The Logical Fallacy of Argumentum ad Hominem

Definition:

An argument based on the perceived failings of an adversary rather than on the merits of the case; a logical fallacy that involves a personal attack.

In their book Commitment in Dialogue: Basic Concepts of Interpersonal Reasoning (SUNY Press, 1995), Douglas Walton and Eric Krabbe identify three types of argumentum ad hominem:

1) The personal or abusive ad hominem alleges bad character for veracity, or bad moral character generally.
2) The circumstantial ad hominem alleges a practical inconsistency between the person and his or circumstances.
3) A third type of ad hominem, the bias or 'poisoning the well' variant, alleges that the person has a hidden agenda or something to gain and is therefore not an honest or objective arguer.

See Examples and Observations below. Also see:

Etymology:
From the Latin, "against the man"

Examples and Observations:

"C.P. Snow’s essay provoked a roaring, ad hominem response from the Cambridge critic F. R. Leavis, who called Snow 'intellectually as undistinguished as it is possible to be.'"
(Peter Dizikes, “Our Two Cultures.” The New York Times, March 19, 2009)
The Abusive Ad Hominem
"The abusive ad hominem is not just a case of directing abusive language toward another person. . . . The fallacy is committed when one engages in a personal attack as a means of ignoring, discrediting, or blunting the force of another's argument.

"Although some faulty arguers may call attention to distasteful features of their opponents in order to manipulate the responses of their audience, most abusers apparently believe that such characteristics actually provide good reasons for ignoring or discrediting the arguments of those who have them. Logically, of course, the fact that any of these characteristics might fit an opponent provides no reason to ignore or discredit his or her arguments or criticisms."
(T. E. Damer, Attacking Faulty Reasoning. Wadsworth, 2001)

Ad Hominem Attacks on British Prime Minister Gordon Brown
"Do we live in an age of hatred? Or has the language of political insult simply become more extreme? Tap the words 'I hate Gordon Brown' into Google, and it comes up with 1,490,000 entries. . . .

"Here is one sentence, culled from a recent national newspaper leader . . .: 'They [British voters] know their Premier to be a neurotic, dysfunctional mediocrity; an insecure Stalinist who worships power but cannot take a decision; a moral and political coward who tries to fill the vacuum at the heart of his leadership with blustering rhetoric and adolescent bullying.'

"Lampooned figures in the past . . . were, from time to time, enveloped in crises every bit as damaging as those that confront Mr Brown. But never were they subjected to such woundingly ad hominem attacks."
(M. Linklater, "The Age of Personal Vitriolic Abuse," The Times, May 16, 2008)
Lincoln's Use of Ad Hominem
"A story is told about [Abraham] Lincoln as a young lawyer. In one of his first jury cases, he showed his political shrewdness by an adroit and quite non-malicious use of ad hominem. His opponent was an experienced trial lawyer, who also had most of the fine legal points on his side. The day was warm and Lincoln slumped in his chair as the case went against him. When the orator took off his coat and vest, however, Lincoln sat up with a gleam in his eye. His opponent was wearing one of the new city-slicker shirts of the 1840's, which buttoned up the back.

"Lincoln knew the reactions of frontiersmen, who made up the jury. When his turn came, his plea was brief: 'Gentlemen of the jury, because I have justice on my side, I am sure you will not be influenced by this gentleman's pretended knowledge of the law. Why. he doesn't even know which side of his shirt ought to be in front!'

"Lincoln's ad hominem is said to have won the case."
(Stuart Chase, Guides to Straight Thinking. Harper & Row, 1956)
Legitimate Uses of Ad Hominem Arguments
"[T]here may be cases in which an ad hominem argument is a legitimate rhetorical tool. When the special interests or associations of an individual or group appear to have a direct impact on their position on an issue, it is fair to raise questions about their lack of objectivity on that basis. For example, the organizer of a petition to build a state-supported recycling center may seem reasonably suspect if it is revealed that he owns the land on which the proposed recycling center would be built. While the property owner may be motivated by sincere environmental concerns, the direct relationship between his position and his personal life makes this fair game for a challenge."
(Gary Goshgarian, et al., An Argument Rhetoric and Reader. Addison-Wesley, 2003)