It’s logical fallacy time, here in Skeptictionary corner.
An argumentum ad hominem might look like this:
Is a convicted drunk driver really qualified to comment on a matter of such importance?
Since when did we start listening to suggestions from communists on how to improve our healthcare system?
You’re wrong because you’re an idiot.
It’s an attempt to discredit an argument, by attacking the person (the hominem in “ad hominem“) who is making it, rather than the logic of the argument itself. It’s fallacious because it’s irrelevant; it doesn’t matter who is claiming something, the facts are what’s important in deciding what’s true.
For instance, I hope to get around to discussing the authenticity of the Bible on this blog before long. If you want to defend your Holy Book, it’s not going to do you any favours to rant about how a godless heathen like me can’t be trusted, or to bring up those baby seals I hunted to death that one time, without actually addressing my points. The historical evidence stands as it stands, regardless of my personal history.
And, fair’s fair; if someone brings up a point about Biblical prophecy or some such which they think supports the Bible’s divine origins, it doesn’t help me to simply declare that nobody so blinded by ideology could possibly have anything useful to say. An argument should be judged on its own merit, and irrelevant ad hominem arguments don’t advance anyone’s cause.
They tend to come in three (or maybe four) flavours:
An abusive ad hominem is a straight-forward insult or an aspersion cast against your opponent, in an illogical attempt to damage the logic of their own standing. (“Sure, he sounds like he knows what he’s talking about, but can you ever really trust a convicted jay-walker?”)
A circumstantial ad hominem is something I’ve seen described in two different ways. It can be an attempt to undermine someone’s credibility, and thus the credibility of their argument, by pointing out an apparent source of bias. (“Of course he says women should get paid more; he needs the women’s vote to get elected.”) It may also be a way of encouraging acceptance of a claim based on your audience’s personal biases, rather than on the facts that support or refute it. (Asking “What would Jesus do?” only has a chance of being persuasive to a certain demographic of the population.)
And a tu quoque (“you too!”) argument is an attempt to undermine the reasoning of an argument by accusing its opponent of acting inconsistently with it. (“If Al Gore’s so worried about the environment, why does he keep flying around on his private jet to tell us all about it?”)
Making accusations about a speaker’s character isn’t always irrelevant and inappropriate, though. Climate change either is or isn’t a problem we should be doing something about, regardless of Al Gore’s personal carbon footprint – but if you wanted to make the case that Al Gore is a hypocrite, then this would be a relevant point to bring up, even if it doesn’t make him wrong about our impact on the environment. Similarly, gender equality seems to be a fine thing, whatever any politicians are saying – but a claim that can’t be objectively verified, which relies more on the arguer’s personal opinions or authority (“You should vote me, because I’ll do something about this”) can reasonably be called into question by bringing up their personal biases.
And, so long as you’re not confusing it with logic, sometimes hurling abuse at people is just fun.