Logical Reasoning · Flaw

PrepTest 155 · Section 1 · Question 13

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

A feature of the parts does not automatically pass to the whole made of them.

The basic logic

Track the chain. A feature moves from one thing to a second thing, and then from those parts to a whole built out of them. The weak link is the jump from parts to whole: a whole can lack a feature that every one of its parts has.

Tiny words that decide it

“composed”
“purely”
“must also”

Playground logic

Owen says the blue bricks are light, red bricks are no different, and the castle is made only of red and blue bricks (patty), so the castle is light (top bun). That last step is a Team-to-Star Mix-Up: tiny light parts can add up to one heavy whole.

Step 2 · The kid story

The Feather-Light Castle

Owen and Nadia built a giant brick castle in the art room. It used every brick in the bin. Owen picked up one blue brick. 'Blue bricks are super light. And red bricks weigh the same as blue ones, so red bricks are light too. The castle is made of nothing but red and blue bricks. So the castle is light! I will carry it to the hallway.' Priya sipped her juice box. 'Owen, one brick is light. Five thousand bricks together are not.' Owen bent down, grabbed the castle, and pulled. Nothing moved. His face turned the color of a red brick. Lulu wrote: 'Parts can be light. The whole can still be heavy.' Biscuit sniffed the castle and walked away.

Step 3 · Try the model question

Owen: Blue bricks are light. Red bricks weigh just the same as blue bricks, so red bricks are light too. Our castle is built entirely from red and blue bricks. Therefore, our castle is light.

Which one of the following describes a mistake in Owen's reasoning?

Step 4 · The courtroom tale

The Tiny Stones Defense

A builder was sued for blocking a creek with a wall. Mr. Whitfield represented the builder. 'Your Honor, each pebble in that wall is tiny. A tiny pebble could never block a creek. The wall is made purely of pebbles. So the wall could never block the creek!' The judge looked at the photo of a very tall wall and a very dry creek bed. 'Mr. Whitfield, every pebble is tiny. Thousands of tiny pebbles stacked together are not.' Mr. Whitfield tugged a suspender. 'A fair point, Your Honor.' 'It is the whole point,' said the judge. 'A thing can be missing a quality even when every piece of it has that feature.'

Step 5 · Work the real question

Open PrepTest 155, Section 1, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Map the chain step by step: which feature moves, and from what to what. Check each jump. When a feature passes from the parts to the thing built from them, ask whether wholes really must share that feature with their parts. The right answer names that gap in abstract terms. Be careful with choices that attack a step the argument handles fine, or that talk about wrong experts or other things built from the same parts.