The argumentum ad hominem (orad hominem for short) is a fallacy that consists in attacking a person instead of his argument or belief. It is, perhaps, the most common fallacy committed on the Internet today.
Here are three examples of the argumentum ad hominem:
1. Mr. Jones believes that Jesus Christ was a real historical figure; he claims that the New Testament documents are the best-attested documents of antiquity and that they were written within the first generation after the death of Christ. But Mr. Jones is neither a historian nor a Scripture scholar, so what business he has speaking as an authority on these issues is beyond me.
2. In a moment, Mr. Jones will offer arguments for why we should believe that Jesus Christ was a real historical figure. But keep in mind, Mr. Jones’ is a grown man who believes that an invisible God listens to his prayers and lives in his heart. Can we really trust what he has to say on anything of importance?
3. Mr. Jones wants us to believe that Jesus Christ was a real historical figure. He points out that I know very little about the New Testament and the Christian religion. But when I asked him a moment ago which of the four Gospels depicts many people rising from the dead and appearing to people in Jerusalem, he was unable to tell us!
You’ll notice that in all three examples the arguer did not attack Mr. Jones’s argument but instead attacked him as a person. In the first example we are led to believe that Mr. Jones’s argument is unreliable because he is not a historian or a Scripture scholar. Now, there is nothing wrong with requiring evidence or expertise before accepting something as true, but Mr. Jones’s opponent doesn’t do that; instead he dismisses Mr. Jones’s on account of his lack of formal training. But Mr. Jones’ lack of formal training has no bearing on whether his argument was sound. Perhaps Mr. Jones also has a low IQ, bad breath, and is an alcoholic. None of that is evidence that his argument is unsound.
Poisining the Well
Now, take a look at the second example. Here we see a variation of the ad hominem fallacy usually called “poisoning the well.” This fallacy is a preemptive attack on the character of a person before they have had the change to make an argument. The term was first used by Cardinal John Henry Newman in his work Apologia Pro Vita Sua, where he wrote, “What I insist upon here . . . is this unmanly attempt of his, in his concluding pages, to cut the ground from under my feet;—to poison by anticipation the public mind against me, John Henry Newman, and to infuse into the imaginations of my readers suspicion and mistrust of every thing that I may say in reply to him. This I call poisoning the wells.” Again, this fallacy lies not in questioning the reliability of Mr. Jones but in making him appear deluded and a fool before he’s even had a chance to make his argument.
The third example is also a type of ad hominem argument, called tu quoque (literally, “you too”). The tu quoque fallacy consists in accusing your opponent of the same thing he has accused you of. “Well maybe I am a thief, but so are you!” Well, perhaps he is, but that fact in and of itself does not attempt to deal with—much less refute—his argument.
If you find yourself on the receiving end of the ad hominem fallacy, I would suggest calmly pointing out to the one you're dialoging with that attacking you (your character, intelligence, etc.) doesn't deal with the argument at hand, then invite himto spend his energy on that instead.