Common goods are defined in economics as goods that are rivalrous and non-excludable. Thus, they constitute one of the four main types based on the criteria:
whether it is possible to prevent people who have not paid for it from having access to it
One modern example is climate stability. Classic examples of common goods are water and air. Water and air can be polluted: water flows can be tapped beyond sustainability, and air is often used in combustion, whether by motor vehicles, smokers, factories, wood fires. In the production process these resources and others are changed into finished products such as food, shoes, toys, furniture, cars, houses and televisions. Another example of a private exploitation treated as a renewable resource and commonly cited have been trees or timber at critical stages, oil, mined metals, and crops. Fish stocks in international waters are also cited. In this latter example, when fish are withdrawn from the water without any limits being imposed, living stocks of fish are likely to be depleted for any later fishermen. To describe situations in which economic users withdraw resources to secure short-term gains without regard for the long-term consequences, the term tragedy of the commons was coined. For example, forest exploitation leads to barren lands, and overfishing leads to a reduction of overall fish stocks, both of which eventually result in diminishing yields to be withdrawn periodically. Debates about sustainability can be both philosophical and scientific. However, wise-use advocates consider common goods that are an exploitable form of a renewable resource, such as fish stocks, grazing land, etc., to be sustainable in the following two cases:
As long as demand for the goods withdrawn from the common good does not exceed a certain level, future yields are not diminished and the common good as such is being preserved as a 'sustainable' level.
If access to the common good is regulated at the community level by restricting exploitation to community members and by imposing limits to the quantity of goods being withdrawn from the common good, the tragedy of the commons may be avoided. Common goods that are sustained through an institutional arrangement of this kind are referred to as common-pool resources.
