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Logical Reasoning · Flaw

PrepTest 141 · Section 2 · Question 5

Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.

Step 1 · Pre-Question Guidance · read this first

Question type
Flaw
The one idea it tests
Two explanations predict the same thing. The arguer treats that shared success as a reason to reject one of them.
The basic logic
When an explanation predicts what we see, that is a point in its favor. If a rival also predicts it, the evidence is a tie, not a strike against either one. Look for an argument that knocks out an explanation just because a rival fits the same facts.
Tiny words that decide it
  • “yet”
  • “because”
  • “also”
  • “must”

Playground logic

Priya's idea (Biscuit took the cookies) predicted crumbs by the dog bed, and there were crumbs. Marcus says the idea fails because the cat idea also predicts crumbs there. A tie is not a loss.

Step 2 · The kid story

Crumbs by the Dog Bed

Grandpa's cookies vanished from the kitchen. Priya has a theory: 'Biscuit took them. If so, we should find crumbs by the dog bed.' Lulu checks with her flashlight. Crumbs! Right by the dog bed. Then Marcus bursts in wearing his detective hat. 'Not so fast. Mr. Whiskers sleeps on that dog bed too. If the cat took the cookies, there would also be crumbs there. So the Biscuit theory is no good!' Priya blinks. 'Marcus, the crumbs fit my theory. They also fit yours. That makes it a tie, not a loss for me.' Lulu writes: a clue both ideas predict cannot knock out either one. Grandpa just laughs and opens a second box, which is how Tommy finds out where the candy should have been hidden.

Step 3 · Try the model question

Marcus: Priya's theory says that if Biscuit took the cookies, there will be crumbs by the dog bed. There are crumbs there. But the theory that Mr. Whiskers took the cookies predicts the very same crumbs. So Priya's theory is wrong.

Marcus's argument is flawed because it

Choices

Step 4 · The courtroom tale

Who Muddied the Hallway?

At the town museum, muddy prints ran down the hall. The guard's lawyer told Judge Reyes, 'The curator says the gardener walked in. If so, there would be mud on the hall floor. There is mud. But the delivery driver also walks that hall, and that theory predicts the same mud. Therefore the gardener theory is worthless.' The judge tapped her pen. 'Counselor, the mud fits both stories. That makes the mud useless for choosing between them. It does not make either story worthless.' The lawyer looked at his notes and found nothing else. 'Bring me a clue,' the judge said, 'that only one story predicts.'

Step 5 · Work the real question

Open PrepTest 141, Section 2, Question 5 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the two explanations and the observation each one predicts. Notice that the arguer rejects one explanation only because the other also fits. Ask: does a rival fitting the same data count against the first explanation? No, it only means the data cannot pick a winner. Choose the answer that names this move. Reject answers that describe an expert problem, a word shift, a cause leap, or a false choice unless the argument actually does that.