The ad hominem is a logical fallacy 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. Typically, this fallacy involves two steps. First, an attack against the character of person making the claim, her circumstances, or her actions is made (or the character, circumstances, or actions of the person reporting the claim). Second, this attack is taken to be evidence against the claim or argument the person in question is making (or presenting). This type of "argument" has the following form:
Person A makes claim X.
Person B makes an attack on person A.
Therefore A's claim is false.
The reason why an Ad Hominem of any kind is a fallacy is that the character, circumstances, or actions of a person do not have a bearing on the truth or falsity of the claim being made in most cases or the quality of the argument being made.
Additionally, the ad hominem is used when the person making it has no ability to refute the argument being made.
