fallacy

Ad hominem

Also called: Personal attack, Argumentum ad hominem

An ad hominem attacks the person making an argument instead of the argument itself. Whether a claim is true does not depend on who says it.

How it works

The speaker shifts attention from the reasons on the table to the character, motives or circumstances of the opponent. If the audience comes to dislike or distrust the person, the argument feels refuted even though nothing in it has been answered.

Examples

"Why should we listen to her plan for the city budget? She can't even keep her own business afloat."

Her business record says nothing about whether the budget figures in this plan add up.

"Of course the professor supports the new policy. Academics never had a real job."

The reply dismisses the support by mocking the speaker's profession and never engages with the policy.

How to spot it

The reply is about the speaker, not the claim.
Insults, labels or motives stand in for reasons.
Ask: if someone else said exactly this, what would the answer be?

How to respond

Separate the two: "That may be true of me, but what is wrong with the argument?"
Restate the claim and ask for a reason it fails.
Note when a person's record is genuinely relevant, such as testimony from a witness with a history of lying, and when it is not.