Ad hominem (Latin for "to the person"), short for argumentum ad hominem, typically refers to a fallacious argumentative strategy whereby genuine discussion of the topic at hand is avoided by instead attacking the character, motive, or other attribute of the person making the argument, or persons associated with the argument, rather than attacking the substance of the argument itself. The terms ad mulierem and ad feminam have been used specifically when the person receiving the criticism is female.
However, the term's original meaning was an argument "calculated to appeal to the person addressed more than to impartial reason". Under this meaning the term does not refer to a logical fallacy. This meaning survives in philosophical usage, as explained by the philosopher R. Scott Bakker: "Classical Pyrrhonians argued ad hominem, not in the sense of the logical fallacy of that name, but in the sense that their dialectical strategy necessitates the exclusive utilization of the beliefs, convictions, and assumptions of their interlocutors. In other words, they construct their arguments on the basis of what other people hold to be true."
Ad hominem tu quoque (literally: "You also") 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 invalid because it does not disprove the premise; 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 the 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.
Circumstantial ad hominem points out that someone is in circumstances such that they are disposed to take a particular position. It 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 invalid; this overlaps with the genetic fallacy (an argument that a claim is incorrect due to its source).
The circumstantial fallacy does not apply where the source is taking a position by using a logical argument based solely on 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.
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:
An example of this fallacy could be "My opponent for office just received an endorsement from the Puppy Haters Association. Is that the sort of person you would want to vote for?"
In philosophical usage ad hominem may refer to a dialectical strategy involving the exclusive utilization of the beliefs, convictions, and assumptions of those holding the position being argued against, i.e., arguments constructed on the basis of what other people hold to be true.
When a statement is challenged by making an ad hominem attack on its author, it is important to draw a distinction between whether the statement in question was an argument or a statement of fact (testimony). In the latter case the issues of the credibility of the person making the statement may be crucial.
An ad hominem fallacy occurs when one attacks the character of an interlocutor in an attempt to refute their argument. Insulting someone is not necessarily an instance of an ad hominem fallacy. For example, if one supplies sufficient reasons to reject an interlocutor's argument and adds a slight character attack at the end, this character attack is not necessarily fallacious. Whether it is fallacious depends on whether or not the insult is used as a reason against the interlocutor's argument. An ad hominem occurs when an attack on the interlocutor's character functions as a response to an interlocutor's argument/claim.
Canadian academic and author Doug Walton has argued that ad hominem reasoning is not always fallacious, and that in some instances, questions of personal conduct, character, motives, etc., are legitimate and relevant to the issue, as when it directly involves hypocrisy, or actions contradicting the subject's words.
The philosopher Charles Taylor has argued that ad hominem reasoning (discussing facts about the speaker or author relative to the value of his statements) is essential to understanding certain moral issues due to the connection between individual persons and morality (or moral claims), and contrasts this sort of reasoning with the apodictic reasoning (involving facts beyond dispute or clearly established) of philosophical naturalism.