The Ad Hominem Fallacy

A fallacy is any mistake in reasoning. Some fallacies occur so often we give them special names, as with the ad hominem (Latin for to the man).

In short, you commit an ad hominem when you attack the person rather than the argument.

This is common on the internet and in political debates today. Here are familiar examples from both the Right and the Left in the US:

The Right: You think Trump is evil? You have Trump-derangement syndrome!

The Left: You deny systemic racism? You’re so racist!

People on the Right or the Left may have some good arguments, but these are not among them. This is a low, shabby business. Why?

It is because the character of the person attacked is not relevant to truth of the claims at issue.

Even if someone has Trump-derangement syndrome—whatever that is—her points might still be true and her arguments persuasive. And the same applies if someone is a racist. Racist people can hold true beliefs and make convincing arguments. If Hitler says that 2 and 2 makes 4, this is not false just because a racist says it. If the Grand Wizard of the KKK offers an elaborate statistical argument against systemic racism, his argument is not bad just because a racist makes it. Now, make no mistake, Hitler and the Grand Wizard deserve a hearty rebuke (and more); but the point here is that the rebuke does not show their claims are false or their arguments uncompelling. Ad hominem attacks are therefore members of a broader class of fallacies of relevance—they are fallacious because they advance claims not relevant to the matters at hand.

Three sub-types

The ad hominem admits of three sub-types:

Ad hominem abusive. This fallacy is committed when you directly insult someone as if this were a sufficient rebuttal. The examples above—racist! Trump derangement!—illustrate the ad hominem abusive.

Ad hominem tu quoque. This fallacy is committed when you accuse someone of being a hypocrite, as if this were a sufficient rebuttal. For instance: “You think it’s wrong to eat meat? I saw you eat a burger just yesterday!” But of course, it could well be that it is wrong to eat meat, even if the person who says this doesn’t follow the maxim in his own life. That a certain vegetarian eats meat, and that it is wrong to eat meat, are entirely compatible.

The genetic fallacy. This fallacy is committed when you exhibit the origins of a belief, as if this were a sufficient rebuttal. For instance: “You only believe in God because it’s a crutch; you can’t face the harsh realities of life!” Of course, even if that were a good explanation of why some person or other believed in God, it wouldn’t show that belief in God is false or even unreasonable. For all this objection says, maybe there are great arguments for God’s existence. More generally, how someone’s belief originated is one thing; whether that belief is true, and whether there are good reasons for it, is quite another.

Ad hominem albus?

A further pattern of ad hominem attack is so common these days it deserves special mention. Jonathan Haidt, a psychologist at NYU, calls it ad hominem albus (in Latin, albus means white).

Here’s how it works. When a white person says something you disagree with—typically about politics or social issues—assert that he is motivated by racism, power, or the desire to protect his privilege.

You can do something similar when a male is making claims about women’s issues (“You, as a man, have no right to speak about abortion!”).

Perhaps those who argue like this believe themselves to be unmasking or exposing the insidious motives of their opponents. Or perhaps they think that the truth of a claim doesn't matter, only the power advancing behind it. Or perhaps they think what matters is not whether a claim is true but whether someone has the right to say it. Whatever the answer, this is a textbook case of the ad hominem fallacy. Even if someone has no right to say something, for instance, it may well be true. Think of secrets told in breach of confidence. And even if someone has insidious motives for saying something, it may well be true. Darth Vader is Luke’s father, though he says this only to corrupt him.

More generally, if we care about social justice issues, we should also care about the truth, because truth is exactly what justice needs. Justice isn't served if you've got the wrong guy. Accordingly, those who, in the name of justice, commit the ad hominem fallacy, are actually betraying their own values.

An exception

Is it always a fallacy to bring up someone’s motives or character? No. Character can be mentioned when it is relevant to the claim at issue. So, imagine the following dialogue:

Witness: Trust me, Judge, she is definitely the culprit. I saw it with my own eyes.

Attorney: Objection! The witness is a known liar!

Does the attorney commit an ad hominem fallacy? Not in this case. For here the character of the witness is relevant, since we are being asked to take his word for it. Of course, if the witness is a liar, that doesn’t mean what he said is false. And it doesn’t mean there are no other reasons to believe it. It simply means that his testimony is not among them.