In logic and mathematics, statements
are said to be logically equivalent if they have the same truth value in every model.
, depending on the notation being used.
However, these symbols are also used for material equivalence, so proper interpretation would depend on the context.
Logical equivalence is different from material equivalence, although the two concepts are intrinsically related.
In logic, many common logical equivalences exist and are often listed as laws or properties.
The following tables illustrate some of these.
represents XOR.
The following statements are logically equivalent: Syntactically, (1) and (2) are derivable from each other via the rules of contraposition and double negation.
Semantically, (1) and (2) are true in exactly the same models (interpretations, valuations); namely, those in which either Lisa is in Denmark is false or Lisa is in Europe is true.
(Note that in this example, classical logic is assumed.
Some non-classical logics do not deem (1) and (2) to be logically equivalent.)
are logically equivalent if and only if the statement of their material equivalence (
) is itself another statement in the same object language as
This statement expresses the idea "'
On the other hand, the claim that two formulas are logically equivalent is a statement in metalanguage, which expresses a relationship between two statements
The statements are logically equivalent if, in every model, they have the same truth value.