Having lived in Cambridge, Massachusetts for many years, I’ve periodically had a generic conversation that that either amuses or annoys me. It proceeds via the following formula:

Me: “I think [insert statement that does not follow the conservative party-line].”

Other person: “Well, you would say that; you live in Cambridge.”

Sometimes, I respond by pointing out that I voted for George W. Bush (twice), trying to draw attention to the fact that the place where a person lives has no essential connection to his political views. That doesn’t always work, though. Maybe it’s just too tempting to believe that my opinions are the fruit of sojourning for too long among the tents of people who voted for Ralph Nader in the 2000 election ^{[ 1 ]}.

Although not a strict example of argumentum ad hominem – the fallacy in which one asserts or implies that an argument is false because the person making the argument has some negative quality – exchanges of this type illustrate a close relative, the circumstantial ad hominem. Whereas an ad hominem often attacks the arguer directly (the “abusive ad hominem“),

Joe is clearly wrong about Richard Wagner’s being a better musician than Tiny Tim. How can you believe a guy who smokes pot all day?

the circumstantial ad hominem tries to undermine an argument by attacking the arguer’s circumstances:

Joe is clearly wrong about Richard Wagner’s being a better musician than Tiny Tim. I mean, what does Joe know about music, he’s from the backwoods of West Virginia!

Sometimes, an ad homninem is combined with guilt-by-association:

Joe is clearly wrong about Richard Wagner’s being a better musician than Tiny Tim. You know, the Nazis also thought Wagner was a great composer; how can you agree with someone who takes the side of the Nazis?!

There’s also a version of this fallacy known as the tu quoque (Latin for “you also”). In a tu quoque, the person making the argument is attacked because he behaves in a manner that appears inconsistent with the position he has taken:

Joe is clearly wrong about Richard Wagner’s being a better musician than Tiny Tim. How can you believe that, when you know Joe owns every album Tiny Tim ever recorded?

What makes each of these attacks against Joe’s argument logically incorrect is this: none of them actually has anything to do with the argument itself. Each substitutes verbal sleight-of-hand for sound reasoning. Instead of addressing whether the arguer’s logic is sound, these responses ignore the argument and seek to undermine the reputation of the person making the argument.

Ad hominem plays to our feelings; if I can make you dislike the arguer, you’ll probably be willing to transfer your dislike to his argument, too. The fallacy is also appealing because such attacks are often funny (albeit at someone else’s expense).

In practice, ad hominem is most effective when there is something obviously bad about the person making the argument. Alternatively you can use it to exploit some bias or prejudice held by others against the person whose argument you wish to discredit. Thus, the examples above argue, respectively, that:

People who smoke pot all day are always wrong about music,

People from the backwoods of West Virginia are always wrong about music,

People who favor composers who were admired by the Nazis are always wrong about music, and

People who appear to be inconsistent are not capable of making sound arguments.

Phrased this way, it’s easier to see the fallacious nature of each assertion.

The fact that Joe smokes marijuana has no bearing at all on the relative merits of Wagner vs. Tiny Tim, nor is Joe’s personal background relevant to that question. In the third example, we have a double-dose of logical shenanigans: it’s possible for Joe to like Wagner while having nothing else in common with Nazis – but even if Joe were an actual Nazi, that wouldn’t have any bearing on the real question at hand: who is the better musician?

The fourth example is especially tempting for us to swallow, because we instinctively like being made to feel morally superior to others. It’s pleasing to believe that a person who is inconsistent (or a hypocrite) must also be wrong. But it ain’t necessarily so. Not only does the fourth statement have no bearing on the argument, it might be that Joe simply loves Tiny Tim’s music in spite of the fact that Wagner was a better musician.

These are all versions of the same trick. One moment we’re talking about music, then suddenly, the focus of attention is Joe, and we are convinced that any argument he makes must be wrong.

The frequency with which we slide into fallacious statements in the ad hominem family is remarkable; I probably commit this error myself at least a few times a week. To make matters worse, if you try to point out to someone that he has committed the fallacy of ad hominem, it’s often the case that he will not understand why that is so. He may insist that doing so does not invalidate his argument. Worse, he may even accuse you of trying to use a trivial technicality to evade the argument.

The fact is, we often find ad hominem arguments persuasive, especially if the arguer has properly targeted one of our existing biases. We use such arguments because they work! A clever trial lawyer may even use ad hominem to sway a jury if he has a weak case.

Here’s an example of an ad hominem that might work nicely for a Reformed audience:

The doctrine of the Immaculate Conception is one of many obviously false Roman Catholic errors.

We might find this persuasive, even though the only argument presented is an ad hominem. But now imagine that we swap a few words:

The doctrine of the Trinity is one of many obviously false Roman Catholic errors.

Oops – now we can see more easily that this is a fallacious argument. But both doctrines are taught by the Roman Catholic Church. If one of the statements is logically unsound, both of them must be. In fact, the truth or error of a doctrine can never be determined by asking who believes in it. For example, Jehovah’s Witnesses believe in inerrancy; the fact that they embrace certain other doctrines that are heretical doesn’t make everything they believe false.

The ad hominem also presents some subtleties. Not every attack against an arguer constitutes an ad hominem.

Suppose someone says that he is a physics expert, and that the theory of relativity means that time travel is impossible. In this instance, you happen to know that this person last studied physics in junior high school. The fact that he has lied about his level of expertise does not automatically prove that his statement about time travel is false; it might still be true. But you would not be committing an ad hominem if you responded by pointing out that he has no expertise in physics. He may be right, but his opinion is not supported by the expert knowledge he has claimed.

Or consider the Bill Clinton-Monica Lewinsky scandal. Early on, when Clinton said, “I did not have sex with that woman,” it would not have been an ad hominem attack for someone to say, “well, of course you would say that, since admitting it would be politically devastating for you.” The fact that he was highly motivated to deny the accusation would not prove or disprove whether he had sex with her, of course, but would weaken the credibility of his denial. If he had been a person for whom admitting that he had sex with Lewinsky would (somehow) have been a highly desirable thing, his denial would have had more force. Moreover, this is not the fallacy of ad hominem because it does not involve a logical proposition; it’s simply a question of whether an event happened or did not happen.

Closely related to the ad hominem is a device that has come to be known as “poisoning the well.” Strictly speaking, it is not a type of argument, but a pre-emptive way to discredit someone even before he has made his case.

So one might say, “Only a fool would borrow money to buy a house.” Anyone subsequently seeking to argue in favor of borrowing to buy a house must first turn his energy to proving that he is not a fool before he can even approach the question at hand. Once the well has been poisoned, the very act of expressing a contrary opinion brands him a fool.

There are even more subtle ways of poisoning the well: “No true American should ever be so cowardly as to criticize our nation.” Would-be critics must now first show that they are not cowards before speaking a discouraging word.

Well-poisoning can also be done by claiming, even before a person has a chance to speak, that he will take a certain position – not because it’s valid, but because he’s biased, e.g., “He’s a doctor, so of course he’ll be in favor of vaccination.” Again, the topic of debate has been subtly shifted to a new point: a doctor will always defend vaccination even if it is a terrible idea, solely because he is a doctor.

If the use of fallacious logic can be deemed a form of mental laziness, then the ad hominem fallacies may be the laziest of all. Sadly, it’s a type of fallacy that one can take up without even knowing it.

Unlike a simple missed thread in an argument, the ad hominem is typically little more than a personal attack against a person who puts forward an argument with which I disagree. Thinking about this in this way has caused me to realize something that never struck me before – the use of ad hominem, in some instances, is a form of bearing false witness against one’s neighbor. ^{[ 2 ]}.

In summarizing the requirements of the ninth commandment, the Westminster Larger Catechism teaches that these include

…the preserving and promoting of truth between man and man, and the good name of our neighbor, … a charitable esteem of our neighbors; loving, desiring, and rejoicing in their good name; …freely acknowledging of their gifts and graces, defending their innocency; a ready receiving of a good report, and unwillingness to admit of an evil report, concerning them; discouraging talebearers, flatterers, and slanderers…. (WLC Answer #144)

Often, when I attack an argument with an ad hominem, I divert attention from the real matter under consideration while undermining my neighbor’s good name. Is it possible to do these things while preserving and promoting the truth?