PrepTest 140 · Section 1 · Question 21 (Flaw)

Step 1 · Pre-Question Guidance · read this first

Question type

Flaw

The one idea it tests

Showing that one way of doing wrong didn't happen does not show that no wrong happened.

The basic logic

Spot the conditional: if A, then wrong. Then watch the arguer deny A and conclude 'not wrong.' That treats A, which is enough for wrong, as if it were required for wrong. It is the Flipped Switch.

Tiny words that decide it

“if”
“so”
“not”

Playground logic

House rule: if you stay up past bedtime, you broke a rule. Kevin went to bed on time, so Marcus declares he broke no rule. Flipped Switch: being late to bed is one way to break a rule, not the only way (hello, pineapple pizza crumbs on the couch).

Step 2 · The kid story

Kevin's Perfect Record

At Kevin's house, staying up past bedtime counts as breaking a house rule. Last night, Kevin was in bed at exactly eight o'clock. Marcus, visiting for a sleepover report, announced, "So Kevin broke zero house rules yesterday. Perfect record!" Lulu raised her clipboard. "Staying up late is one way to break a rule," she said. "Is it the only way?" Everyone looked at the couch. There were pineapple pizza crumbs all over it, and eating on the couch is also against the rules. Kevin started explaining a very long theory about how a T. rex would have eaten on couches too. Priya smiled. "Marcus, you showed he skipped one door to trouble. You never checked the other doors."

Step 3 · Try the model question

At Kevin's house, staying up past bedtime counts as breaking a house rule. Kevin went to bed on time last night. So Kevin did not break any house rule last night.

Which of the following best describes an error in Marcus's argument?

Step 4 · The courtroom tale

The Parking Spot Defense

Mr. Whitfield faced the jury. "The town code says: parking in a fire lane is a violation. My client did not park in a fire lane. Therefore, my client committed no violation." The prosecutor stood up. "Your Honor, the charge is not about a fire lane. It is about parking on the town's tulip bed." The jury giggled. Judge Okafor peered over her glasses. "Mr. Whitfield, the fire lane rule tells us one thing that is enough for a violation. It does not say it is the only thing." Mr. Whitfield tugged his suspenders sheepishly. "I see, Your Honor. I closed one door and declared the house locked."

Step 5 · Work the real question

Open PrepTest 140, Section 1, Question 21 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the conditional in the premises: if a certain condition holds, then the action is wrong. Next, notice the arguer shows the condition does not hold and concludes the action is not wrong. That treats a sufficient condition as a necessary one. Pick the answer that describes treating something that is enough for wrongness as if it were required for wrongness. Cross off answers that describe moves the argument never makes.