The English word "fallacy" comes from the Latin noun "fallacia", meaning "trick" or "fraud". "Fallacia" is related to the Latin verb "fallere", meaning "deceive", so that a "fallacia" is a deception. The English words "false", "fail", "fallible", and "fault" are all derived from these same Latin words.
Source: John Ayto, Dictionary of Word Origins (1991)
The English word "fallacy" is both vague and ambiguous. Its general meaning is "mistake" or "error", especially a common one. For instance, "fallacy" is frequently used to mean a common factual error, and a number of books that use it in the titlesuch as Tad Tuleja's Fabulous Fallacies and the Reader's Digest book Facts & Fallaciesare collections of common factual mistakes with corrections. This is not the type of fallacy catalogued in The Fallacy Files; rather, it is a collection of logical fallacies.
"Logical fallacy" shares with "factual fallacy" the genus "common error", that is, both are types of error commonly committed by people. Factual fallacies, of course, are mistakes about factual matters, whereas logical fallacies are not errors of fact, but errors of reasoning. Thus, a logical fallacy is a common error in reasoning.
- There is an ambiguity in the phrase "logical fallacy" between type and instance:
- Type: In this sense, a logical fallacy is a type of error, that is, a class of many similar instances of bad reasoning, so that it would be a mistake to say of a particular instance of bad reasoning that it is a "logical fallacy".
- Instance: In this sense, a logical fallacy is an instance of bad reasoning, that is, a specific bad argument rather than a class of them.
Both of the above senses of the phrase "logical fallacy" are used commonly, even by logicians and mathematicians, without distinguishing the two meanings. However, context will usually indicate whether "fallacy" refers to a fallacious argument or a type of error in reasoning―such as the fallacy of affirming the consequent. Nonetheless, throughout The Fallacy Files I restrict the term "logical fallacy"or just "fallacy", for short, since I'm not concerned with factual mistakesto sense 1, and not sense 2. For sense 2, I say that a particular argument "commits" a fallacy, or that it is "fallacious", which means that the argument is an instance of a fallacy, in sense 1.
In sum, throughout The Fallacy Files, I will use the following definition:
logical fallacy = a common type of error in reasoning.
- In the phrase "logical fallacy", "fallacy" tends to retain some of the meaning of its Latin origin, "fallacia", in that a logical fallacy is often thought to be a deliberate deception. However, while it is certainly true that some people intentionally use fallacious arguments to deceive, the deceptiveness of logical fallacies is such that many who use such arguments are themselves deceived by them. For this reason, in The Fallacy Files I do not assume that a person who argues fallaciously is engaged in intentional deception.
- With the first letters capitalized, "Logical Fallacy" refers to the most general type of common error in reasoning, that is, a logical fallacy―first letters in lowercase―committed by every fallacious argument. Thus, in the taxonomic tree of logical fallacies, Logical Fallacy is the root from which all more specific fallacies sprout―to see the Taxonomy, use the Main Menu to your left.
Fallacious arguments will usually commit some more specific fallacy than Logical Fallacy. The only exceptions to this rule would be arguments that have not yet been classified, or which commit a fallacy that has not yet been named or for which there is no entry. However, it might be misleading to simply classify such an argument as a Logical Fallacy, since this could lead the reader to think that it does not commit some more specific error. For this reason, I do not provide here any examples. However, there is no dearth of examples: all those arguments listed as Examples under any of the more specific fallacies in the Taxonomy are also examples of Logical Fallacy. Alternatively, you can access the Examples page from the Main Menu.