Logical Fallacy: "ad hominem"

The most common fallacy is "ad hominem" or "argumentum ad personam". Someone's opinion, arguments, statements are invalidated because it is that someone stating it.

"As long as you act this way, don't expect anyone to take you seriously."

"I know you are not sincere and are merely trolling."

"No, the only sham is your failed intellect concerning these matters."

"I suspect you simply are spouting things you know nothing about."

"So you are still in denial"

"You don't have a single argument in that post. Nothing but facile, unargued, and incorrect assertions."

"Is that all you can say? More lies and innuendo about my "emoting"?"

"...as always you resort to lying"

"...once again shows our inability to distinguish between..."

"S/he is a professional disrupter."

In common language, any personal attack, regardless of whether it is part of an argument, is often referred to as ad hominem.

Personal attacks can be ad hominem fallacy:

"You can't believe Jack when he says there is a God because he doesn't even have a job."

"Charles Manson wrote this song, so it must be unlistenable".

"Candidate Jane Jones's proposal X is ridiculous. She was caught cheating on her taxes in 2003."

"What John said should not be believed because he has red hair"

But it doesn't need to be a personal attack. Circumstancial ad hominem fallacy, is when one's arguments are invalidated because the person claiming so is disposed to take that position.

"Tobacco company representatives should not be believed when they say smoking doesn't seriously affect your health, because they're just defending their own multi-million-dollar financial interests."

"He's physically addicted to nicotine. Of course he defends smoking!"

"What do you know about politics? You're too young to vote!"

"Well, he would [say that], wouldn't he?"

Ad hominem tu quoque: "you too..."

"So what if I used an ad hominem? You insulted me earlier."

"How can you tell me not to experiment with drugs when you did the same thing as a teenager?"

"Based on the arguments I have presented, it is evident that it is morally wrong to use animals for food or clothing."

"But you are wearing a leather jacket! How can you say that using animals for food and clothing is wrong!"

"I find it amusing and completely predictable that the responses here would be so vitriolic, and really nothing more than personal attacks and more empty assertions."

A reductio ad Hitlerum argument is a special case of an ad hominem argument. If Hitler said it, did it or supported it, it must be wrong, because Hitler was a bad person.

(The same way: reductio ad Nazium)

Guilt by association can sometimes also be a type of ad hominem fallacy, if the argument attacks a person because of the similarity between the views of someone making an argument and other proponents of the argument.

This form of the argument is as follows:

Person A makes claim P.
Group B also make claim P.
Therefore, person A is a member of group B.

Example:

"You say the gap between the rich and poor is unacceptable, but communists also say this, therefore you are a communist"

This fallacy can also take another form:

Person A makes claim P.
Group B make claims P and Q
Therefore, Person A makes claim Q.

Examples:

"You say the gap between the rich and poor is unacceptable, but communists also say this, and they believe in revolution. Thus, you believe in revolution."

A similar tactic may be employed to encourage someone to renounce an opinion, or force them to choose between renouncing an opinion or admitting membership in a group. 

For example:

"You say the gap between the rich and poor is unacceptable. You don't really mean that, do you? Communists say the same thing. You're not a communist, are you?"

Guilt by association may be combined with ad hominem abusive. For example:

"You say the gap between the rich and poor is unacceptable, but communists also say this, and therefore you are a communist. Communists are unlikeable, and therefore everything they say is false, and therefore everything you say is false."

-- Wikipedia; Ad hominem
http://en.wikipedia.org/wiki/Ad_hominem