Unfortunately, all of the implications of any action must be known. Each sub-system that provides a product for the end result must be sustainable as well. It becomes difficult to understand all the components of any action that consumes resources. When a resource is used, and the use of that resource is not accounted for in the economics of the transaction, an externality is created. In order to take actions that are fair and equitable for all parties, the externalities must be accounted for.
Sustainable development is the incorporation of externalities into an economic transaction.
When these externalities are accounted for in the economic transaction, then the true cost of actions can be determined. Permaculture attempts to be conservative in its accounting for these externalities. Assuming that deleterious side-effects will be present, until it is known otherwise, actions are taken with the presumption that minimal impact on the evolved ecosystems is the appropriate place to begin. This means that the impacts of individual persons actions is strongly concentrated. People in urban areas are far removed from visualizing the effects of their actions on the evolved ecosystem, accustomed instead to satisfying their daily needs with economic transactions involving considerable externalities.