这个问题随便找一本美国初级的教科书早已经有说明。
我也曾经在这论坛回复过这样的帖。
但这里永远捧着一本200年前过时而且满是谬误的事,
兴高采烈口沫横飞在讨论,
What is a Public Good?
To see what makes a good a public good, we distinguish two features of all goods: the extent to which people can be excluded from consuming them
and the extent to which one person’s consumption rivals the consumption of others.
空气是公共品,决定公共品的两个要素,一个人不能阻止别人呼吸空气,而空气也不会因为一个人呼吸空气而减少。
Excludable A good is excludable if it is possible to prevent someone from enjoying its benefits. Brink’s security services, East Point Seafood’s fish, and a U2 concert are examples. People must pay to benefit from them. A good is nonexcludable if it is impossible (or extremely costly) to prevent anyone from benefiting from it.
Rival A good is rival if one person’s use of it decreases the quantity available for someone else.