Ad hominem

“Why listen to his plan for the budget? He failed maths at school.” The plan’s numbers are still there to be checked, whoever wrote them.

Attacking the speaker instead of the claim

Ad hominem is Latin for “to the person.” The Stanford Encyclopedia of Philosophy describes the fallacy as bringing negative aspects of an arguer, or of their situation, to bear on the view they are advancing. The claim is left untouched while the person is pushed aside. Since a true statement stays true when a disliked person says it, and a false one stays false when a respected person says it, the move gives no reason about the claim itself.

Three versions

The Stanford Encyclopedia separates three kinds. The abusive ad hominem says a view should be rejected because the person has some unfavourable trait: “She is rude, so her report on the bridge is wrong.” The circumstantial ad hominem claims the person holds the view out of self-interest rather than evidence: “Of course the bakery owner says bread prices must rise; he sells bread.” The third, tu quoque (Latin for “you too”), points out that the speaker does not follow their own advice: “My uncle told me smoking is harmful, but he smokes, so I can ignore him.”

The uncle example shows why the move fails. The uncle’s smoking tells you something about the uncle, perhaps that he finds it hard to stop. It tells you nothing about whether smoking is harmful, which is a question about lungs and hearts. In the same way, a bakery owner may well have an interest in higher prices, and the argument for higher prices may still be sound; the way to find out is to look at flour and rent costs.

When the person does matter

Not every remark about a person is a fallacy. When a claim rests on someone’s word alone, for example a witness who says they saw the accident, evidence that the witness has lied before or could not see the road is relevant, because the only support for the claim is their testimony. A conflict of interest is also a fair reason to check a claim more carefully. The mistake is to treat “this person has a reason to say it” as if it meant “what they say is false.” The question to ask is whether the comment about the person changes the evidence for the claim, or only changes how we feel about the speaker.