Ad hominem is a Latin expression that means “to the man.” It is an informal logical fallacy of which attempts to refute or discredit a person’s argument by means of an appeal to some irrelevant personal characteristic of the arguer.
Like all informal fallacies, an ad hominem is fallacious not because of its form, but because of its content. That is to say, in order to establish that an ad hominem fallacy has occurred, it is necessary to show this in each case, because an appeal to personal characteristics of a person is not always fallacious.
People often confuse insults and unwelcome descriptions or observations for ad hominem fallacies, but this is a misunderstanding of the fallacy. Consider the following three statements:
1. Your argument about tax policy is invalid because you are an ugly, foul-smelling, unpleasant person.
2. You have trouble getting dates because you are an ugly, foul-smelling, unpleasant person.
3. You are an ugly, foul-smelling, unpleasant person.
Of these three, only Statement 1 is an ad hominem fallacy. The fact that a person is ugly, foul-smelling, and unpleasant is irrelevant to the truth of their views about taxation. It is this irrelevance that makes the fallacy fallacious—an ad hominem could be described as an “appeal to irrelevant personal characteristics.”
Statement 2 is not an ad hominem because being ugly, foul-smelling, and unpleasant is relevant to a person’s ability to get dates. The argument presented in Statement 2 may be unsound, the premise that the person is ugly, foul-smelling, and unpleasant may be false, but the argument is not fallacious.
Statement 3 is not an ad hominem fallacy, because it is not an argument at all. It is simply an insult, or a description, or an observation (even if the description or observation is unwelcome). Insulting someone may be impolite, but it cannot be fallacious.
For a charge of ad hominem to be justified, it must pass two tests:
- There must be a claim that a person’s argument is refuted, undermined, or weakened on the basis of some personal characteristic.
- This personal characteristic must be irrelevant to the matter at hand.
If you are going to charge someone with committing an ad hominem fallacy, you need to show both these things to be the case.
If an ad hominem charge fails either of these tests it is a false charge and may be dismissed.
Consider some other cases:
A. A politician argues that a lucrative government contract should be awarded to company C. It is discovered that the politician is major shareholder in company C, and that if company C is awarded the contract in question, he personally stands to make a significant amount of money. Is it an ad hominem to make people aware of the politician’s vested interest in the matter? Answer: It is not, because a person’s motives in making a practical argument (an argument for doing something) are often highly relevant to our assessment of his argument. But where the personal characteristic(s) appealed to are relevant, there is no fallacy. The fact that the politician has a vested interest in company C being awarded the contract does not automatically refute his argument that company C should be awarded the contract, but it does factor into our reasoning about the issue.
B. In matters of jurisprudence, we hold judges to have an ethical obligation to recuse themselves from any case in which there exists a conflict of interest. We expect a judge not to sit in judgment at the trial of, for example, his own son, or involving a company in which he is a major shareholder. Is the concept of recusal unreasonable? What if a person were to argue that the expectation of recusal is based on an implicit ad hominem fallacy, namely, that a judge’s judgment will be invalid based on some personal characteristic of his, e.g. his familial connection to his son, or his financial connection to the company under indictment? Answer: The idea of recusing oneself where there is a conflict of interest is reasonable. We know in such cases that a human being’s judgement (judges are human beings) is likely to be biased in cases where deciding one way and not the other is directly in his self-interest; even if the man or woman in question is of exemplary moral character, we recognize that biases can sometimes operate subconsciously. The existence of a conflict of interest does not just by itself prove bias; but it is enough, for example, if a previously undisclosed conflict of interest comes to light, to warrant a retrial. A conflict of interest in a matter is relevant to the assessment of a judgment or decision, just as it was in example A.
C. A history professor wants to argue the rather unobjectionable claim that “Hitler was a person of bad moral character.” Unfortunately for her, one of her students is also taking a philosophy class, in which they have just done a reading about informal fallacies. The student charges the professor with committing an ad hominem fallacy, on the grounds that talking about Hitler’s personal characteristics appeals to Hitler’s personal characteristics, and any appeal to personal characteristics is an argument “to the man” and therefore an ad hominem. Answer: It is an absurd misunderstanding to think that the existence of ad hominem fallacies rules out talking about persons or personal characteristics. A person’s personal characteristics are supremely relevant to their personal characteristics; they are them. Hitler’s moral character is entirely relevant to Hitler’s moral character; it is it.
ADDITION: Sargon of Akkad has helpful video distinguishing INSULTS and AD HOMINEMS.