Logical Reasoning · Flaw

PrepTest 113 · Section 3 · Question 18

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

Showing that one relevant feature is hard to judge does not show that the whole category is hard to judge, since other features might settle membership.

The basic logic

Track the jump: evidence is about one property that is fuzzy; the conclusion is about whether something belongs to a whole group. Ask what else could decide membership. The flaw is ignoring that some other feature might do the job cleanly.

Tiny words that decide it

“therefore”
“cannot be any”
“some”

Playground logic

Marcus says being funny matters for joke cards, and funny depends on who reads it, so there is no clear way to tell what counts as a joke card. His patty is about one fuzzy feature, but his top bun is about the whole club. He never rules out that something else, like the official joke stamp, could decide it clearly.

Step 2 · The kid story

Marcus and the Joke Card Mystery

The Joke Club posts a Joke Card on the board every morning. Marcus, wearing his detective hat, made an announcement. 'Some Joke Cards are funny, and being funny matters to how we enjoy them. But whether a card is funny depends on who is reading it. Kevin laughs at anything with a dinosaur. Lulu only laughs at puns. There is no clear test for funny. So there can be no clear test for what counts as a Joke Card at all!' Priya held up a card. 'Marcus, every real Joke Card has the club's purple star stamp on the back. That is a clear test.' Marcus looked at the stamp. Then at Priya. Then at the ceiling. 'I was getting to that,' he said. He was not getting to that.

Step 3 · Try the model question

Being funny is something that matters to how we enjoy some Joke Cards. Whether a card is funny depends on the reader. There is no clear test for features that depend on the reader. So there is no clear test for whether a card counts as a Joke Card.

Marcus's reasoning is weak because he overlooks the chance that

Step 4 · The courtroom tale

The Case of the Fancy Cake

A baking contest was sued for 'having no clear rules.' Attorney Lark Benson argued, 'Your Honor, beauty matters to how judges enjoy a cake. Beauty is in the eye of the beholder. So there is no clear rule for what counts as a contest cake!' Mr. Whitfield rose and adjusted his suspenders. 'Your Honor, every entry must be registered by Friday and carry a numbered ticket. That decides what counts as an entry, cleanly. Counsel looked at one wobbly feature and forgot the others.' Judge Ruiz smiled. 'The fuzzy part does not make the whole thing fuzzy. Case dismissed.' Someone in the gallery ate a numbered cupcake.

Step 5 · Work the real question

Open PrepTest 113, Section 3, Question 18 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Spot the evidence: it is about one property that is relevant but hard to pin down. Then spot the conclusion: it is about whether anything belongs to the whole group. Notice the gap between one property and the whole test for membership. Look for the answer naming a possibility the argument fails to rule out that would let membership be decided some other way. Skip answers about side details that do not touch that gap.