Logical Reasoning · Flaw

PrepTest 124 · Section 2 · Question 11

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

Backwards Arrow: 'A leads to B' does not mean 'B came from A.'

The basic logic

The premise says a certain cause brings a certain result. The conclusion sees the result and assumes that cause. Look for the answer that says the result could have other sources.

Tiny words that decide it

“must”
“will”
“hence”
“can be assumed”

Playground logic

Tommy's patty: a big breakfast makes him zoom at recess. His top bun: he zoomed today, so he ate a big breakfast. He flipped the arrow and forgot that other things (a new pair of sneakers, a sugar rush from hidden candy) can also make him fast.

Step 2 · The kid story

Tommy's Breakfast Theory

Tommy announces a scientific discovery at the lunch table. "To run at all, I need my sneakers tied. And a giant breakfast always makes me super fast at recess. So today, since I was super fast, I must have eaten a giant breakfast." Lulu flips open her clipboard. "You had half a toast." "Impossible. I was lightning." Priya sets down her juice box. "Tommy, the big breakfast is one way to get fast. It is not the only way. You also ate three candy bars from your sock drawer." Tommy freezes. "Who told you about the sock drawer?" Lulu taps her clipboard. "Your arrow goes breakfast to fast. You read it backwards: fast to breakfast." Marcus nods slowly. "Classic backwards arrow. I've done that twice this week."

Step 3 · Try the model question

Tommy needs his sneakers tied to run at recess. Eating a giant breakfast always makes Tommy run fast. Tommy ran fast today. So Tommy must have eaten a giant breakfast today.

Tommy's argument goes wrong mainly by ignoring that

Step 4 · The courtroom tale

The Shiny Bicycle

The prosecutor points at Mr. Dobbs. "Polishing a bicycle every night always makes it shine. This bicycle shines. Therefore Mr. Dobbs polished it every night, as he swore to the bike club." Mr. Whitfield rises, suspenders creaking. "Your Honor, my learned friend has his arrow on backwards. Polishing leads to shine. But shine can come from elsewhere. This bicycle was bought new on Tuesday." He holds up a receipt. "Brand-new bikes shine without a single rag." The judge peers over her glasses. "So the shine proves nothing about the polishing." "Exactly. A cause that works is not the only cause there is."

Step 5 · Work the real question

Open PrepTest 124, Section 2, Question 11 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the premise that says one thing reliably brings about another. Then read the conclusion: does it start from the result and claim the cause must have happened? If so, you have a Backwards Arrow. The right answer will say the result can occur without that cause. Confirm your pick speaks to the direction of the reasoning.