I have had quite a few interactions with people of various philosophical, scientific, and theological positions. Knowing the formal and informal fallacies are pertinent to intelligent, reasonable debate and dialogue. In my experience, I’ve found that very few people actually know what an ad hominem fallacy is.
An ad hominem fallacy is one that argues to the man. The conclusion of the argument is rejected due to an “attack” made against the person. For instance, if I argue that Preston is an idiot and therefore his argument is wrong then that would be fallacious.
Preston: I’m better at climbing trees than you are because I only use sturdy branches and take my time whereas you just use any branch and just fly up as fast as you can.
Sandy: No, you’re not better. You’re so mean and rude. You’re such an idiot. [Because] you’re a mean, rude idiot you’re wrong and I’m actually better than you.
Most interactions aren’t as explicitly laid out as this but I did it for the sake of pointing out the linguistic constructs. Now, this wouldn’t be fallacious if Sandy called him a mean, rude idiot but rejected Preston’s argument because of other reasons (e.g. she actually does check for strong branches and uses a safety harness).
Just because someone calls someone stupid, an idiot, ignorant, a fool, doesn’t mean an ad hominem has been used. I often find that people take so much offense to having something they said be labeled as ignorant. Ignorant just means lack of knowledge. So, what I’ve tried to do is just say “lacks knowledge” in order to avoid false ad hom claims. Here’s a great example from William Lane Craig.
So, please be careful how you throw around ad hominem claims and accusations. Only ignorant people misuse the ad hominem fallacy.