Logical Reasoning · Flaw

PrepTest 124 · Section 2 · Question 23

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

Because one event has always come before another, the argument treats the first as required for the second, so blocking the first is said to block the second.

The basic logic

Find the pattern in the evidence: event X has always come before event Y. Then look at the conclusion: stop X and Y will not happen. That treats a usual forerunner as a must-have condition. Y might come without X.

Tiny words that decide it

“preceded”
“clearly”
“every”
“will”

Playground logic

Tommy's patty: every time Mr. Whiskers knocked over the fishbowl, he first meowed loudly. His top bun: keep him from meowing and the fishbowl is safe. The hidden sauce is that meowing is required for knocking, which nobody showed.

Step 2 · The kid story

Tommy's Quiet Cat Plan

Tommy had kept a log. "Every time Mr. Whiskers knocked the fishbowl off the shelf, he meowed really loud first. Every single time." Lulu checked the log on her clipboard. Four meows, four tipped fishbowls. "So," Tommy said, "if I give him a treat whenever he starts to meow, he'll stay quiet, and the fishbowl will be safe forever." Priya sipped her juice box. "Tommy, does a cat need to meow to push a bowl?" Tommy paused. "I guess paws work without sound." "So the meow came first," Priya said, "but that doesn't mean no meow, no crash." That afternoon Mr. Whiskers, full of treats and silent as a ninja, tapped the fishbowl right off the shelf. Biscuit caught the goldfish. Nobody knows how.

Step 3 · Try the model question

Tommy: Every time Mr. Whiskers has knocked over the fishbowl, he has meowed loudly just before. Clearly, then, keeping him from meowing will keep the fishbowl safe.

Which of the following describes a flaw in Tommy's reasoning?

Step 4 · The courtroom tale

The Case of the Silent Doorbell

A homeowners' club wanted to ban doorbells. Its lawyer explained, "Your Honor, before every break-in on our street, someone rang a doorbell first. So with no doorbells, no break-ins." Judge Haddad blinked. Mr. Whitfield stood and snapped his suspenders. "Your Honor, the bell came first each time. That does not make the bell a required step. A burglar can skip the bell and simply try the back window." "So," said the judge, "you are saying the doorbell is a usual opening act, not a ticket that must be bought." "Precisely. Remove it, and the show may go on without it." The ban was denied, and the doorbells kept ringing.

Step 5 · Work the real question

Open PrepTest 124, Section 2, Question 23 in your official copy from LSAC (for example in LawHub) and read it there.

Your answer

How to apply it

Find the evidence pattern: one kind of event has reliably come before another. Then look at the conclusion and ask whether it says that preventing the first will prevent the second. If so, the argument treats a forerunner as a necessary condition. The right answer describes that leap in general terms. Pass over answers that complain about other topics, side details, or the reverse direction.