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