Abstract

The most important problem is philosophical deontic logic is to determine the logical form of expressions of conditional obligation. The dissertation shows first that this problem is closely related to David Lewis's well-known "problem about permission"--a problem concerning the characterization of changes in normative systems. The dissertation contains a solution to the problem about permission, as well as an argument that expressions of conditional obligation cannot be represented satisfactorily by means of some combination of monadic deontic operators and a counterfactual conditional . ;Lewis formulates the problem about permission in terms of a language game for imperative and permission sentences. In order to solve the problem it is necessary first to point out the shortcoming in Lewis's game that it cannot be used satisfactorily to provide semantics for "contrary to duty" imperatives. Having revised the game it is possible to define the concept of relative defeasibility between imperatives. It is in terms of this concept that a solution to the problem about permission is formulated. ;The "revised game" is used as well to discuss problems concerning reasoning from sets of rules, principles, and facts to "all things considered" conclusions. It is argued that two distinct senses or uses of 'ought' must be distinguished in order to represent such reasoning satisfactorily. The role of the notion ceteris paribus is discussed. The notion is given a precise definition in terms of the possible worlds semantics of the "revised game" and it is argued that the notion is not as significant in normative reasoning as commonly is supposed. An alternative notion is defined and defended