The Ad Hominem (Source) Flaw on the LSAT
Attacking the arguer instead of the argument is the ad hominem flaw. Learn to distinguish a relevant credibility point from an irrelevant personal attack on the LSAT.
2026-06-01 · 6 min read
What the flaw is
An ad hominem (or "source") flaw rejects a claim by criticizing the person or source making it, rather than addressing the claim itself. Whether someone is biased, hypocritical, or unlikeable usually has no bearing on whether their argument is sound.
Worked example
"The senator argues that the new bridge is unsafe, but she has always disliked the construction company. So the bridge is perfectly safe."
The senator's motives do not establish that the bridge is safe. The argument dodges the evidence about safety and attacks the source instead. That is the flaw.
The subtle part: when a source point is legitimate
Not every mention of a source is a flaw. If an argument is explicitly about whether to trust a source's testimony, then the source's reliability can be relevant. The flaw arises when the truth of a claim is rejected purely because of who said it.
Answer choices name it as "rejects a claim by criticizing its proponent rather than addressing the claim" or "treats the source's bias as proof the claim is false." Verbloom helps you separate the argument from the arguer so you do not get baited by the personal detail.
Frequently asked questions
Is pointing out bias always an ad hominem flaw?
No. Noting bias can be relevant when the question is whether to rely on someone's testimony. It becomes a flaw when bias is treated as proof that the claim itself is false.
How is ad hominem different from appeal to authority?
Ad hominem rejects a claim because of who said it; appeal to authority accepts a claim because of who said it. They are mirror images of the same relevance error.
What words signal a possible source flaw?
Watch for the argument pivoting to the speaker's character, motives, or history right before drawing a conclusion about the claim's truth.
Related Verbloom guides
Sources
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