"Ad Hominem" means "against the man" or "against the person." The person presenting an argument is attacked instead of the argument itself.
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, his circumstances, or his 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:
Person A presents an argument.
Person B attacks person A's character.
Therefore person A's argument is invalid.
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."
There are three major forms of Ad Hominem fallacies:
- ad hominem abusive:
Instead of attacking an assertion, the argument attacks the person who made the assertion.
- 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.
- ad hominem tu quoque:
This form of attack on the person notes that a person does not practise what he preaches.
(i) You may argue that God doesn't exist, but you are just following a fad.
(ad hominem abusive)
(ii) We should discount what John Kerrry says about taxation because he won't be hurt by the increase.
(ad hominem circumstantial)
(iii) You say I shouldn't drink, but you haven't been sober for more than a year.
(ad hominem tu quoque)
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 defended.
Barker: 166, Cedarblom and Paulsen: 155, Copi and Cohen: 97, Davis: 80