Logical Reasoning · Flaw

PrepTest 101 · Section 2 · Question 8

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

An argument cannot use its own conclusion, in different clothes, as one of its reasons.

The basic logic

Line up the top bun and each premise side by side. If a premise says the same thing as the conclusion in other words, the argument runs in a Circle and proves nothing new.

Tiny words that decide it

“false”
“for”
“since”
“likely”

Playground logic

Marcus says it is false that Kevin often wears his dinosaur costume. His reason: Kevin wearing it is rare, so photographers love to snap it. But 'rare' is just his conclusion dressed up as a reason: a Circle.

Step 2 · The kid story

The Yearbook Dinosaur

Nadia flipped through the class yearbook. 'Kevin is wearing his dinosaur costume in half these photos. He must wear it all the time.' Marcus put on his detective hat. 'False! You only think that because of all the photos. But Kevin almost never wears it, and when something almost never happens, the yearbook kids rush to take a picture.' Nadia squinted. 'How do you know he almost never wears it?' 'Because,' said Marcus, 'the idea that he wears it all the time is false.' Lulu tapped her clipboard twice. 'Marcus, you said it's rare to prove it's not common. Those are the same thing.' Priya nodded. 'You walked in a circle and ended up where you started.' Kevin, in full costume, roared softly. Marcus thought about this. 'It was a very confident circle, though.'

Step 3 · Try the model question

Marcus says the claim that Kevin often wears his dinosaur costume is false, because that claim rests only on the many yearbook photos of him in it. But since Kevin wearing the costume is very rare, the yearbook team is likely to photograph it whenever it happens.

Marcus's reasoning is flawed because it

Step 4 · The courtroom tale

The Honest Witness

Counsel Brandt stood before the jury. 'My witness is honest. You can trust him because he told you he is honest, and an honest man would never lie about that.' Mr. Whitfield rose, smiling. 'Your Honor, counsel asks us to believe the witness is honest because of what the witness said, which we should believe only if he is honest. That is a merry-go-round.' The judge nodded. 'Counsel, you have used your conclusion as your proof. Bring me something from outside the circle: a record, another witness, a document.' Brandt flipped through his notes. 'I may need a short recess, Your Honor.' 'Granted.'

Step 5 · Work the real question

Open PrepTest 101, Section 2, Question 8 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the top bun: a popular claim is called false. Then find the reason offered against it, usually after a word like 'since.' Hold that reason next to the conclusion and ask: is this just the conclusion said another way? If yes, pre-phrase the flaw as a Circle: the argument assumes what it sets out to prove. Pick the choice that says so. Eliminate choices that name a flaw the argument never commits (Phantom Pieces) and ones that add a worry the argument never relies on (Strangers).