I wrote this in 2002, and don’t have any plans to publish it. It has been cited however, so I maintain the final draft here.
Paper begins:
Analytic philosophers usually think about modality in terms of possible worlds. According to the possible worlds framework, a proposition is necessary if it is true according to all possible worlds; it is possible if it is true according to some possible world. There are as many possible worlds as there are ways the actual world might be. Only one world is actual.