I didn’t say that an ad hominem changes what is being said or that truth is dependent of the person uttering it. You’d have no reason to doubt what I said if you had addressed what I actually said.

“Theorists are careful to distinguish between simple attacks on character – X is a known drunk so X is a bad person – and the questioning of a person’s argument or advocacy of a proposition because of some characteristic or circumstance of the person. Brinton draws attention to three elements that might be confused: the person, the person’s advocacy of a proposition or claim, and the proposition or claim itself. A nonfallacious case of ad hominem argument would then be one that tried to influence an audience’s attitudes to the person’s advocacy of the proposition or claim by introducing relevant information about the person.”

From the Cambridge edition of Fallacies and Argument Appraisal. It’s just a more lengthy version of what I initially said.

If a psychopath claims that dropping a ball will cause gravity to pull it toward the ground we can agree that him being a psychopath is not relevant.

If however a city planner, to use another example from the aforementioned book, were to advocate building a new road along route A rather than route B, we can very well question his motives if he himself happens to live along route B. The planner may present a very good case for why the road should follow route A, so an evaluator would have to consider carefully the degree to which the circumstantial factor should play a role in the reasoning.