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Ad Hominem

Description of Ad Hominem [from this site]
Translated from Latin to English, “Ad Hominem” means “against the man” or “against the person.”

An 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. 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:

1 Person A makes claim X.
2 Person B makes an attack on person A.
3 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 (in most cases) have a bearing on the truth or falsity of the claim being made (or the quality of the argument being made).

Example of Ad Hominem

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.”