 Standard Deontic Logic (SDL) is the most cited and studied system of deontic logic, and one of the first deontic logics axiomatically specified. SDL includes classical propositional logic, and is essentially a distinguished member of the most studied class of propositional modal logics, “normal modal logics”. SDL is a “monadic” deontic logic, since its primitive deontic operator is a one-place operator (like \(\neg\), and unlike &amp;): it applies to a single sentence to yield a compound  sentence.[16]  This operator, \(\OB\), is taken as primitive within SDL; as explained in  Section 1.2,  other operators such as \(\PE\) and \(\IM\) can be defined from it following the Traditional Definitional Scheme.