 Now, is negation a normal modal operator of impossibility or unneccessity? According to Berto (2014), the meaning of negation is grounded in the notion of compatibility, together with its opposite concept of incompatibility. Moreover, Berto takes compatibility and incompatibility to be symmetric relations and, as a result, holds that nothing can be justifiably called a negation if it does not satisfy the contraposition rule and the double negation introduction rule. According to Berto and Restall (2019), “because incompatibility is modal, negation is a modal operator as well.” This view is contentious, in light of non-contraposable negations in systems such as Ł3, K3, LP, and N4, and it has been argued by De and Omori 2018 that there is more to negation than modality.