Wikipedia defines it like this:
Ad hominem reasoning is normally described as a logical fallacy, more precisely an informal fallacy and an irrelevance.
My view of this fallacy reduces this issue to three interconnected points.
1: Ad Hominem is a way of reasoning. This means, ultimately, that an argument or a discussion involving some kind of reasoning process has to be the context of the ad hominem fallacy. Insults, name calling, and other sorts of personal attacks - if they are not used to further an argument or as part of the reasoning, are not an ad hominem attack. Calling someone a moron on a message forum, calling someone to repentence, insisting that they are liberal, fundamentalist, socialist, communist, atheist, anti-mormon - if it isn't used to reflect on or dismiss a point of view or an argument, it isn't ad hominem. (It may still be highly inappropriate, way out of line, and so on). The moment we move to you are wrong because you are an X, is the moment that line has been crossed (and sometimes this is an implicit statement).
2: Ad Hominem is a "logical" fallacy. When we talk about ad hominem attacks, unlike with polemics or apologetics, the truth of propositions doesn't have an impact on our identification of the fallacy. If, in a discussion on apologetics, someone asserts that everything written by FARMS apologists is an ad hominem attack, this charge isn't itself an ad hominem. The fact that it isn't a factually accurate statement doesn't make it an instance of ad hominem - it just means that the charge is wrong. And an appropriate response is to show how it is wrong (and not to simply label it as ad hominem itself).
3: Ad hominem is the argument from irrelevancy. If the reasons invoked are relevant to the issue - even if they focus on the person making the argument, then they are not usually an ad hominem attack. If the issues being reasoned about have absolutely nothing to do with the issue, then it can be identified as an ad hominem fallacy.
Examples (which I have stolen from this book because it was easier than trying to produce them myself).
The first couple of examples follow the simple form:
Person 1 is claiming Y.
Person 1 is (or is doing) X.
Therefore, Y is not true.
My opponent suggests that lowering taxes will be a good idea – this is coming from a woman who eats a pint of Ben and Jerry’s each night!
The fact that the woman loves her ice cream, has nothing to do with the lowering of taxes, and therefore, is irrelevant to the argument. (Unless, of course, the tax is one that specifically impacts just ice cream or even more narrowly, just Ben and Jerry's, when it might become relevant.)
Tony wants us to believe that the origin of life was an “accident”. Tony is a godless SOB who has spent more time in jail then in church, so the only information we should consider from him is the best way to make license plates.
Tony may be a godless SOB. Perhaps he did spend more time in the joint than in church. But all this is irrelevant to his argument or truth of his claim as to the origin of life. Sometimes the third part of this formula can simply be inferred. If someone makes a claim and the response is "Did you take your meds this morning" - with the intention of dismissing the claims as being crazy, that is ad hominem (the wiki article has some examples of this referring just to women).
The next couple of examples follow a slightly different form in which motives are presented):
Person 1 is claiming Y.
Person 1 has a vested interest in Y being true.
Therefore, Y is false.
Salesman: This car gets better than average gas mileage, and is one of the most reliable cars according to Consumer Reports.
Customer: I doubt it – you obviously just want to sell me that car.
The fact that the salesmen has a vested interest in selling you the car, does not mean that he is lying. He may be, but this is not something you can conclude solely on his interests. It is reasonable to assume that salespeople sell the products and services they do because they believe in them. Claiming that the church's (or church leaders) perspectives are bad only because they want to create tithe paying adherents is an example of an ad hominem argument.
Of course your minister says he believes in God. He would be unemployed otherwise.
The fact atheist ministers are about as in demand as hookers who, “just want to be friends”, does not mean that ministers believe in God just because they need a job. Likewise, claiming that those individuals who work for the Church or BYU are saying what they are saying just to keep their job, is an example of ad hominem. But the line here can be blurred a bit as sometimes motivation is a valid line of inquiry - particularly if it is raised by the other party.
Obviously, as the bias or conflict of interest becomes more and more relevant to the argument, it becomes less of a fallacy and more as a legitimate question to that motive (its good in these cases to be upfront about personal motivations when possible). If a man is accused of having an affair, of course he is going to deny it. But, the only situation he might be more inclined to deny that an affair happened is if he hadn't really had an affair. And this is why these arguments are often considered ad hominem. The narrowness of the ad hominem charge only applies here in cases where the original premise is based on a more broadly recognized issues - if it is all about an appeal to personal authority or experience, that can swing the ideas away from ad hominem. However, to use this to attack while avoiding other kinds of evidence that is being presented is often ad hominem (unless again, as I note, bias is a part of the original discussion). If the guy accused of having an affair can produce evidence that he wasn't there, and the response is that of course he would deny it, that is ad hominem.
The next two examples show how this works in guilt by association.
Person 1 states that Y is true.
Person 2 also states that Y is true, and person 2 is (or does) X.
Therefore, person 1 must also be (or be doing) X.
Delores is a big supporter for equal pay for equal work. This is the same policy that all those extreme feminist groups support. Extremists like Delores should not be taken seriously – at least politically.
Dismissing Delores as an extreme feminist simply because she supports a policy that many people also support, is ad hominem.
Pol Pot, the Cambodian Maoist revolutionary, was against religion, and he was a very bad
man. Frankie is against religion, therefore, Frankie, too, must be a very bad man.
The fact that Pol Pot and Frankie share one particular view does not mean they are identical in other ways unrelated, specifically, being a very bad man. Pol Pot was not a bad man because he was against religion, he was a bad man for his genocidal actions. Of course, there are exceptions to this - if the shared trait is relevant to the article. If instead of comparing the views on religion, we compared the two people based on a genocidal trait - that is, Polt Pot was a genicidal maniac, and was a very bad man, and Frankie also is a genocidal maniac and is thus must also a very bad man might not be such a stretch. It is more of a publicly accepted premise that being genocidal makes you bad, as opposed to having anti-religious tendencies that make you bad.
The next two show the personal inconsistency form of the fallacy.
Person 1 is claiming that Y is true, but person 1 is acting as if Y is not true.
Therefore, Y must not be true.
Helga: You should not be eating that... it has been scientifically proven that eating fat burgers are no good for your health.
Hugh: You eat fat burgers all the time, so that can’t be true.
It doesn’t matter, to the truth claim of the argument at least, if Helga follows her own advice or not. While it might appear that the reason she does not follow her own advice is because she doesn’t believe it’s true, it could also be that those fat burgers are just too darn irresistible.
Jimmy Swaggart argued strongly against sexual immorality, yet he has had several affairs with prostitutes, therefore, sexual immorality is acceptable.
The fact Jimmy Swaggart likes to play a round of bedroom golf with some local entrepreneurial ladies, is not evidence for sexual immorality in general, only that he is sexually immoral. (The exception to this, of course, is when he argues that his actions were in line with his sexual morality - and then it becomes a very relevant part of the discussion).
Edited by Benjamin McGuire, 29 June 2012 - 07:39 AM.