Ad hominem

Overview
An ad hominem argument (formally: argumentum ad hominem) or attack is where the arguer criticizes of the source of the opposing argument as proof that the argument itself is incorrect.

The logic generally follows this form:
 * premise: Person A asserts X.
 * premise: Person A is not a valid source.
 * conclusion: X must be false.

It is often presented:
 * as a choice between discredit or disavowal: If A asserts X, then they must not be a valid source.
 * as a statement directly to the source: If you assert X, then you must not be a valid source.

Issuepedia refers to the combination of these two presentations as the "coercive form" of ad hominem: not just attacking an argument by undermining the messenger, but attempting to get the messenger to recant by threatening their social standing. The coercive form is essentially a carrot-and-stick argument on top of ignoring the substance.

Counters
Even the most untrustworthy person imaginable can still make a true statement; the classic literary example is "The Boy Who Cried Wolf".

Examples

 * "What lying commie-traitor blog did you get that from?"

These combine the "choice" and "direct statement" presentation:
 * "You must be an idiot if you think that."
 * "Of course you would say that, being a high-school dropout."

Significance
When used in public discourse, the coercive form of ad hominem is a direct attempt at coercion in the public sphere and consequently runs counter to the ideals of a free, democratic society. A functional democracy requires that its citizens be able to make claims about their desires and needs in regards to each other, and that those considerations be fairly weighed and evaluated (This is exactly the goal that the Constitution -- and the Lockean ideal -- pursue.) Ad hominem attacks are intended to prevent the legitimate consideration of certain claims in the public sphere by discrediting the speaker without due process, or threatening the speaker with same if s/he continues to express certain ideas. This is not fundamentally different in its effects than, say, preventing the consideration of someone's claims by threatening to kill the listener if s/he listens.

Related

 * Ad hominem is a form of reasoning by reputation, in that it presumes destruction of an opposing arguer's reputation also destroys the opposing argument.
 * An ad hominem attack is often part of an argument by ridicule; the two are closely related.
 * Name-calling is an ad hominem technique.

Reference

 * (see Conservapedia) (emphasis added):
 * "Conservatives understand that the basic moral character of a person is always relevant to an argument. Liberals and Atheists are outraged by examination of an individual's character, considering it to be a personal affront, mainly because they are moral relativists. Conservatives understand how important it is that those debating an issue be trustworthy, otherwise a true debate/discussion cannot happen. An immoral person is of course incapable of making a legitimate, intellectual, argument because they come from deceit, like Richard Dawkins."
 * Sadly, it seems to be true that US conservatives believe this.
 * This belief is probably part of the conservative meme's defense mechanism. It is vital that conservatives not listen to anyone not infected by the meme, since only someone infected by it would believe many of the things conservatives believe; anyone else is much more likely to attack those beliefs and therefore threaten the meme's livelihood. (This is a good example of a memetic defense mechanism.)
 * It also makes the more interesting point that, in the legal courtroom, ad hominem attacks are considered acceptable arguments against the credibility of witness testimony. The implication that this is a valid basis for ad hominem attacks on a rational argument is utter nonsense, however.
 * gives an example of Conservapedia's ad hominem philosophy in action.
 * The Nizkor Project: |ad hominem| |ad hominem tu quoque|
 * gives an example of Conservapedia's ad hominem philosophy in action.
 * The Nizkor Project: |ad hominem| |ad hominem tu quoque|
 * The Nizkor Project: |ad hominem| |ad hominem tu quoque|

Other

 * 2008-11-25 The Complete Idiot's Guide to Ad Hominem: an edge case pinpointing the line between ad hominem and legitimate argument