In accounts of deontic logic, a unary operator $\text{P}$ such that the intended interpretation of a formula $\text{P}\phi$ is ‘it is permissible that $\phi$ occur’. When a logic has an obligatory operator $\text{O}$, permissibility is frequently treated as the dual of $\text{O}$, that is, the operator defined by $\neg \text{O}\neg$. The scheme represents the fact that to say that one is not obliged to refrain from some action is an appropriate account of what it means for that action to be permissible.