In logic, a tautology is a statement which is true by its own definition, and is therefore fundamentally uninformative. Logical tautologies use circular reasoning within an argument or statement.
In linguistics, a tautology is a redundancy due to superfluous qualification.
A logical tautology is a statement that is true regardless of the truth values of its parts. For example, the statement "All crows are either black, or they are not black," is a tautology, because it is true no matter what color crows are. As a humorous example, tautology is famously defined as "that which is tautological". (That definition is, of course, tautological.) In a more realistic example, if a biologist were to define "fit" in the phrase "survival of the fittest" as "more likely to survive", that definition would be 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.
What is or isn't a logical tautology can be a matter of bitterly-contested controversy. Consider the below use of the example "helpful assistance." If this was a key term in the systematic thinking of some philosopher, a critic might think it a devastating objection to point out its tautological character. Yet the philosopher might reply, "it plainly isn't a tautology, because unhelpful assistance also exists -- most of us have been the victims of enough bad luck to receive it now and again!"
Less hypothetically, the question whether "God is good" is a tautology or a substantive assertion about God is a contentious issue among theologians, and the question whether such statements as "only the free market can find the true value of a commodity" are tautological or substantive have been much debated in social philosophy.
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.
A linguistic 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), 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", or "LCD display". (See RAS syndrome.) Other examples of linguistic tautologies include "in this day and age", "helpful assistance", and "new innovation".
A linguistic 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. For these reasons, although tautologies are technically unnecessary, and may be considered incorrect, they are nonetheless common in some contexts.