Introduction
The Ad Hominem or "Against the Man" fallacy, as it is sometimes known, seeks to dismiss someone's argument or credibility on the basis of a personal attack against him instead of addressing the actual argument advanced. The propagator of this fallacy is really seeking to exempt himself from the burden of responding to the points raised against his position by heaping contempt or ridicule on his opponent. A variant form of this fallacy is sometimes known as tu quoque or "you're another". The perpetrator of this kind of ad hominem attack will appeal to his opponent's guilt in order to deflect the fact that he himself is guilty of a transgression. Another kind of ad hominem is known as "poisoning the well" where the speaker will attempt to cast doubt on the viability of an argument by directing the audience to the motives of his opponent.
Definition
The fallacy which utilizes a negative trait or characteristic of an opponent in order to mitigate or cancel the argument advanced against a proposition.
Illustrations
Argument #1: The Papacy cannot be authentic. Look at all of the corrupt Popes throughout history!
Argument #2: How can the Catholic Church insist on clerical celibacy when many of her own priests cannot seem to abide by her teachings?
Diagnosis
Ad Hominem attacks in apologetics are rather common, but they are also quite ineffective as most people, obviously, do not accept the simple form of this fallacious argumentation. However, there are various forms of this argument which are not as easy to dismiss quite that easily. There is something just about someone telling another person to remove the plank from his own eye before he attempts to remove it from his brother's. On the other hand, you cannot argue against a proposition simply by demonstrating that the other person is as guilty of the charge as you are. After all, the cop is still going to write you up for speeding even though the guy ahead of you was going 10 mph faster than you were.
Antidote
The most effective way of exposing this argument is not to use it in response to someone who does! As a purely tactical matter, however, it is always good not to be a hypocrite even when you might be right!
John Pacheco
The Catholic Legate
June 7, 2002