A provocative modern treatment this question by a philosopher is Margaret   
Gilbert's work, *Living Together* and *On Social Facts*. She argues for the   
irreducibility of what she calls the Plural Subject that we create through   
mutual commitments to one another in varied contexts all the time. The key   
point for economists is that goals of the Plural Subject create reasons for   
action for the individuals who make it up, reasons that are unmediated by   
references to beliefs and preferences of the individuals concerned.  She   
convincingly argues that a convention cannot be understood along Lewisian   
lines  as  an equilibrium in a coordination game  and that only something   
like the idea of a plural subject will do.  
  
  
Kevin Quinn