LSAT Flaw Practice Question: Ad Hominem
An original flaw question from our free Question of the Day rotation, built around a trap the LSAT reuses constantly: ad hominem. Try it first, then read the explanation — it teaches the move, not just the answer.
Your job:Name what's wrong with the argument.
City councilor Ibarra has proposed converting an unused rail spur into a walking-and-cycling bridge, presenting traffic studies and cost projections in support of the plan. Councilor Voss urged colleagues to reject the proposal, pointing out that Ibarra owns a bicycle shop two blocks from the proposed bridge and thus stands to profit if it is built.
Voss's response to Ibarra's proposal is most vulnerable to criticism on the grounds that it
Show the answer & full explanation
The correct answer is C
The JBDM line
Just because Ibarra might profit from the bridge doesn't mean her evidence for it is wrong.
The maybes
- MaybeMaybe the traffic studies hold up regardless of who presented them.
- MaybeMaybe a self-interested proposal is still the best proposal on the table.
Why C is right
Voss goes after the person, not the argument. Ibarra offered traffic studies and cost projections; Voss answered with her motives and never touched the evidence. An argument's quality doesn't depend on who benefits from its conclusion — that's the ad hominem move, described exactly.
Why the others are wrong
- AVoss never claims the bridge couldn't help both the public and Ibarra — he simply never engages the merits at all.
- BVoss says nothing about how the studies were conducted; he ignores them entirely.
- DNo generalization about the whole council appears anywhere.
- EVoss's conclusion is 'reject the proposal,' not a claim about the bridge's profitability.
Sammy's note
Motive is not a rebuttal. If the evidence goes unanswered, the argument stands untouched.
What's a JBDM line?
Just Because Doesn't Mean (JBDM) is the Burton Method's foundational framework for attacking LSAT arguments: identify the premise, identify the conclusion, then say "just because [premise] doesn't mean [conclusion]" and generate "maybes" — alternative explanations that expose the gap between them. That gap is what weaken, flaw, strengthen, and assumption questions all test, so every explanation on this page starts with the JBDM line. It's a micro-lesson in the system, not just an answer key.
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