Ad Hominem Fallacy: what it is and isn’t

I’ve found this principle is very commonly misunderstood by folks, especially by supposed internet logic experts.
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A personal (or ad hominem) attack can be mean.  But if someone you describe as mean  is actually cruel and insulting it may be fully appropriate to describe that person as mean.  To call someone mean is itself not a logical fallacy.  It’s a description of behavior.  Debaters can, after all, describe behaviors.
But unless you’re connecting a personal quality (ugly, skinny, fat, attractive) to an argument (therefore you can’t understand understand the nature of God) you can’t commit an ad hominem fallacy.  The fallacy is committed when an argument doesn’t follow.  For example, see below:
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People who are wrong are not wrong because they’re ugly.  The claim — you’re wrong — doesn’t follow from the claim that you’re ugly.  The person could be ugly (missing 1 eye, have no teeth, have scars all over the face, etc.) — and it would be mean to say so — but it isn’t a logical fallacy to describe someone as wrong and ugly.
If you feel someone is wrong and ugly, you have a right to explain yourself & tell the person they are wrong and ugly.  Might be impolite, but saying someone is wrong and ugly isn’t itself a logical fallacy.
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Often the very people who shriek about others committing personal attack fallacies are themselves (ironically) doing the same.  Worth 3 minutes:

William Lane Craig explains what an ad hominem fallacy argument is vs. what a characterization or description is (even if that descriptions is considered impolite by some):


A few pages showing examples of ad hominem logical fallacy:  here, here, and here.  These pages also explain what is not an ad hominem fallacy.


In order for an argument to be ad hominem, it has to actually attack its opponent. Second, not all ad hominem attacks are ad hominem fallacies.
In reality, the ad hominem fallacy is unrelated to sarcasm or personal abuse. Argumentum ad hominem is the logical fallacy of attempting to undermine a speaker’s argument by attacking the speaker instead of addressing the argument.
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The mere presence of a personal attack does not indicate use of the ad hominem fallacy: the attack must be used for the purpose of undermining the argument, or otherwise the logical fallacy isn’t there. It is not a logical fallacy to attack someone; the fallacy comes from assuming that a personal attack is also necessarily an attack on that person’s arguments.
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Therefore, if you can’t demonstrate that your opponent is trying to counter your argument by attacking you, you can’t demonstrate that he is resorting to ad hominem. If your opponent’s sarcasm is not an attempt to counter your argument, but merely an attempt to insult you (or amuse the bystanders), then it is not part of an ad hominem argument.
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Actual instances of argumentum ad hominem are relatively rare. Ironically, the fallacy is most often committed by those who accuse their opponents of ad hominem, since they try to dismiss the opposition not by engaging with their arguments, but by claiming that they resort to personal attacks.
Those who are quick to squeal “ad hominem” are often guilty of several other logical fallacies, including one of the worst of all: the fallacious belief that introducing an impressive-sounding Latin term somehow gives one the decisive edge in an argument.
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Pointing out a problem in an opponent’s argument, asking for their sources, criticizing their sources, etc. does not count as ad hominem. You cannot accuse someone of an ad hominem fallacy just because they disagreed with you.
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Asking someone to back up their claims with facts and logic is itself neither rude nor offensive. If you’re polite in your request this isn’t a logical fallacy. If you’re rude in this request this still isn’t a logical fallacy, though it is rude. So please stop being so thin-skinned.
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Further, you do not have the right to write/say anything without being subject to criticism or ridicule. Yes, you are entitled to an opinion. And, yes, we should all be tolerant of other people’s beliefs. But when you are making a factual claim, you are neither expressing an opinion nor belief. And you should be held accountable for the accuracy of that claim.
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Imagine, for example, how ridiculous it would be if someone who didn’t accept gravity became upset when people ridiculed their absurd views. Even so, when you make factually incorrect statements about vaccines, genetics, economics, etc. you aren’t expressing an opinion, you’re just wrong. And no one should be tolerant of your nonsense.
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In the case of my sarcastic title — Stop accusing me of ad hominem fallacies, you stupid idiots!  —  I got your attention by using hostile language, then proceeded to actually explain the logic of how these fallacies work; therefore, I did not commit an ad hominem fallacy (i.e., my insult was just an insult, not an argument).