What is a logical fallacy?

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, which does not imply that I never discuss factual errors or that they are unimportant, but that they are not the focus.

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"1.

There are two types of mistake that can occur in arguments, both of which are important and cause an argument to fail:

  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 particular argument that is said to be a "fallacy", so that in this sense a "fallacy" is an argument that uses incorrect 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 is ungrammatical2, for your argument is not a type of argument. 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 logical error in arguments.

History

Aristotle was the first formal logician―codifying the rules of correct reasoning―and the first informal logician―cataloging types of incorrect reasoning, specifically, informal fallacies. He was both the first to name types of logical error, and the first to group them into categories in his book On Sophistical Refutations3.

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 dialogue "Euthydemus"4 preserves a collection of fallacious arguments by putting them in the mouths of two sophists, that is, itinerant teachers. For this reason, fallacious arguments are sometimes called "sophisms" and bad reasoning "sophistry". Aristotle, in On Sophistical Refutations, refers to a few of these examples as instances of fallacies.

In the centuries since Plato and Aristotle, many great philosophers and logicians have contributed to the study of fallacies, among them John Locke5, Jeremy Bentham6, John Stuart Mill7, and Arthur Schopenhauer8.

Last century, an Australian philosopher, logician, and computer scientist, Charles L. Hamblin, wrote a highly-influential book on fallacies9. The first part of the book is a history of the 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 late psychologists Daniel Kahneman and Amos Tversky10. 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, most 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 Grootendorst11.

Why study fallacies?

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

  1. 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 logical fallacies are for: 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, but only a supplement.
  2. You might be tempted to study fallacies so that you can use them to fool others. I don't recommend this, but there's no way to prevent people from doing so.
  3. Even if you could count on reasoning correctly 100% of the time―that is, if you could navigate the logical landscape without ever needing the "WRONG WAY" signs―you cannot count on others being able to do so. As mentioned in point 2, above, there's no way to stop con artists, corrupt politicians, and ruthless advertisers from studying fallacies. In logical self-defense, you need to be able to spot poor reasoning. Moreover, to be able to correct others' mistakes, or to explain them convincingly, you need to understand why they are wrong.

Notes:

  1. ↑ I spell "premiss" with two esses instead of one, for reasons explained in the Glossary; in other words, this is not a misspelling.
  2. ↑ Technically, it commits a category mistake.
  3. ↑ Aristotle, On Sophistical Refutations (350 B.C.?).
  4. ↑ Plato, Euthydemus (380 B.C.?).
  5. ↑ John Locke, An Essay Concerning Human Understanding, Book IV, chapter xvii, sections 19-22.
  6. ↑ Jeremy Bentham, The Handbook of Political Fallacies, revised & edited by Harold A. Larrabee (1971).
  7. ↑ John Stuart Mill, A System of Logic, Ratiocinative and Inductive, Book V, chapters 1 & 2 (1892).
  8. ↑ Arthur Schopenhauer, The Art of Controversy, translated by Thomas Bailey Saunders, edited by Axel Wendelberger (2008).
  9. ↑ C. L. Hamblin, Fallacies (1970).
  10. ↑ See, especially: Daniel Kahneman, Paul Slovic & Amos Tversky, editors, Judgment Under Uncertainty: Heuristics and Biases (1982).
  11. ↑ See, for instance: Frans H. van Eemeren, Rob Grootendorst & Francisca Snoeck Henkemans, Argumentation: Analysis, Evaluation, Presentation (2002).

Acknowledgment: Thanks to Nicholas Joll for corrections.

Revised: 5/22/2025