Distinguishing ad hominem arguments

Here's a discriminating discussion:

A debater commits the Ad Hominem Fallacy when he introduces irrelevant personal premisses about his opponent. Such red herrings may successfully distract the opponent or the audience from the topic of the debate.

Ad Hominem is the most familiar of informal fallacies, and—with the possible exception of Undistributed Middle—the most familiar logical fallacy of them all. It is also one of the most used and abused of fallacies, and both justified and unjustified accusations of Ad Hominem abound in any debate.

The phrase "ad hominem argument" is sometimes used to refer to a very different type of argument, namely, one that uses premisses accepted by the opposition to argue for a position. In other words, if you are trying to convince someone of something, using premisses that the person accepts—whether or not you believe them yourself. This is not necessarily a fallacious argument, and is often rhetorically effective.

For instance, ad hominem is one of the most frequently misidentified fallacies, probably because it is one of the best known ones. Many people seem to think that any personal criticism, attack, or insult counts as an ad hominem fallacy. Moreover, in some contexts the phrase "ad hominem" may refer to an ethical lapse, rather than a logical mistake, as it may be a violation of debate etiquette to engage in personalities. So, in addition to ignorance, there is also the possibility of equivocation on the meaning of "ad hominem".

For instance, the charge of "ad hominem" is often raised during American political campaigns, but is seldom logically warranted. We vote for, elect, and are governed by politicians, not platforms; in fact, political platforms are primarily symbolic and seldom enacted. So, personal criticisms are logically relevant to deciding who to vote for. Of course, such criticisms may be logically relevant but factually mistaken, or wrong in some other non-logical way.

Subfallacies:

Abusive: An Abusive Ad Hominem occurs when an attack on the character or other irrelevant personal qualities of the opposition—such as appearance—is offered as evidence against her position. Such attacks are often effective distractions ("red herrings"), because the opponent feels it necessary to defend herself, thus being distracted from the topic of the debate.

Circumstantial: A Circumstantial Ad Hominem is one in which some irrelevant personal circumstance surrounding the opponent is offered as evidence against the opponent's position. This fallacy is often introduced by phrases such as: "Of course, that's what you'd expect him to say." The fallacy claims that the only reason why he argues as he does is because of personal circumstances, such as standing to gain from the argument's acceptance.

This form of the fallacy needs to be distinguished from criticisms directed at testimony, which are not fallacious, since pointing out that someone stands to gain from testifying a certain way would tend to cast doubt upon that testimony. For instance, when a celebrity endorses a product, it is usually in return for money, which lowers the evidentiary value of such an endorsement—often to nothing! In contrast, the fact that an arguer may gain in some way from an argument's acceptance does not affect the evidentiary value of the argument, for arguments can and do stand or fall on their own merits.

http://www.fallacyfiles.org/adhomine.html