One of the less well understood fallacies that is committed often enough to have a Latin name is the argumentum ad hominem, the argument “against the person.” Many people take it for granted that an ad hominem occurs any time you say something negative about another person or group. Certainly it does not help when popular references – that are supposed to be authoritative – fail to accurately characterize matters.

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Thus, for example, the online Merriam-Webster dictionary gives two definitions of the ad hominem: The first defines it by saying, “appealing to feelings or prejudices rather than intellect,” the second definition given says, “marked by or being an attack on an opponent’s character rather than by an answer to the contentions made.” (The link may be found HERE.) But definition one is patently wrong; what they are describing is the argumentum ad misericordiam, not the ad hominem. The second is closer, but invites the error of use that I want to mention here. Because, you see, merely by being an evident attack on another’s character is not, by itself, sufficient to mark a statement as an ad hominem fallacy.

Informal fallacies almost invariably suffer from one or both of two kinds of error: they are (broadly) untrue, &/or they are irrelevant. (I emphasize that the discussion here is regarding informal fallacies, because the formal one’s involve a different class of errors that won’t concern us here.) Conversely, it is nearly impossible to have committed a fallacy if what one asserts is both true and relevant. For, on the one hand, if what one says is true, then no factual errors have been committed. Care should still be exercised here, in that the truth stated should be known via legitimate means. Just as a broken clock is on time twice a day, it is possible for someone to state a truth by accident only, which makes what has been stated deeply problematic. But it is not enough that what has been said be true, it must also be pertinent to the discussion or inquiry at hand.

Thus, for example, if someone says something like, “You can’t believe anything Al Gore says about climate change: he’s a liberal!” we can readily see that there are multiple ways in which this statement is problematic. For one thing, it is not immediately obvious how “liberal” Al Gore really is; a person on the further left side of the political spectrum might be inclined to dismiss Gore precisely because he’s barely more than a moderate. On the other hand, Gore himself (and most liberals to boot) would not find the charge of being a liberal an attack on their character. Nevertheless, for the hypothetical individual issuing the above claim, we can safely take it to be the case that the person views the term “liberal” as a damning pejorative. Let us then, grant that the “charge” is true, and recognize its intention as an attack on Gore’s character. But that is only half the battle: is Gore’s political orientation relevant?

Here the answer is clearly, “no.” One’s political orientation, religious inclinations, eye color, sartorial tastes, etc., have no bearing on whether or not one has accurately portrayed the scientific understanding of contemporary climate change and the prominence of anthropogenic forcings. In Al Gore’s case, his presentations of the science have been recognized by the scientists themselves as being faithful to the evidence and the overwhelming consensus within the scientific community itself. Similarly, one does not have to be Catholic, or agree with Pope Francis on every issue to appreciate the scientific accuracy of his recent encyclical.

Other attacks on Gore have focused more upon Gore’s lifestyle, and claimed that he has a rather poor track record when it comes to his personal consumption of fossil fuel resources, in matters such as his constant jetting around the world to various meetings, or the energy consumption at his home/office complex in Tennessee. One might defend some of Gore’s consumption on a variety of grounds, but doing so would be its own kind of logical error. (Rhetorically, as opposed to logically, such moves might be both necessary and useful.) Because the problem of relevance still remains. This type of fallacy is a sub-category of the basic ad hominem known as the tu quoque. Gore’s personal habits have no more bearing on the accuracy of his presentations regarding global warming than do his political leanings. As an example, consider that, simply because a person is a drunkard who is constantly consuming alcohol to excess, that does not mean that person’s recommendations against such behavior are inaccurate or ill-founded.

Suppose now, instead, that someone comes to you excitedly describing the latest refutation of climate science that can be found at Fred’s Blog, and you respond, “You can’t believe what you read there. Fred is an habitual liar.” Let us suppose that your statement is true, and you know it to be true on well-established grounds. Certainly it is the case that calling Fred “an habitual liar” is to sharply impugn Fred’s character. But have you committed an ad hominem fallacy? No, in point of fact, you have not, because Fred’s truthfulness is directly and immediately relevant to his credibility as a presenter of arguments against the established scientific consensus. So your statement is both true and relevant, and hence not a fallacy. Under the circumstances, it is perfectly reasonable to require that a more respectable and legitimate source for Fred’s claims be presented before one wastes any effort in addressing those claims.

Finally, a disclaimer: there are some fairly infamous sources of misinformation in the blogosphere on the science of climate change, but I quite deliberately choose not to name any of them. On the other hand, to my knowledge, there is no such thing as “Fred’s Blog” out there. My choice of the name is driven entirely by the fact that I not only do not know anyone named “Fred,” but I’ve never even met such a person. If there really is such a thing as “Fred’s Blog” out there, be assured I’ve no knowledge of it, and am in no way referring to it.