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Definition: also known as argumentum ad hominem (Latin: "argument to the person", "argument against the man") consists of replying to an argument by attacking or appealing to the person making the argument, rather than by addressing the substance of the argument. It is most commonly used to refer specifically to the ad hominem abusive, or argumentum ad personam, which consists of criticizing or personally attacking an argument's proponent in an attempt to discredit that argument.
Other common subtypes of the ad hominem include the ad hominem circumstantial, or ad hominem circumstantiae, an attack which is directed at the circumstances or situation of the arguer; and the ad hominem tu quoque, which objects to an argument by characterizing the arguer as being guilty of the same thing that he is arguing against.

An ad hominem fallacy consists of asserting that someone's argument is wrong and/or he is wrong to argue at all purely because of something discreditable/not-authoritative about the person or those persons cited by him rather than addressing the soundness of 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 his 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. However, this is not how the meaning of the term is typically introduced in modern logic and rhetoric textbooks, and logicians and rhetoricians are in agreement that this use is incorrect.


pedantry_02.jpgExamples:
A prosecutor asks the judge to not admit the testimony of a burglar because burglars are not trustworthy.

Francis Bacon's philosophy should be dismissed since Bacon was removed from his chancellorship for dishonesty.
  • Prof. Smith says to Prof. White, "You are much too hard on your students," and Prof. White replies, "But certainly you are not the one to say so. Just last week I heard several of your students complaining."
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* I can't see that we should listen to Governor Smith's proposal to increase the sales tax on automobiles. He has spent the last twenty years in state government and is hardly an unbiased source.


http://en.wikipedia.org/wiki/Ad_hominem
http://philosophy.lander.edu/logic/person.html