ACCESSIBILITY, CAUSATION, AND NECESSITY

Laws of nature, we have argued, should be explicated in terms of a particular accessibility relation among possible worlds, a relation we call nomic accessibility. Counterfactuals should be explicated in terms of degrees of the same accessibility relation among possible worlds.

How, then, are we to explicate degrees of accessibility? We have urged a supervenience thesis for accessibility. The degree of accessibility between two worlds should be determined by the intrinsic natures of those worlds. We recommend that some sort of combinatorial world-property theory offers the best chance of providing a satisfactory theory both of the worlds and of the degree of accessibility between them. Each world arises from a recombination of individuals in the actual world with the various quantitative properties and relations in the actual world, such as mass, charge, relative velocity, and force. More specifically, each world is a structural universal, standing in a host of internal relations to its own constituent universals and to other possible worlds. The degree of accessibility between worlds will be a function of the proportions holding between the different quantities assigned to the same individuals in these different worlds. This holds not only for the combinatorialist's world books, but also for the world properties which, we have argued, correspond to them.