- De Morgan's laws
- The equivalences (i) not (A or B) if and only if not-A and not-B, and (ii) not (A and B), if and only if not-A or not-B, are known as De Morgan's laws, although De Morgan himself gave the equivalent statements in Boolean algebra . Each equivalence is a theorem of the propositional calculus.

*Philosophy dictionary.
Academic.
2011.*

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