An ad hominem (Latin for “to the man” or “to the person”), short for argumentum 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. Fallacious Ad hominem reasoning is normally categorized as an informal fallacy, more precisely as a genetic fallacy, a subcategory of fallacies of irrelevance. Ad hominem reasoning is not always fallacious, for example, when it relates to the credibility of statements of fact.
Ad hominem arguments are the converse of appeals to authority, and may be used in response to such appeals.
Abusive ad hominem usually involves attacking the traits of an opponent as a means to invalidate their arguments. Equating someone’s character with the soundness of their argument is a logical fallacy. Mere verbal abuse in the absence of an argument, however, is not ad hominem nor any kind of logical fallacy.
Ad hominem circumstantial points out that someone is in circumstances such that they are 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).
The circumstantial fallacy applies only where the source taking a position is only making a logical argument from premises that are generally accepted. Where the source seeks to convince an audience of the truth of a premise by a claim of authority or by personal observation, observation of their circumstances may reduce the evidentiary weight of the claims, sometimes to zero.