There are many more ways of reasoning poorly than there are of reasoning well, just as there are vastly more ways of getting lost than there are of proceeding directly to your destination. (A quick note on that last analogy: not every circuitous path is a mistake; depending on time and tide, sometimes there are aesthetic values other than efficiency of travel and timeliness of arrival at play in an actual journey.) Even the most detailed catalog of fallacies must content itself with providing little more than a generic list. My favorites such list is The Fallacy Files; besides their basic list, the Files also provides a well worked out taxonomy. But what is a fallacy? I can give a list of bird species without ever saying what a bird is. The Files do offer an answer to this question, but I wish to propose a slightly different approach. Where I am going is definitely outside the mainstream when it comes to saying what a fallacy is, but I believe a substantive argument can be made for the case I present here.

At its simplest, “a “logical” fallacy is a mistake in reasoning.” (Quoted directly from the above link to Fallacy Files.) This is probably the most common, and almost certainly the most defensible, short-form definition of a fallacy out there. It is difficult to disagree with this definition, but I wish to offer an initial attempt at doing so here. You see, the part I would disagree with is calling a fallacy a “mistake.” There are mistakes that occur in reasoning, to be sure, but I am uncomfortable with labeling all, or most, such errors as “fallacies.” Rather, I would argue that a fallacy is more in the way of a misuse, than a “mistake.” Errors come into play even when the methods and instrumentalities of both formal and informal logic are properly used. But, qua errors, they are easily corrected. But when the methods of logic are misused – whether by ignorance or malfeasance – fallacies are committed, and these fallacies will oftentimes become rigidly impervious to genuine reason and inquiry.

A simplified taxonomy that will prove useful in most circumstances is the following. A fallacy can occur when someone asserts something that is

1. False
2. Irrelevant
3. Derived by illegitimate means.

These categories are not mutually exclusive, which is to say, they each will frequently involve one or both of the others. Indeed (mutilating Claudius’ line from Hamlet), fallacies never come as lone spies, they always arrive in battalions. After I gloss items 1 – 3, I will present some illustrative examples. Also, it must be born in mind that this highly simplified taxonomy will certainly fail in some (possibly many) circumstances. This is especially true of point #1, to which I now turn.

Per #1, asserting a Falsehood: Stating a falsehood is one of the commonest errors that humans can commit, and in part because of this commonness I am inclined to argue that this is not sufficient unto itself to qualify as a fallacy. Honest mistakes are easily corrected, and can often be attributed to faulty information. Faulty information can be used in a formally impeccable manner to derive a false conclusion. Such errors are errors of fact, not of logic or reasoning, and as such do not qualify as fallacies. Fallacious reasoning occurs when the falsehood is elevated to the status of ideological, hence unquestioned, “truth.” At this point what one has is no longer a mere error, because the falsehood has been removed from the very possibility of inquiry.

Per #2, asserting irrelevancies: This is probably the largest single category of fallacies. Stating as a reason for X some claim C that has no formal or factual connection to the subject at hand is the single most common misdirection one finds in fallacious reasoning. Irrelevancies are smoke-and-mirrors distractions that lead away from the pertinent facts and invite us to accept conclusions which, even if true, have no bearing on the matter at hand, and as such no logical standing.

Per #3, illegitimate conclusions: In logic, being lucky is not the same as being right. Indeed, saying something true by accident is the same as saying something false. The bases upon which our conclusions stand are as important as those conclusions themselves, and those conclusions must follow from those bases by means of legitimate methods of inference, or those conclusions do not follow from those bases at all.

Examples are easy to come by, but pure examples (involving only one of the above cases) are nearly impossible. I have previously discussed a situation where, from one falsehood an even greater falsehood was concluded (the supposed “implication” of the fact that no right to privacy is mentioned in the Constitution.) The nature of the fallacy (as opposed to simple error) in this case is the fact that the falsehood is to be found in a premise that is never explicitly stated. When the premise is stated, the falsehood is so manifest to anyone who has troubled to read the Constitution and its Amendments, that one can scarcely avoid the uncomfortable suspicion that the unmentioned premise is unmentioned for a reason.

Another example is the argumentum ad hominem, the “argument against the person.” This is actually a widely misunderstood fallacy, as we will see in a moment. But common examples of it can easily be found. Some years back, Al Gore’s An Inconvenient Truth was widely dismissed because, after all, “You can’t believe what Al Gore says! He’s a liberal!” To begin with, it is not entirely clear how liberal Gore really is; he is a moderate, and only appears liberal to radical extremists. (He also appears liberal to many socialists, who disdain him for being too moderate. But that is a rather different subject.) The fact is, that Gore’s politics are irrelevant (#2 above); the only matter of relevance is whether Gore has accurately reported the scientific facts of global warming. When real scientists review Gore’s presentation, they tend to give it high marks. Rest assured that the denialists will dismiss these facts since “obviously” the scientists are liberals too …

An example of a type #1 and #2 failure is the amusingly named “fallacy fallacy.” This is commonly (though not universally) taken to mean accusing someone of having committed a fallacy when they have not. One frequently encounters this with the above mentioned argumentum ad hominem; as I said, it is widely misunderstood. Dictionary (as opposed to logical) definitions seem to indicate that any mention of a person’s character is fallacious, but this is itself false. When the claim is true, when it is relevant, and when it is derived by valid means, it is all but impossible for the claim to be a fallacy. For example, if I tell you that Fred is an habitual liar and you should not take what he tells you at face value, then if it is true that Fred is an habitual liar, and it is relevant to the topic at hand (because you are inclining to take what he has told you at face value), and I have come to this conclusion from logically valid means (through experience, through multiple lines of evidence, etc.) then I have committed no fallacy in calling Fred a liar, even though I “appear” to have made an argument based upon Fred’s character. Indeed, there is no “appearance” here: I have done exactly that. But the point about Fred’s character is (per hypothesis) true, relevant, and derived by valid means.

Even if one were to successfully master the rather daunting task of memorizing the full taxonomy of fallacies, in all its particularized glory, one would not necessarily gain an insight into the nature or “essence” of the fallacies themselves. Developing one’s intuitions around items #1, #2, and #3 above (as well as the admittedly vague and thoroughly unworked out difference between mere errors and decidedly fallacious reasoning) might go much further in that direction.