This paper explores David Lewis’s four theses on possible worlds. It is argued that these constitute a doctrine called extreme realism about possible worlds, which is deemed false. However, these theses need not be accepted or rejected as a package. The independence of the more plausible parts of the package is shown to defend the coherence of a more moderate form of realism about possible worlds, one that may be justified by common modal opinions and defended as a foundation for a theory about the activities of rational agents.
Keywords: possible worlds, David Lewis, extreme realism, coherence, rational agents,