A system is a delineated part of the universe which is distinguished from the rest by an imaginary boundary.
One of the basic concepts in the systems approach is that all systems interact with their environment. How can we then identify what a system is? Aren't we always making an artificial boundary? In order to perceive or know anything, one must make a distinction. The key idea of "system" is that once a system is identified (the boundary described) then one describes:
Thus, it is not necessary to assume that a system is isolated from or independent of the environment, it is simply a task of the describer to identify the way in which the system is interdependent with the environment.
Generally, a useful definition of a system is one that makes the interactions or relationships with the environment weaker or simpler than a different definition. In some cases one can usefully begin by studying the system in isolation, in others one is focused first on the interactions / relationships.
Often the identification of a system arises from a specified region of space. This region of space can change over time. The identification of the region of space and the way it changes over time is the definition of the system. For example, a physical picture of a person considers the person as a physical region of space delineated by a dynamic surface (roughly corresponding to the skin surface) which moves through space. However, it is also possible to identify a system in ways which do not correspond to a spatial separation. For example, one might consider the economic system of a society even though it overlaps spatially with other aspects of the society.
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