Many trolls, demitrolls, and flamers believe that "You are making an ad hominem argument" means "Any statement in which you say something unpleasant about me is automatically invalid." They like that idea. They also like using Latin because it sounds impressive.

Unfortunately, they've got it wrong. The argument ad hominem, as Wikipedia will tell you, "consists of replying to an argument or factual claim by attacking or appealing to a characteristic or belief of the source making the argument or claim, rather than by addressing the substance of the argument or producing evidence against the claim." That is: it's arguing about the man, not his proposition. Thus, if Ferdy makes an argument, and Bonzo replies to it by calling him a wan-fukkit ignorant fule, it's not an ad hominem; it's just rude. However, if Ferdy makes a case for greater public funding of education, and Bonzo replies that he's a dirty hippie communist who's using education as an excuse to raise taxes, Bonzo has fallen into the ad hominem fallacy.

Note that an argument ad hominem isn't always inappropriate. Let's say Ferdy is making a sales pitch for his scientifically advanced new engine that runs on water. Lulu points out that Ferdy has a long history of running scams and cons that have twice landed him in prison -- most recently, for swindling investors with a different engine that was also supposed to run on water. That's an ad hominem argument, but it's not a fallacy, because Ferdy's long history as a con artist is pertinent to the discussion.

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