The impossibility of withholding some part of the benefits of an activity from those who do not pay is of course precisely the definition of the neo-classical concept of external economies. The impossibility of exclusion which characterises a public good is therefore simply an extreme case of external economies, such that, when one individual consumes a unit of service, an identical-quality service unit is consumed by all other individuals without payment (21). In the case of Polaris submarine deterrent, for example, the defence-producing firm is completely unable to charge for any part of the given-quality service unit fully and equally consumed by all individuals. This can therefore be regarded as an extreme case of external economies of Scitovsky’s producer-consumer variety (22). The same is true of the draining of a malarial swamp. Alternatively, we might envisage a case such as that of a vaccination against some communicable disease, which when consumed by A provides similar protection to B. Allhough it seldom quite fits in practice, this case clearly provides in principle an example of extreme external economies of Scitovsky’s consumer-consumer variety. Here the producing « public health firm » can readily exclude a single consumer of an injection of vaccine, but once the health benefit has been made available to one 21 22
cit-’ PP- 200-10. Tn his original treatment of these problems, Musgrave (op. cit., 1059. p. 10. n. 1) maintained that exclusion problems necessarily entail joint supply. He has now reversed his position. See op. cit., 1966.
      In his excellent recent discussion of the joint supply characteristic, Buchanan (op. cit., 1966) also attempts to demonstrate that the existence of exclusion problems necessarily implies the existence of joint supply. In fact all he shows Is that exclusion problems will not be «observed » unless joint supply (either Marshallian or Samuelsonian) is privately profitable. Where exclusion problems are not « observed », market failure is of course even more striking.
      (21)        As in the case of the special joint supply characteristic, it is therefore obvious that the public good concept can readily be generalised in its non-exclusion aspects. See, for example, Head, op. cit., pp. 205-6. Thus, in one dimension, it may be recognised that only a part of the benefit is strictly non-excludable, e.g. the « general » as against the « vocational» aspects of education. In another dimension, it may be recognised that exclusion is seldom literally impossible for any aspect, of a service, but only more or less difficult and uneconomic. it should be noted in this latter connection that some degree of excludability may be achieved either through the adoption of a «pricing system» (such as tolls, or complicated legislation on property rights) which may be very costly to the supplier and/or the consumer, or through the employment of a relatively inefficient but more readily excludable technology (such as locks and night-watchmen rather than police patrols etc.). The vast body of literature on external economies is effectively concerned with these more general cases.
      (22)        See T. Scitovsky, « Two Concepts of External Economies », Journal of Political Economy, April 1954, pp. 143-51.