PRIVATE GOOD, EXTERNALITY, PUBLIC GOOD*

In two recent investigations into the economic problems of externality the authors have noted in passing that the welfare or optimality conditions in the case of a consumption externality seemed identical with the welfare conditions in the case of public goods as originally stated by Samuelson.1 The purpose of this paper is to demonstrate that the welfare conditions for a public good are a special case of the welfare conditions for a consumption externality where a public good is defined as a good ‘which all enjoy in common in the sense that each individual's consumption of such a good leads to no subtraction from any other individual's consumption of that good’ (Samuelson [4]). Since the welfare conditions for a private good are also a special case of the welfare conditions for a consumption externality, it follows that we have a range of externality with the pure private good and the pure public good as polar cases.