 Lewis’s actual theory of content is more complex, and has to be reconstructed from several sources. It has three stages, in each of which naturalness plays some role. The first is an account of what it is for a person to have certain beliefs and desires, or to be more exact, a certain credence function (mapping properties to real numbers in the unit interval) and valuation function (mapping maximally specific properties to real  numbers).[50]  The second is an account of what it is for a certain “Language” (function from sentences to propositions) to be “used by” (admissible for) a given population. The third stage extends the second stage to interpretation-functions that map arbitrary expressions (not just sentences) to semantic values of the appropriate type. At each stage, the resources employed include those analysed at the previous stage.