Logical Reasoning · Flaw

PrepTest 151 · Section 3 · 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 rule that runs one way (if A, then B) does not run backwards (if B, then A).

The basic logic

Map the rule as an arrow: whenever the condition is present, the signal shows up. Then the argument sees the signal and concludes the condition is present. That is the Backwards Arrow. The right answer describes mixing up 'condition guarantees signal' with 'signal guarantees condition.'

Tiny words that decide it

“whenever”
“must”
“so”

Playground logic

Biscuit barks every single time the mail carrier comes. Today Biscuit is barking, so Marcus declares the mail is here. The patty is a one-way arrow (mail → bark), but the top bun rides it backwards (bark → mail). Biscuit also barks at squirrels, the vacuum and his own reflection.

Step 2 · The kid story

Biscuit's Bark

"Every time the mail carrier comes up the walk, Biscuit barks," said Marcus. "I've studied it for weeks. Never misses." Just then Biscuit started barking like crazy. Marcus jumped up. "The mail is here! Tommy, go get the package!" Tommy ran to the door. No mail carrier. Just a squirrel on the fence, looking smug. Lulu wrote on her clipboard: 'Mail comes → Biscuit barks. NOT the same as: Biscuit barks → mail comes.' Priya nodded. "Biscuit barks every time the mail comes. That doesn't mean the mail comes every time Biscuit barks. He also barks at squirrels, the doorbell on TV and the vacuum." Marcus sat down slowly. "So my detective rule only works in one direction." "Exactly," said Priya. "Arrows have pointy ends for a reason."

Step 3 · Try the model question

Biscuit barks whenever the mail carrier comes to the house. Biscuit is barking right now. So the mail carrier must be at the house.

What is the main error in Marcus's reasoning?

Step 4 · The courtroom tale

The Sneeze Detector

"Your Honor," said the prosecutor, "Mrs. Lin sneezes whenever a cat is in the room. Witnesses agree. On the night of the dinner party, Mrs. Lin sneezed. Therefore, the defendant smuggled a cat into the party." Mr. Whitfield rose and tugged his suspenders. "Your Honor, does Mrs. Lin sneeze only when there is a cat?" Mrs. Lin raised her hand. "Oh, no. Pepper, dust, sunshine. I sneeze at everything." Mr. Whitfield turned to the jury. "The prosecution has shown that a cat guarantees a sneeze. It has not shown that a sneeze guarantees a cat. That arrow points one way only." The judge sighed. "Case dismissed. Somebody get Mrs. Lin a tissue."

Step 5 · Work the real question

Open PrepTest 151, Section 3, Question 13 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Translate the general rule in the evidence into an arrow: which part guarantees which. Then translate the conclusion into an arrow. If the conclusion runs the rule in reverse, you have the Backwards Arrow. Look for the choice that describes confusing the two directions. Cross out choices about evidence quality, sample groups or other issues the argument does not actually turn on.