x I wish to express my thanks to all contributors to this volume for their collaboration, and especially to Professor laakko Hintikka, Editor of Synthese Library, for his help in editing this book. THE EDITOR INTRODUCTION TO THE SECOND IMPRESSION The articles included in this collection represent what may be called the standard modal approach to deontic logic (the logic of normative concepts), in which deontic logic is treated as a branch of modal logic, and the normative concepts of obligation, permission (permissibility) and prohibi­ tion are regarded as analogous to the 'alethic' modalities necessity, possi­ bility and impossibility. In his recent paper [16] Simo Knuuttila has shown that this approach can be traced back to late medieval philosophy. Several 14th century philosophers observed the analogies between deontic and alethic modalities and 4iscussed the deontic interpretations of various laws of modal logic. A relatively simple deontic system of this kind (called the system D or K D; cf. Lemmon and Scott [17], pp. 50-51, Chellas [10], p. 131) is obtained by adding to propositional logic two deontic axioms (or axiom schemata), (K) O(A :::J B) :::J (OA :::J OB) and (D) OA :::J ~ 0 ~ A, where '0' is the obligation operator, and the deontic variant of the 'rule of necessitation' (0) From A, to infer ~A.