Ad Hominem

I often find that I do not understand charges of ad hominem. (For the novice: ad hominem is a fallacy of a certain sort, i.e. it is a kind of mistake made in the attempt to establish the truth of some conclusion. The phrase 'ad hominem' is from the Latin, and means roughly 'against the man'. Ad hominem, we will find, is a fallacy that illegitimately substitutes a judgment about the character of a man for a judgment about the truth of what he says.) I don't mean that I don't understand what's meant by 'ad hominem'. About that I'm tolerably clear. What I often don't understand is why in debate this or that person thinks his opponent is guilty of ad hominem. I suspect that many charges of ad homimen are misplaced.

In the course of my discussion of ad hominem, I'll explain why I think charges of ad homimen are often misplaced.

First, let us place ad hominem on the fallacy map. Ad hominem is an informal fallacy. It has to do not with the form of an argument but rather with the content of its premises and conclusion. It is, moreover, a fallacy of irrelevance. The premises of an argument guilty of ad hominem are not relevant to its conclusion, i.e. the truth of those premises (if in fact they are true) should in no way contribute to our confidence in the truth of the conclusion. Last ad hominem is a species of genetic fallacy. One who commits ad hominem invites us to reject an opinion for no other reason than that its source is tainted in some way.

Next let us have a definition.

An argument commits the fallacy of ad hominem just if it infers from premises to do solely with the character of a person that that person's views about a certain irrelevant matter are false.

The definition makes quite clear just what ad hominem is. It is a fallacy of irrelevance in which we are to conclude that what a man says is false because he and not his argument has some defect. (Excuse the sexist language. I promise next post to shift to 'she' and 'her'.)

Why take the time to define this particular form of fallacy of irrelevance? Why not simply counsel that when one argues one must be careful never to commit any fallacy of irrelevance? Of course all arguments in which the premises are irrelevant to the conclusion are fallacious. Why not simply say so and leave it at that? The reason is that ad hominem is really quite common. Indeed we humans seem to have a predilection to commit it. We do it again and again. Why is this? Do we perhaps feel a need to believe that those we condemn must be through and through corrupt, so corrupt that even their very words are always false? Do we perhaps feel a need to tear down those we condemn so that we cannot even grant the truth of what they say? These are but guesses. I'll speculate no further. This question is, in all strictness, not a logical one at all; it has nothing to do with logic per se.

If you've ever had a logician explain to you what ad hominem is, likely that logician will not have inserted 'irrelevant' into her definition as I have into mine. Why insert it? Why not leave it out? If we leave it out, we must conclude that ad hominem is not always a fallacy. For we might well encounter an argument in which both premises and conclusion are about the character of a man. Indeed such a argument might take this form:

Issac is quick to anger but slow to forgive. Thus Issac is a hypocrite.

In this little argument, both the premise and the conclusion concern Issac's character. But the premise is quite clearly relevant to the conclusion. (Those who are quick to anger sin against others and thus are in need of their forgiveness. But Issac only very reluctantly gives forgiveness to others.) Thus the argument commits no fallacy. We must conclude then that if we wish to say that ad hominem is in all cases fallacious, the claims made within the premises about a man's character must be irrelevant to the claim made within the conclusion.

Of course instances of ad hominem are likely not as brazen as the definition might lead us to suspect. As with other sorts of fallacy, one who commits ad hominem will likely try to hide his mistake. He will not say any such thing as this: "Issac is a mean son-of-a-bitch. Thus his proof of the Poincare conjecture must be mistaken." Rather one who commits the fallacy of ad hominem is likely to hide his error by misdirection. Often he begins with a discussion of another's views. But he quickly leaves off discussion of those views and begins a rant about the other's faults. In this way we are invited to reject the other's views though we are never explicitly told that his views must be false because he has this or that fault of character. The moral here is that we must take care both when we read the arguments of others and when we construct our own. Ad hominem can be subtle. It can attempt to hide itself. Beware, then, when an argument - whether your own or the argument of another - begins to delve into the life of a man. This is no sure sign that the argument is fallacious. (Recall the little argument about Issac.) But it is a danger sign.

I'll say just a few words about mistakes to avoid when a charge of ad hominem is made. (i) One does not commit ad hominem simply because one says something critical of the character of another. Criticism of character is quite obviously often permissible, indeed sometimes it is required. But one may not infer anything irrelevant about the truth of a person's views simply because one has found fault not with those views but with them. (ii) In cases where one wishes to determine whether a certain person is a genuine authority in some area of inquiry, one may quite legitimately say something about their character. An environmental scientist - a climatologist, say - who radically altered his views after he began employment in the oil industry quite naturally and inevitably comes under suspicion. He looks to have sold out. Now, we cannot say that his views are for that reason false - if we did, that would be ad hominem. But we can for that reason find him no longer a legitimate authority. He has likely compromised his objectivity and thus we can no longer look to him as a source of unbiased, expert opinion.