In “Eldred and Fair Use,” Posner comments that awarding a monopoly over Mickey Mouse avoids the congestion problem that occurs with open access highways. Is the public domain a commons like an open access highway? No, it is not.

We characterize equilibria of general equilibrium models with externalities and taxes as solutions to optimization problems. This characterization is similar to Negishi's characterization of equilibria of economies without externalities or taxes as solutions to social planning problems. It is...