What is a logical fallacy?

El sueño de la razon produce monstruos.

A "fallacy" is a mistake, and a "logical" fallacy is a mistake in reasoning. There are, of course, other types of mistake than mistakes in reasoning. For instance, factual mistakes are sometimes referred to as "fallacies". However, The Fallacy Files is specifically concerned with logical errors, not factual ones.

A logical error is a mistake in an argument, that is, a mistake in an instance of reasoning formulated in language. As the term is used in logic, an "argument" is a group of statements one of which is called "the conclusion" and the rest are called "premisses"―by the way, I spell "premiss" with two esses instead of one, for reasons explained in the Glossary; in other words, this is not a spelling mistake.

There are two types of mistake that can occur in arguments:

  1. A factual error in the premisses. As mentioned above, factual "fallacies" are not usually a question of logic; rather, whether a premiss is true or false is a matter for history or a science other than logic to determine.
  2. The premisses fail to logically support the conclusion. A logical fallacy is usually a mistake of this type.

In logic, the term "fallacy" is used in two related, but distinct ways. For example:

  1. "Argumentum ad Hominem is a fallacy."
  2. "Your argument is a fallacy."

In 1, what is called a "fallacy" is a type of argument, so that a "fallacy" in this sense is a type of mistaken reasoning. In 2, it is a specific argument that is said to be a "fallacy", so that in this sense a "fallacy" is an argument which uses bad reasoning.

Clearly, these two senses are related: in 2, the argument may be called a "fallacy" because it is an instance of Argumentum ad Hominem, or some other type of fallacy. In order to keep these two senses distinct, I restrict the term "fallacy" to the first sense. For me, a fallacy is always a kind of argument.

For the second sense, I will say that a specific argument "commits" a fallacy, or is "fallacious". So, in my terminology, 2 above commits a category mistake, for there is no way that your specific argument could be a fallacy. I would say, instead: "Your argument commits a fallacy" or "it's fallacious."

However, not just any type of mistake in reasoning counts as a logical fallacy. To be a fallacy, a type of reasoning must be potentially deceptive, that is, it must be likely to fool at least some of the people some of the time. Moreover, in order for a fallacy to be worth identifying and naming, it must be a common type of logical error.

To sum up, in these Fallacy Files a logical fallacy is a common, deceptive type of error in arguments.


History

Aristotle was both the first formal logician—codifying the rules of correct reasoning—and the first informal logician—cataloging types of incorrect reasoning, namely, fallacies. He was both the first to name types of logical error, and the first to group them into categories. The result is his book On Sophistical Refutations.

However, Aristotle's teacher, Plato, deserves credit for being the first philosopher to collect examples of bad reasoning, which is an important preliminary piece of field work before naming and cataloging. Plato's "Euthydemus" preserves a collection of fallacious arguments in dialogue form, putting the perhaps exaggerated examples into the mouths of two sophists, that is, itinerant teachers of rhetoric. For this reason, fallacious arguments are sometimes called "sophisms" and bad reasoning "sophistry". Aristotle refers to a few of these examples as instances of his named fallacies.

In the centuries since Plato and Aristotle, many great philosophers and logicians have contributed to fallacy studies, among them John Locke, John Stuart Mill, Jeremy Bentham, and Arthur Schopenhauer. Last century, an Australian philosopher, logician, and computer scientist, Charles L. Hamblin, wrote the highly-influential book Fallacies, which is unfortunately hard to obtain nowadays.

The first half of Fallacies is a history of the general concept of logical fallacy and the development of particular named fallacies. However, the most influential part of the book was probably the first chapter, which criticized the "standard treatment" of fallacies―that is, their discussion in textbooks of the time―and his criticisms seem to have inspired much subsequent research. Of less lasting influence were Hamblin's efforts, in the latter part of the book, to develop a formal treatment of dialectical argument as a basis for a theory of fallacies.

All of the above were efforts by those in the philosophical and logical tradition to understand mistakes in reasoning, but in the decades since Hamblin's history a separate research program has developed in psychology, associated especially with the psychologists Daniel Kahneman and the late Amos Tversky. Psychologists have mainly concentrated on mistakes in reasoning about probabilities, or what logicians call "induction".

Another recent development outside of logic, philosophy, and psychology, is in the field of rhetoric. As I mentioned above, the philosophical tradition of logical fallacies began by criticizing the arguments of the sophists of ancient Greece, many of whom taught rhetoric. As a result, there has been something of a split between philosophy and rhetoric ever since, and rhetoricians have developed their own distinct treatment of fallacies, though there is considerable overlap with that of logicians. Perhaps the most influential recent work is the pragma-dialectical approach most often associated with the rhetoricians Frans Van Eemeren and the late Rob Grootendorst.

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Why study fallacies?

Why study how to reason incorrectly; why not just study how to reason correctly? There are two reasons:

  1. Even if you could count on reasoning correctly 100% of the time, you cannot count on others doing so. In logical self-defense, you need to be able to spot poor reasoning, and—more importantly—to understand it. To be able to correct others' mistakes, or to refute them convincingly, you need to understand why they are wrong.
  2. Studying formal logic and the rules of correct reasoning is like having a road map that shows how to get from point A to point B. However, even the best navigators sometimes get lost, and it helps if the roads that go nowhere are clearly labeled "DEAD END", "WRONG WAY", or "DO NOT ENTER".

That is what fallacy studies is all about: marking the wrong turns that reasoners are likely to take. Thus, studying fallacies is no substitute for studying the positive principles of good reasoning—learning to navigate through logical space, so to speak. You would not set out on a trip without a road map, hoping to rely upon the "DEAD END" signs to get to your destination. Similarly, The Fallacy Files are no replacement for the study of formal and informal logic, only a supplement.