Double Negation/Double Negation Introduction/Proof Rule

Proof Rule
The rule of double negation introduction is a valid argument in types of logic dealing with negation $\neg$.

This includes propositional logic and predicate logic, and in particular natural deduction.

As a proof rule it is expressed in the form:
 * If we can conclude $\phi$, then we may infer $\neg \neg \phi$.

It can be written:
 * $\ds {\phi \over \neg \neg \phi} \neg \neg_i$

Also see

 * Double Negation Elimination: Proof Rule