 What that means for belief and desire is easiest to express using the language of possible worlds. The standard view is that propositions are (or at least determine) sets of possible worlds, and that the content of a belief is a proposition. To believe something then is to locate yourself within a class of possible worlds; to believe that you inhabit one of the worlds at which the proposition is true. Lewis’s view is that properties are (or at least determine) sets of possible individuals, and that the content of a belief is a property. To believe something then is to locate yourself within a class of possible individuals; to believe that you are one of the individuals with the property. More simply, beliefs are the self-ascriptions of properties.