An argumentum ad hominem (Latin: "argument against the man"), is a logical fallacy consisting of denigrating one's opponent or otherwise introducing irrelevant premises about one's opponent, instead of dealing with the flaws in the form and function of the opponent's argument. Note that the statement made in an ad hominem argument does not have to be false for this type of fallacy to have been committed; it just has to be irrelevant to the topic of the debate.
Classically, this fallacy is aimed at casting doubt on what a person is saying because of (imputed) flaws in the person's character, or circumstances peculiar to the person, that are unrelated to the matter at hand. Its usual structure is:
- A makes claim X.
- A has a flaw in his character, or else his circumstances mean that X somehow serves a special interest of A, etc.
- Therefore X is false.
Why Ad Hominem is Objectionable
The ad hominem is objectionable and usually fallacious practice because:
Forms of Ad Hominem
The ad hominem fallacy can take several forms, the most common of which are personal abuse and the introduction of an irrelevant circumstance. Some debaters use the ad hominem attack pre-emptively, a technique known as poisoning the well.
In a special form of ad hominem that is closely akin to special pleading, a debater attempts to deflect criticism of himself by asserting that his opponent has the identical character flaw. This form is called tu quoque (Latin, "thou too"). The proverbial expression of this fallacy is "It takes one to know one" or "Let the pot call the kettle black."
Example: Atheist scientist A might present arguments that there is no God. Opponent C might respond by stating that A is fat and should be able to use science to control his weight. This would be an example of the tu quoque fallacy.
- 'Richard Wagner was an opera composer much admired by the Nazis'
might be considered an ad hominem attack on Wagner's music. It is true, and in some contexts might be a relevant comment. But if someone says it in order to suggest that Wagner was a bad composer, then it's an ad hominem attack, because whether some bad people liked his music is not a relevant comment on the quality of his music. Additionally, the Nazis' misdeeds had nothing to do with their ability to critique opera. However, even if a tone-deaf person, who is not a good judge of music, said he liked Wagner, it would still be an ad hominem attack to conclude Wagner is bad.
Examples of non-ad hominem arguments
- The ad hominem attack is usually fallacious, and becomes reasonable only when the consideration being advanced is directly relevant to whether the other person is generally truthful or reliable, or whether he is telling the truth in this instance, or whether he is engaged in a logical fallacy of his own. The usual case occurs when the other person is engaging in special pleading. Exposure of special pleading is a valid counterargument--so long as the accusation itself is valid. Technically, this is not an example of ad hominem, because it is an attack on the specific argument and/or the evidence advanced to support it.
- In the modern legal system, most judges allow the prosecutor to attack the credibility of those who testify on the stand; such witness impeachment can call into question the character, impartiality, honesty, or competence of a witness. Again, however, this is not technically an ad hominem argument.
Other meanings of ad hominem
American Heritage Dictionary says:
- The expression now also has a looser use in referring to any personal attack, whether or not it is part of an argument, as in "It isn't in the best interests of the nation for the press to attack him in this personal, ad hominem way." 
- The Nizkor Project, Fallacy: Ad Hominem (retrieved April 9, 2007)
- Lander University, Department of Philosophy, Philosophy 103, Introduction to Logic: Argumentum ad Hominem. (Retrieved April 9, 2007)
- Definition of ad hominem from The American Heritage® Dictionary of the English Language, 4th ed., Houghton Mifflin Company, 2006 (retrieved April 9, 2007)
- S. Morris Engel, With Good Reason: An Introduction to Informal Fallacies, 5th ed., St. Martin's Press, 1994, pp. 198-206. Retrieved April 9, 2007, from The Fallacy Files
- Engel, op. cit., pp. 206-209. Retrieved April 9, 2007, from The Fallacy Files
- Nigel Warburton, Thinking from A to Z, 2nd. ed., "Politician's Answer", pp. 103-104. Retrieved April 9, 2007, from The Fallacy Files