Translated from Latin to English, "Ad Hominem" means "against the man" or "against the person."
An Ad Hominem is a category of fallacies in which an argument is rejected on the basis of some irrelevant fact (character, nationality, motive, etc.) about the person presenting the argument rather than on the substance of the argument. Typically, this fallacy involves two steps. First, an attack against the character of person making the argument is made. Second, this attack is taken to be evidence against the argument the person is making. 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.
There are three major forms of Attacking the Person:
(1) ad hominem (abusive): instead of attacking an assertion, the argument attacks the person who made the assertion. For example:
"You claim that animal rights activists can be moral to towards humans -- yet I happen to know that you divorced your wife."
This is a fallacy because the truth of an assertion doesn't depend on the virtues of the person asserting it. A less blatant argumentum ad hominem is to reject a proposition based on the fact that it was also asserted by some other easily criticized person. For example:
"Therefore I should stop eating meat? Hitler would have agreed with you."
(2) ad hominem (circumstantial): instead of attacking an assertion the author points to the relationship between the person making the assertion and the person's circumstances. For example:
"Therefore it is perfectly acceptable to kill animals for food. I hope you won't argue otherwise, given that you're quite happy to wear leather shoes."
This is known as circumstantial argumentum ad hominem. The fallacy can also be used as an excuse to reject a particular conclusion. For example:
"Of course you'd argue that meat is unhealthy. You want restaurants to serve more vegetables."
This particular form of Argumentum ad Hominem, when you allege that someone is rationalizing a conclusion for selfish reasons, is also known as "poisoning the well."
It's not always invalid to refer to the circumstances of an individual who is making a claim. If someone is a known perjurer or liar, that fact will reduce their credibility as a witness. It won't, however, prove that their testimony is false in this case. It also won't alter the soundness of any logical arguments they may make.
(3) ad hominem (tu quoque): this form of attack on the person notes that a person does not practice what he preaches.
link: Ad Hominem Tu Quoque
The reason why an Ad Hominem (of any kind) is a fallacy is that the character, circumstances, or actions of a person do not (in most cases) have a bearing on the truth or falsity of the claim being made (or the quality of the argument being made).
"How can you believe the theory of relativity -- Einstein couldn't comb his hair?"
Bill: "I believe that abortion is morally wrong."
Dave: "Of course you would say that, you're a priest."
Bill: "What about the arguments I gave to support my position?"
Dave: "Those don't count. Like I said, you're a priest, so you have to say that abortion is wrong. Further, you are just a lackey to the Pope, so I can't believe what you say."
You may argue that God doesn't
exist, but you are just following a fad. (ad hominem abusive)
We should discount what Premier Klein says about taxation because he won't be hurt by the increase. (ad hominem circumstantial)
We should disregard Share B.C.'s argument because they are being funded by the logging industry. (ad hominem circumstantial)
You say I shouldn't drink, but you haven't been sober for more than a year. (ad hominem tu quoque)
You don't have a life. You're following a fad. You are already a vegetarian. You don't work for an animal abuser.
Identify the attack and show that the character or circumstances of the
person has nothing to do with the truth or falsity of the proposition being