 Once there is more than just a single negation connective available, the interplay between these operations can be considered. Although classical and intuitionistic logic as well as the familiar systems of modal logic comprise only one negation operation, there are also very naturally arising logical systems with more than just one negation, and the motivation for taking into account multiple negations not only comes from natural language semantics but also from the field of knowledge representation, see, for instance, Wagner 1994.