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