 On the Simple Picture, belief is a binary relation between subjects and possible-worlds propositions (or something like them—more on this in a moment), and the interesting and theoretically important doxastic states of believers are states of believing P for possible-worlds propositions P. (So, in particular, all the states required to fully account for what’s happening with people doxastically, to explain people’s doxastic similarities and differences, and to give explanations of people’s behavior in terms of their doxastic states, will be states of believing P for propositions P, with propositions understood as sets of possible worlds.)