 Such a treatment was only possible after the development of general modal logic. Indeed, what we are dealing with here is a bimodal logic, which is properly treated only after a deeper understanding of the semantical subtleties of mixing alethic and deontic logics. Moreover, according to some philosophers who have argued that it is not possible to link ‘is’ and ‘ought’ (that is, who defend Hume’s thesis that no non-trivial ‘is-ought’ deductions are possible), it is mandatory to use combinations of first-order, alethic and deontic logics (see e.g., Stuhlmann-Laeisz 1983 and Schurz 1997).