Logical Reasoning · Weaken

PrepTest 151 · Section 2 · Question 16

Phase 1 (your first ~400 questions): read the guidance, then answer. No clock.

Step 1 · Pre-Question Guidance · read this first

Question type

Weaken

The one idea it tests

"We never saw it" only counts against a cause if we would have seen it had it been there.

The basic logic

The speaker rejects an obvious cause for two reasons: no one has observed the cause in action, and another kind of variation could explain the effect. The first reason leans on No Footprints. Weaken by showing that the cause would be invisible to our tools anyway, so not seeing it proves little.

Tiny words that decide it

“but”
“should not conclude”
“no”

Playground logic

Marcus refuses to blame Biscuit for the missing sandwich: nobody saw Biscuit near it, and sandwiches sometimes just get moved. Pop it with this: Biscuit only sneaks food when the kitchen is empty, so no one could ever have seen him do it.

Step 2 · The kid story

Not Biscuit?

Tommy's peanut butter sandwich was gone. Crumbs led toward the dog bed. Biscuit has a long record of sandwich crimes. Lulu pointed. "Case closed." Marcus held up a hand. "Not so fast. A detective doesn't jump. Nobody saw Biscuit take it. And sandwiches get moved around this kitchen all the time. Grandpa moves stuff." Priya sipped her juice box. "Marcus, when does Biscuit steal food?" Marcus paused. "Only when the kitchen is empty. He waits until everybody leaves." "So nobody could have seen him, ever?" "Well. No." Priya smiled. "Then 'nobody saw him' isn't a clue. It's just how Biscuit works." Biscuit licked peanut butter off his nose. Lulu wrote: no footprints, because he wipes his paws.

Step 3 · Try the model question

Marcus: Tommy's sandwich is missing, and crumbs lead toward Biscuit's bed. Biscuit has stolen sandwiches before. But we should not conclude that Biscuit took it. Nobody saw him near the sandwich, and things in this kitchen often get moved around.

Which choice, if true, does the most harm to Marcus's reasoning?

Step 4 · The courtroom tale

The Invisible Fox

The farmer's chickens were missing, and fox tracks circled the coop. The county's lawyer argued, "Your Honor, we cannot blame a fox. Our night cameras show no fox, and chickens wander off on their own." Mr. Whitfield stood, suspenders snapping. "May I ask the camera expert a question?" The expert took the stand. "Can your cameras see a fox at night?" "No, sir. Their range ends at the fence, and foxes stay low." Mr. Whitfield turned. "So the cameras would show no fox whether a fox came or not. Their silence tells us nothing." Judge Mbeki nodded. "The 'no fox seen' point is set aside."

Step 5 · Work the real question

Open PrepTest 151, Section 2, Question 16 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Spot that the speaker is arguing against a likely cause, and list the reasons given. Flag any reason of the form "we have not observed it." Ask: would we have observed it if it were happening? Look for a choice saying our tools or methods could not detect the cause anyway. Reject Saboteurs that help the speaker, and Bystanders.