# Equivalence

Equivalence is a 1. (material equivalence) the combination of two formulas with the connective <-> (if and only if, iff), which is only true if both formulas have the same truth value. phi <-> psi can also be defined as the conjunction of two implications: phi -> psi and psi -> phi. For this reason, the connective of material equivalence is sometimes called the biconditional. The truth table for material equivalence is as follows:

```(i)	phi		psi	 phi <-> psi
1		 1	      1
1		 0	      0
0		 1	      0
0	 	 0	      1
```

See Connective. 2. (logical equivalence) a relation obtaining between two formulas phi and psi if their material equivalence phi <-> Psi is a tautology. In other words, two formulas which are logically equivalent have the same truth value for every possible model.

### Example

phi -> psi is logically equivalent with Neg [ phi & Neg psi ] in propositional logic and ThereIs(x) [ P(x) ] is equivalent with Neg All(x) [ Neg P(x) ] in predicate logic. When two expressions are logically equivalent, it is possible to substitute them for each other, without changing the truth values of the proposition they are contained in.