sweeping claim about many people.