David Lewis's Semantics for Deontic Logic

Abstract

Numerous attempts to provide'semantic interpretations for deontic logic are based on the idea that a given state of affairs ought to be the case in this world if and only if it is the case in every morally perfect world. In classical versions of this theory, a 'morally perfect world' is simply defined as one in which all the particular obligations obtaining in the actual world are fulfilled.' Such theories have two disadvantages. First, as Richard Purtill has pointed out, it is extremely difficult to spell out the requirement that all obtaining obligations be satisfied in a morally perfect world.2 If a world contains no agents at all, are all our obligations fulfilled in it? If it contains agents who are not counterparts to the agents in this world, are our obligations fulfilled in it? Second, the value of such theories lies solely in their elucidation of the logical form of obligation statements, e.g., their explanation of what makes a set of obligation statements consistent. The application of such theories depends on prior possession of the list of true statements of particular obligations ('Jones ought to do A', 'Smith ought to do B', etc.). Since they do not offer an independent method for determining whether or not a given obligation obtains, they fail to provide illuminating reductive or eliminative definitions for statements of particular obligation. David Lewis has proposed a semantic theory which falls within this general tradition but employs an alternative definition of the morally best worlds.3 Because of this difference, his theory may escape Purtill's direct objections.4 In addition, this aspect of his theory makes possible the derivation of particular obligations, not from an initial list of such obligations, but rather from a statement of abstract moral principles together with the general notion of a possible world. Thus it may be interpreted as providing not only an account of the logical form of statements of particular obligations, but also a reductive definition of them -in terms of these other two concepts. Promising as this theory appears, I shall argue that it does not succeed, because it neglects the fact that