All societies exist in a state of dynamic equilibrium with their environments: this relationship constitutes a system. Systems are infinitely complex and can be approached at any level. Thus, at a very simple level, a climatic change affecting the environment could lead a community to change its agricultural regime over a period of years but that change would probably lead to some readjustment in social organization and this might, in turn, create new demands which could only be met by making further changes to the agricultural practices. In this example a prime mover, climatic change, initiates a modification in the man:landscape system which sets up further reverberations around the system. In other words each changefeeds back into the system causing further change.