Chapters  2 and 3 outline the nature of Boolean versus non-Boolean descriptions. While a Boolean two-valued logic with truth values "true" and "false" is best characterized by the famous "rule of the excluded middle" (or tertium non datur), non-Boolean logic violates this rule. The consequence is incompatible descriptions, which are central to the notion of complementarity. Originally imported into quantum physics by Bohr, complementary descriptions are formally related to non-commutative algebras of observables.