Ad hominem

An ad hominem (Latin: "to the man"), also known as argumentum ad hominem, is an attempt to link the validity of a premise to a characteristic or belief of the person advocating the premise. The ad hominem is a classic logical fallacy, but it is not always fallacious. For in some instances, questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue.

Ad hominem abuse
Ad hominem abuse (also called personal abuse or personal attacks) usually involves insulting or belittling one's opponent in order to invalidate his or her argument, but can also involve pointing out factual but ostensible character flaws or actions which are irrelevant to the opponent's argument. This tactic is logically fallacious because insults and even true negative facts about the opponent's personal character have nothing to do with the logical merits of the opponent's arguments or assertions.

Examples:
 * "You can't believe Jack when he says the proposed policy would help the economy. He doesn't even have a job."
 * "Candidate Jane's proposal about zoning is ridiculous. She was caught cheating on her taxes in 2003."

Ad hominem circumstantial
Ad hominem circumstantial points out that someone is in circumstances such that he is disposed to take a particular position. Ad hominem circumstantial constitutes an attack on the bias of a source. This is fallacious because a disposition to make a certain argument does not make the argument false; this overlaps with the genetic fallacy (an argument that a claim is incorrect due to its source).

Where the source taking a position seeks to convince us by a claim of authority, or personal observation, observation of their circumstances may reduce the evidentiary weight of the claims, sometimes to zero.

Examples:

Mandy Rice-Davies's famous testimony during the Profumo Affair, "Well, he would [say that], wouldn't he?", is an example of a valid circumstantial argument. Her point was that since a man in a prominent position, accused of an affair with a callgirl, would deny the claim whether it was true or false, his denial, in itself, carries little evidential weight against the claim of an affair. Note, however, that this argument is valid only insofar as it devalues the denial; it does not bolster the original claim. To construe evidentiary invalidation of the denial as evidentiary validation of the original claim is fallacious (on several different bases, including that of argumentum ad hominem); however likely the man in question would be to deny an affair that did in fact happen, he could only be more likely to deny an affair that never did.

Ad hominem tu quoque
Ad hominem tu quoque (lit: "You too!") refers to a claim that the source making the argument has spoken or acted in a way inconsistent with the argument. In particular, if Source A criticizes the actions of Source B, a tu quoque response is that Source A has acted in the same way. This argument is fallacious because it does not disprove the argument; if the premise is true then Source A may be a hypocrite, but this does not make the statement less credible from a logical perspective. Indeed, Source A may be in a position to provide personal testimony to support the argument.

For example, a father may tell his son not to start smoking as he will regret it when he is older, and the son may point out that his father is or was a smoker. This does not alter the fact that his son may regret smoking when he is older, and the fact his father was a smoker means he can talk from a position of experience.

Guilt by association
Guilt by association can sometimes also be a type of ad hominem fallacy, if the argument attacks a source because of the similarity between the views of someone making an argument and other proponents of the argument.

This form of the argument is as follows:
 * Source A makes claim B.
 * Group C also makes claim B.
 * Therefore, source A is a member of group C.

Inverse ad hominem
An inverse ad hominem argument praises a source in order to add support for that source's argument or claim. A fallacious inverse ad hominem argument may go something like this:


 * "That man was smartly-dressed and charming, so I'll accept his argument that I should vote for him."

As with regular ad hominem arguments, not all cases of inverse ad hominem are fallacious. Consider the following:


 * "Elizabeth has never told a lie in her entire life, and she says she saw him take the bag. She must be telling the truth."

Here the arguer is not suggesting we accept Elizabeth's argument, but her testimony. Her being an honest person is relevant to the truth of the conclusion (that he took the bag), just as her having bad eyesight (a regular case of ad hominem) would give reason not to believe her. However, the last part of the argument is false even if the premise is true, since having never told a lie before does not absolutely guarantee that she isn't now.

Appeal to authority is a type of inverse ad hominem argument.

Common misconceptions
Gratuitous verbal abuse or "name-calling" itself is not an ad hominem or a logical fallacy.

This is not to be confused with a true fallacy, which would be "X is idiotically ignorant [of politics], so why should we listen to him now?"