 As in the modal case, the thought is that all propositions are divided into three jointly exhaustive and mutually exclusive classes: every proposition is obligatory, impermissible or neither (that is, optional) and no proposition falls into more than one of these three categories. Furthermore, the permissible propositions are those that are obligatory or optional, the omissible propositions are those that are impermissible or optional, and the non-optional propositions are those that are either obligatory or impermissible. This classification too has roots centuries  ago.[13]  The reader can easily confirm that this natural scheme is also perfectly analogous to the threefold classification we gave above for necessary, contingent and impossible propositions.