In logic, a tautology is a statement that is true by its own definition. All true statements of logic are tautologies. Outside logic, it sometimes means a useless tautology, that is, one that is uninformative (or in colloquial terms, stating the obvious). This definition is imprecise, as all statements are informative in some context.
A logical tautology is a statement that is true regardless of the truth of its parts. For instance, "Either it is raining now outside or it is not raining now outside" is a tautology.
The opposite of a tautology is a contradiction, which is a statement that is always false regardless of the truth values of its parts.
Tautologies can be used to introduce a red herring into an argument, but the two are not mutually inclusive.
The mathematical symbol for a tautology is .
Controversies arise because sometimes a logical tautology can be quite subtle. Suppose that a news analyst were to make the following statement:
- All mainstream U.S. Senators agree that the House bill is unacceptable.
This would seem to be a meaningful statement. But suppose further that he were also to reveal his opinion that "Senator K disagrees, and has therefore shown himself to be outside of the mainstream." In this case, the analyst's definition of "mainstream" requires opposition to the House bill. Therefore his original statement was a tautology.
See No true Scotsman.
A grammatical tautology is often a fault of style. It was defined by Fowler as "saying the same thing twice". For example, "three-part trilogy" is tautologous because a trilogy, by definition, has three parts. "Significant milestone" and "significant landmark" are also, if less obviously, tautologous, because milestones and landmarks are again significant by definition (could one imagine an "insignificant landmark"?).
Tautologies sometimes occur when multiple languages are used together, such as "The La Brea Tar Pits" (the the tar tar pits), "Sierra Nevada Mountains" (snowy mountains mountains), "Manos: The Hands of Fate" (hands: the hands of fate), "The Los Angeles Angels" (the the angels angels), or "Shiba Inu dog" (small dog dog). They often appear in conjunction with acronyms or abbreviations, when the original meaning fades through familiarity with the acronym itself; for example, consider "ATM machine", "PIN number", "HTML language", "VIN number", "NT Technology" (an expression actually used by Microsoft), or "LCD display". (See RAS syndrome.) Other examples of linguistic tautologies include "in this day and age", "helpful assistance", "new innovation", "rate of speed", "one (2-, 20-)year anniversary". Many of these examples are also models of pleonasm.
A grammatical tautology may be intended to amplify or emphasize a certain aspect of the thing being discussed: for example, a gift is by definition free of charge, but one might talk about a "free gift" if the fact that no money was paid is of particular importance. A tautology could also be used if a non-tautologous expression might not be taken at face value: for example, a business might offer its customers a "free gift", to distinguish itself from other businesses that claim to offer "gifts" but only give them in conjunction with a purchase. Similarly, a tautology could be used if the non-tautologous expression might be ambiguous or might not be understood: although PIN stands for "Personal Identification Number", one might refer to a "PIN number" if the intended audience is unfamiliar with the acronym, or to avoid confusion with the word pin... or in the southern United States, where "pen" and "pin" are often pronounced nearly identically, hence the use of the apparent tautology "ink pen". For these reasons, although tautologies are technically unnecessary, and may be considered incorrect, they are nonetheless common in some contexts.
The late comedian Alan King used to tell this story: His lawyer asked him if he had ever drawn up a will. Alan said "No". The lawyer, in shock and horror, said, "If you died without a will, you would die intestate!" Alan looked up the word and found that it means "to die without a will". "In other words, if I die without a will, then I'll die without a will. This legal pearl cost me $500!"
One Dilbert cartoon takes this subject to a humorous extreme. Dilbert says he is working on a project that is known as "TTP". When asked what "TTP" stands for, Dilbert responds that it means "The TTP Project". This is not strictly a tautology, but a recursive acronym. The Concise Oxford Dictionary defines tautology as "the saying of the same thing twice over in different words". In the spirit of pedantry, it should be noted that this statement is itself tautological; by using the word "same", it is already implied that the thing has a plural value, so there is no need for the word "twice". It could also be argued that the word "over" is redundant in this context. The definition could instead read "the saying of something twice in different words" or "the saying of the same thing in different words".
- The Columbia Guide to Standard American English: Tautology
- Figures of Speech: Tautologycs:Tautologie