"An Ad Hominem is a general category of fallacies in which a claim or argument is rejected on the basis of some irrelevant fact about the author of or the person presenting the claim or argument."
"Ad hominem is Latin for "to the man." The ad hominem fallacy occurs when one asserts that somebody's claim is wrong because of something about the person making the claim. The ad hominem fallacy is often confused with the legitimate provision of evidence that a person is not to be trusted. Calling into question the reliability of a witness is relevant when the issue is whether to trust the witness. It is irrelevant, however, to call into question the reliability or morality or anything else about a person when the issue is whether that person's reasons for making a claim are good enough reasons to support the claim."
"It is important to note that the label “ad hominem” is ambiguous, and that not every kind of ad hominem argument is fallacious. In one sense, an ad hominem argument is an argument in which you offer premises that you the arguer don’t accept, but which you know the listener does accept, in order to show that his position is incoherent (as in, for example, the Euthyphro dilemma). There is nothing wrong with this type of argument ad hominem."
"An ad hominem attack is not quite as weak as mere name-calling. It might actually carry some weight. For example, if a senator wrote an article saying senators' salaries should be increased, one could respond:
"Of course he would say that. He's a senator.
"This wouldn't refute the author's argument, but it may at least be relevant to the case."
"An Ad Hominem is a general category of fallacies in which a claim or argument is rejected on the basis of some irrelevant fact about the author of or the person presenting the claim or argument."
http://www.skepdic.com/adhominem.html
"Ad hominem is Latin for "to the man." The ad hominem fallacy occurs when one asserts that somebody's claim is wrong because of something about the person making the claim. The ad hominem fallacy is often confused with the legitimate provision of evidence that a person is not to be trusted. Calling into question the reliability of a witness is relevant when the issue is whether to trust the witness. It is irrelevant, however, to call into question the reliability or morality or anything else about a person when the issue is whether that person's reasons for making a claim are good enough reasons to support the claim."
http://www.logicalfallacies.info/relevance/ad-hominem/
"It is important to note that the label “ad hominem” is ambiguous, and that not every kind of ad hominem argument is fallacious. In one sense, an ad hominem argument is an argument in which you offer premises that you the arguer don’t accept, but which you know the listener does accept, in order to show that his position is incoherent (as in, for example, the Euthyphro dilemma). There is nothing wrong with this type of argument ad hominem."
http://www.paulgraham.com/disagree.html
"An ad hominem attack is not quite as weak as mere name-calling. It might actually carry some weight. For example, if a senator wrote an article saying senators' salaries should be increased, one could respond:
"Of course he would say that. He's a senator.
"This wouldn't refute the author's argument, but it may at least be relevant to the case."