Double negation is a rule of inference and rule of replacement for classical propositional logic. It states that the negation of a negation of a proposition is logically equivalent to the affirmation of that proposition.  It is often expressed symbolically as so:
Where means "replaceable".
In standard rule form, this rule may be represented as two subrules:
Please note it is not universally accepted. Some formulations of propositional logic omit this rule of inference (such as intuitionistic propositional logic).
- ↑ Double negation rule. Retrevied January 27, 2017 from http://www.philosophy-index.com/logic/forms/double-negation.php.
- ↑ Double Negation. Retrieved January 27 2017 from http://lc.brooklyn.cuny.edu/smarttutor/logic/dubneg.html.