张五常(1970)对外部性有精辟的理解:
In the simple case, presented, indeed it is impossible to draw a dividing line such that "externalities" can be meaingfully identified. Every economic action has effects.
----------------------------------------the structure of a contract and the theory of a non-exclusive resource
现实世界不存在某种经济行为只有一种影响,因此,对于某种行为的某种影响只要不做分析研究,那么这种影响都可以被称之为相对于研究内容的外部性。