Attacking the source: the LSAT flaw that feels reasonable

A source argument rejects a claim by discrediting whoever made it. The person is biased, inconsistent, unqualified, or has something to gain — therefore what they said is false.

It does not follow. A claim is true or false on the evidence, and someone with every reason to lie can still be right. The flaw is treating a reason to distrust the speaker as a reason to reject the statement.

Why it is hard to catch

Because in ordinary life the reasoning is often sensible. If a study is funded by the industry it exonerates, you should be more skeptical, and being more skeptical is rational. What the argument does is go further — from "treat this cautiously" to "this is false" — and that step is invisible if you are nodding along.

The LSAT builds these to be sympathetic. The person attacked is frequently a hypocrite, or funded by someone obviously interested, and the argument reads as the reasonable position in the exchange. That is the trap: your agreement with the conclusion is doing the work your evaluation should be doing.

The forms it takes

• Bias or interest. They stand to gain, therefore the claim is false.
• Hypocrisy. They do not follow their own advice, therefore the advice is wrong. Extremely common, and the inconsistency is genuinely irrelevant to whether the advice is good.
• Character. They have behaved badly, therefore their argument fails.
• Circumstance. Given their position they would say that — which explains why they said it and says nothing about whether it is true.
• Attacking a different argument. The response rebuts a claim the speaker did not make, which is a straw man rather than a source attack, and the answers distinguish them.

How the answers describe it

• rejects a claim by attacking its proponent rather than addressing the claim itself
• treats the source of an argument as relevant to the argument's merits
• criticizes the argument on the basis of the arguer's motives
• confuses the question of whether a position is held sincerely with whether it is correct

These also appear as Method of Reasoning answers rather than flaw answers, describing what a second speaker did rather than judging it. The description is the same; only the question is different.

When it is not a flaw

Two cases matter. When the conclusion is about the person rather than about the claim — arguing that someone is not a credible witness, not that their statement is false — the reasoning is legitimate. And when the original claim rested entirely on the speaker's authority, undermining the authority genuinely undermines the support.

So read the conclusion carefully before selecting a source-attack answer. "We should not trust her testimony" and "her testimony is false" are different conclusions, and only one of them commits the flaw.