## You are here

Hometautology

## Primary tabs

# tautology

A *tautology* is a statement (or form) which is true solely on account of its logical form rather than because of the meaning of the terms employed.

In propositional logic, a tautology is a statement which is true regardless of the truth value of the substatements of which it is composed. An example would be

It is a sunny day and on a sunny day the rain does not fall so the rain does not fall.

This statement is true no matter what the truth value of the statements of which it is comprised, “It is a sunny day” and “The rain does not fall” may be. More generally, any statement of the form

(P and (if P then Q)) implies Q

or, in symbols,

$(P\wedge(P\rightarrow Q))\rightarrow Q$ |

is true no matter what statements one may substitute for $P$ and for $Q$. (They may even contain connectives themselves. An expression of the sort appearing above in which one can obtain tautologies by substituting arbitrary statements for the variables which appear in the expression is known as a *tautologous form*.)

To test a statement or form to see if it is a tautology, one may construct a truth table. If it turns out that one obtains “T” in every column, then the statement is a tautology.

In predicate logic, a tautology is a statement which is true no matter what choice one makes for the predicates which appear in that statement. A simple example would be

If all cats are black then there exists a black cat.

This is of the general form

$(\forall x)(A(x)\rightarrow B(x))\rightarrow(\exists x)(A(x)\rightarrow B(x))$ |

with $A$ being “is a cat” and $B$ being “is black”.

In the notation of Peano, tautology is denoted by “$\curlyvee$”. (“v” comes from the Latin “verum” = ’true’.)

## Mathematics Subject Classification

03B05*no label found*03B10

*no label found*

- Forums
- Planetary Bugs
- HS/Secondary
- University/Tertiary
- Graduate/Advanced
- Industry/Practice
- Research Topics
- LaTeX help
- Math Comptetitions
- Math History
- Math Humor
- PlanetMath Comments
- PlanetMath System Updates and News
- PlanetMath help
- PlanetMath.ORG
- Strategic Communications Development
- The Math Pub
- Testing messages (ignore)

- Other useful stuff

## Recent Activity

new correction: Error in proof of Proposition 2 by alex2907

Jun 24

new question: A good question by Ron Castillo

Jun 23

new question: A trascendental number. by Ron Castillo

Jun 19

new question: Banach lattice valued Bochner integrals by math ias