Why Not Ad Hominem?

Byran asks why serious scholar eschew ad hominem augments.

First, I think that most ad hominem attacks are not in fact a logical fallacy. By ad hominem I mean attacking an opponent's character, qualifications, motives or experience rather than addressing the substance of her argument.

The logic doesn't go:

A is a bad person therefore A's argument is false.

It goes this way

1) Intelligent and well motivated people are usually correct

2) If A were intelligent and well motivated then this alone would be reason to believe her.

3) However, A is either unintelligent or ill motivated and thus there is no particular reason to believe her.

The underlying assumption is that I, the debater, cannot independently verify some of A's facts. Therefore, A's competence and trustworthiness become important inputs to the debate.

However, a serious scholar will never admit that he or she cannot independently verify the facts. Thus, he avoids ad hominem arguments. To engage in them would draw into question his expertise.

Bryan would know this if he weren't such a jerk ;)