A colleague presents a cost estimate and someone replies, “He is always arrogant, so there is no point looking at that figure.” This does not challenge the data or reasoning in the estimate. It turns dislike of the person into a conclusion. Traditionally, this is called an ad hominem argument. The problem is not that it mentions a person, but that the personal information is irrelevant to the claim being assessed.
Contemporary informal logic also warns against dismissing every reason about a speaker. When a claim depends mainly on testimony, it matters whether the person witnessed the event, has relevant expertise or has repeatedly reported things inaccurately. A conflict of interest may also justify closer checking. Yet none of these facts directly proves that the claim is false. If the source data and method behind the cost estimate can be checked independently, the speaker's personality does not alter the calculation.
My view is that discussion of a person is reasonable only when its relevance can be stated as a premise. “This conclusion relies entirely on his unverified report, and he has repeatedly falsified similar reports” explains how credibility enters the argument. “I dislike him, therefore he is wrong” does not. Mature judgement neither lets dislike replace reasons nor pretends that testimony, expertise and conflicts of interest never matter.
https://plato.stanford.edu/entries/logic-informal/
https://iep.utm.edu/fallacy/
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