The evolution of system theory exhibits three main phases of development. The first phase in the evolution of the theory of systems depends heavily upon ideas developed within physiology. Homeostasis. in particular is the guiding idea: a system is a dynamical whole able to maintain its working conditions.
In order to define a system, one needs (1) components; (2) mutual interactions; (3) the environment in which the system is situated; and (4) a boundary distinguishing the system from its environment.
The main intuition behind this first understanding of dynamic systems is well expressed by the following passage: “The most general and fundamental property of a system is the interdependence of parts or variables. Interdependence consists in the existence of determinate relationships among the parts or variables as contrasted with randomness of variability. In other words, interdependence is order in the relationship among the components which enter into a system. This order must have a tendency to self-maintenance, which is very generally expressed in the concept of equilibrium. It need not, however, be a static self-maintenance or a stable equilibrium. It may be an ordered process of change, a process following a determinate pattern rather than random variability relative to the starting point. This is called a moving equilibrium and is well exemplified by growth” (T. Parsons, The Social System, New York, Free Press, 1951, p. 107).