onegoodmove.org/fallacy/attack.htm

Attacking the Person
(argumentum ad hominem)

Definition:

The person presenting an argument is attacked instead of the argument itself. This takes many forms. For example, the person's character, nationality or religion may be attacked. Alternatively, it may be pointed out that a person stands to gain from a favourable outcome. Or, finally, a person may be attacked by association, or by the company he keeps.

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.
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.
3. ad hominem (tu quoque): this form of attack on the person notes that a person does not practise what he
preaches.

Examples:

1. You may argue that God doesn't exist, but you are just following a fad. (ad hominem abusive)
2. We should discount what Premier Klein says about taxation because he won't be hurt by the increase. (ad hominem circumstantial)
3. We should disregard Share B.C.'s argument because they are being funded by the logging industry. (ad hominem circumstantial)
4. You say I shouldn't drink, but you haven't been sober for more than a year. (ad hominem tu quoque)

Proof:

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.