John Wilkins has a salutary post on fallacies. I wouldn't put everything exactly the same way, in part because I think Whately et al. are slightly misleading themselves (as I've suggested in various attempts on this blog to give a better account of fallacies), but it makes a key point that I've made before. The way I've put it is that we should distinguish between rhetorical tactic and rational error. Appealing to authority is an argumentative tactic, a way of giving your reasons for a conclusion; it may or may not involve an error of argument. The same may be said with ad hominem. When we talk about ad hominem fallacies, we are really talking about instances of a rhetorical tactic, reasoning ad hominem, that commit and conceal a fallacy like ignoratio elenchi (in which the premises are not actually relevant to the conclusion put forward, or not sufficiently relevant to establish it as a reasonable conclusion). So we can never determine, simply by looking at the rhetoric (e.g., whether it is insulting), whether an argument is fallacious.
Of course, a rhetorical tactic doesn't have to be fallacious to be wrong. For instance, Newman, in his famous dispute with Kingsley in the nineteenth century, suggested that Kingsley was using a rhetorical tactic, which Newman dubbed "poisoning the wells", that was wrong, because it was something to which no honest Englishman would resort. This, however, was an ethical criticism. And one finds that often people conflate this two ways of criticizing an argument, so that a criticism of the ethics or etiquette of a way of expressing an argument is treated as if it were a criticism of the logic of the argument itself. I suspect, in fact, that this is one of the key reasons why people make so many mistakes with fallacies.
