Ad hominem
An ad hominem argument, also known as argumentum ad hominem (Latin, literally "argument to the man"), is 1) a logical fallacy that involves replying to an argument or assertion by addressing the person presenting the argument or assertion rather than the argument itself; 2) an argument pointing out an inconsistency between a view expressed by an individual and the remainder of his or her beliefs.
Ad hominem as logical fallacy
A traditional, regular (fallacious) ad hominem argument was identified by Aristotle in his On Sophistical Refutations and has the basic form:
Regular Ad Hominem
A makes claim B;
there is something objectionable about A,
therefore claim B is false.
An inverted (fallacious) ad hominem argument was only identified as such more recently. Traditionally it would have been considered only an appeal to authority fallacy which, it also is. The inversion can be seen as, and called as such when, an implication to "appeal to authority-lacking" regarding the opponent is obvious. Stated as such it is a better reflection of the bind the fallacymonger finds himself in, in presenting this fallacy, especially when self-referencing credentials as a critical point to an argument and, particularly when the claim is, to his own authority or, authority for it's own sake. It has the basic form:
Inverted Ad Hominem
A makes claim B;
there is something desirable about A,
therefore claim B is true.
The first statement is called a 'factual claim' and is the pivot point of much debate. The last statement is referred to as an 'inferential claim' and represents the reasoning process. There are two types of inferential claim, explicit and implicit. Other positive arguments to the person are discussed under appeal to authority.
Ad hominem is one of the best-known of the logical fallacies usually enumerated in introductory logic and critical thinking textbooks. Both the fallacy itself, and accusations of having committed it, are often brandished in actual discourse. As a technique of rhetoric, it is powerful and used often, despite its lack of subtlety.
Usage
A regular ad hominem fallacy consists of saying that someone's argument is wrong and/or they are wrong to argue at all purely because of something discreditable/not-authoritative about the person or those persons cited by them rather than about the argument itself. The implication is that the person's argument and/or ability to argue correctly lacks authority. Merely insulting another person in the middle of otherwise rational discourse does not necessarily constitute an ad hominem fallacy. It must be clear that the purpose of the characterization is to discredit the person offering the argument, and, specifically, to invite others to discount her arguments. In the past, the term ad hominem was sometimes used more literally, to describe an argument that was based on an individual, or to describe any personal attack. But this is not how the meaning of the term is typically introduced in modern logic and rhetoric textbooks, and logicians and rhetoricians are widely agreed that this use is incorrect.
An inverted ad hominem fallacy consists of saying that someone's argument is correct and/or they are correct to argue at all purely because of something creditable/authoritative about the person or those persons cited by them rather than the argument itself. The implication is that the person need not even bother to defend against an attack on the soundness of his arguments because of this very credibility/authority. The other implication which can be seen as either leading to or following from the preceding implication, being that a potential opponent lacks this very same credibility/authority. This "appeal to authority" and/or implied "appeal to authority-lacking" leaves the fallacymonger in a serious bind as it also implies a hidden premise which itself defeats his argument. This premise may then be 'granted' by any victim of this fallacy and her argument is all but made.
Bind example: A recognizably credentialed professional logician (Fallacymonger) vs. a layperson (Fallacymongered); Fallacymonger - "You are wrong to argue with me about matters of logic because I am an expert in this field. Because you lack this expertise yourself, I refuse to waste my time arguing with you at all about this matter." Fallacymongered - "I will grant you this premise: That I would have to be a fool to argue that you are wrong in this regard simply because most, if not all, professional logicians agree that an 'appeal to authority' of this sort is a fallacy." By granting this premise the fallacymongered acknowledges and forces the fallacymonger likewise to acknowledge A) he is already arguing with her and B) a willingness to argue from first principles if required, in order to make their respective cases. She must still present a sound argument, but so must the fallacymonger. Note that if he rejects her offer of the granted premise then he acknowledges, A) her right to cite his own peers to establish the fallaciousness of his argument or B) her right to ignore likewise the rules of logic, and so assume authority fallaciously herself (which of course leads to absurdity). But if he accepts the granted premise then, he has defeated himself undoubtedly, for he acknowledges that he would have to be a fool himself to use his own expertise in this way. Either way she has established her right to question his authority and his need to defend his authority logically. Note also that in this example the fallacymonger is also fallaciously begging the question by claiming his authority by appealing to his own authority. This is of course a rather silly example, but unfortunately it is in principle a common situation. One need only consider the circular arguments of someone defending their religion by invoking the authority of the God of that same religion to defend the authority of the church or bible of that very same religion which, in turn the authority of, are used to defend the authority of that same God and the artifacts themselves. This is of course an inversion of the all too common regular (fallacious) ad hominem arguments of those who discredit secular views as evil and, claim those views are therefore invalid. (Inverted-Ad Hominem is a term first coined by Layman at The Brights Forums (
http://www.the-brights.net/forums/index.php?act=idx). A bright being a person whose worldview is naturalistic.)
Conversely, not all regular ad hominem attacks are insulting. "Paula says it is impossible for her to murder a man, but this is false because Paula has never lost her temper." Nor likewise are all inverted ad hominem defenses self flattering - "I say it is impossible for me to murder a man, and this is true because I often lose my temper."
Subtypes
Three traditionally identified varieties include ad hominem abusive, ad hominem circumstantial, and ad hominem tu quoque.
Ad hominem abusive
Ad hominem abusive (also called argumentum ad personam) usually and most notoriously involves merely (and often unfairly) insulting one's opponent, but can also involve pointing out factual but damning character flaws or actions. The reason that this is fallacious is that--usually, anyway--insults and even damaging facts simply do not undermine what logical support there might be for one's opponent's arguments or assertions.
An example: "Jack is wrong when he says there is no God because he is a convicted felon."
Ad hominem circumstantial
Ad hominem circumstantial involves pointing out that someone is in circumstances such that he or she is disposed to take a particular position. Essentially, circumstantial ad hominem constitutes an attack on the bias of a person. The reason that this is fallacious is that it simply does not make one's opponent's arguments, from a logical point of view, any less credible to point out that one's opponent is disposed to argue that way.
"Tobacco company representatives are wrong when they say smoking doesn't seriously affect your health, because they're just defending their own multi-million-dollar financial interests."
The Mandy Rice-Davies ploy, "Well, he would [say that], wouldn't he?" is a superb use of this fallacy.
It is important to note that the above argument is not irrational, although it is not correct in strict logic. This illustrates one of the differences between rationality and logic.
Ad hominem tu quoque
Ad hominem tu quoque (literally, "at the person, you too") could be called the "hypocrisy" argument. It occurs when a claim is dismissed either because it is inconsistent with other claims that the claimant is making or because the claim is about actions the claimant has engaged in, too.
"You say airplanes fly because of physics, but this is false because you said earlier airplanes fly because of magic."
"You cannot accuse me of libel because what you do is libel as well."
The tu quoque form is often a specific kind of the two wrongs make a right fallacy.