If distorting or distracting from an argument isn't beneficial, the next fallacy often employed is the Ad Hominem, which means “against the man”. It's also known as a personal attack. In an ad hominem argument, the credibility, reputation, or motives of the person is maligned.
It doesn't matter if the attack on the person is legitimate or not if the argument he is making is not addressed.
Mr Johnson: The data suggest that the globe has been in a cooling trend for the last decade; global warming is not what we are observing.
Mrs. Adams: Mr. Johnson can't be trusted to provide an unbiased analysis, he once worked at a gas station, which everyone knows is owned by the oil companies.
In this case, the argument is not addressed, the motives are. Either the data show cooling or they do not, the man's former employer does not change anything.
On the other hand, questioning someone's honesty is not necessarily an ad hominem attack.
Police Officer: Mr. Gates, could you tell me where you were yesterday at the time of the murder of John Modesto?
Mr. Gates: I was fishing on Lake Minnetonka.
Jane Modesto: He's lying officer, he can't be trusted because if he admitted to killing my husband, he'd go away for life.
Whether Mr. Gates was fishing or off murdering John Modesto, Jane's statement is not an ad hominem. Her statement merely shows that evidence independent of Mr. Gates must be considered before he can be believed.
Like the previous two fallacies, Straw Man and Red Herring, the Ad Hominem is often used in politics.
“His opinion on gay marriage is irrelevant, he's an Evangelical Christian”
“He doesn't understand abortion, he's never been pregnant”
“She doesn't understand the need for public transportation, she grew up on a rural farm.”
“It's easy for you to oppose Universal Health Care, you have insurance through your job.”
The worst kind of ad hominem attacks are ones that are solely intended to scare or intimidate people into agreement. They also attempt to completely discredit an opponent without considering their arguments.
“The only reason to oppose gay marriage is if you are a hateful bigot”
“The only reason to oppose affirmative action is if you are a racist”
“Abortion is a women's rights issue, if you oppose abortion, it's because you are a misogynist”
“Those who oppose Universal Health Care want people without insurance to die”
UPDATE: Fellow blogger Matthew Flannagan has been posting on logic and fallacies on his blog, and featured a great post about Ad Hominem. (HT: Apologetics 315 via Twitter)
Next Lesson: Tu Quoque
