Logical Reasoning · Flaw

PrepTest 124 · Section 2 · Question 4

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

Knocking out the only evidence for a claim does not prove the claim false. A lack of good proof that something is so is not proof that it is not so.

The basic logic

Patty: the only support for a worry comes from people who are not experts, and only experts can decide the matter. Top bun: so the worry is false. The leap: the speaker swaps 'not proven' for 'disproven' without offering any expert evidence on their own side. That is the No Footprints flaw.

Tiny words that decide it

“only”
“none”
“hence”
“no”

Playground logic

Marcus says only an expert can count dinosaur toes, and none of Kevin's critics is an expert, so Kevin's drawing must have the right toes. He knocked out the patty for one side but brought no patty for his own top bun. No Footprints: a missing case against is not a case for.

Step 2 · The kid story

Marcus Rules on Toes

Kevin taped up his giant T. rex drawing. Each foot had five toes. Nadia and Owen tilted their heads. 'T. rex did not have five toes,' said Nadia. Owen agreed. Marcus jumped in like a judge. 'Objection! Only a real dinosaur scientist can say how many toes a T. rex had. Nadia, are you a dinosaur scientist?' 'No.' 'Owen?' 'No.' 'Then the only people who complained are not experts. So the drawing is correct. Five toes. Case closed.' Lulu tapped her clipboard. 'Marcus, you are not a dinosaur scientist either. You did not ask one. You just showed that Nadia and Owen can't prove it is wrong.' Priya sipped her juice box. 'Not proven wrong is not the same as proven right.'

Step 3 · Try the model question

Marcus: Nadia and Owen say Kevin's drawing gives T. rex the wrong number of toes. But only a trained dinosaur scientist can settle how many toes T. rex had, and neither of them is one. So the drawing shows the correct number of toes.

Marcus's reasoning goes wrong because it

Step 4 · The courtroom tale

The Case of the Wobbly Bridge Model

At a fictional science fair hearing, two parents claimed a student's model bridge would collapse under a toy truck. The organizer's lawyer rose. 'Your Honor, only a licensed engineer can judge a bridge, and neither parent is an engineer. Therefore the bridge is perfectly strong.' Mr. Whitfield stood up slowly. 'Did my friend bring an engineer who says it is strong?' Silence. 'Then he has shown only that the parents cannot prove it is weak. He has shown nothing about strength.' Judge Park sighed. 'Agreed. Let's just put the truck on it.' The bridge held, by the way, but the argument did not.

Step 5 · Work the real question

Open PrepTest 124, Section 2, Question 4 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the claim the speaker attacks and the conclusion the speaker draws. Notice that the attack removes support for the other side but adds no support for the speaker's own side. Name it as No Footprints. Then match the answer that describes treating missing evidence as proof of the opposite, or the lack of any positive support for the conclusion. Skip choices about issues the argument never raises.