A set of things -people, cells, molecules, or whatever- interconnected in such a way that they produce their own patters or behavior over time. It can be buffered, constricted, triggered or driven by outside forces (…) We are complex systems—our own bodies are magnificent examples of integrated, interconnected, self-maintaining complexity. Every person we encounter, every organization, every animal, garden, tree, and forest is a complex system (…) A system isn’t just any old collection of things. A system is an interconnected set of elements that is coherently organized in a way that achieves something. If you look at that definition closely for a minute, you can see that a system must consist of three kinds of things: elements, interconnections, and a function or purpose (..) Is there anything that is not a system? Yes—a conglomeration without any particular interconnections or function (…) there is an integrity or wholeness about a system and an active set of mechanisms to maintain that integrity (…) Some interconnections in systems are actual physical flows (…) Many interconnections are flows of information—signals that go to decision points or action points within a system (…) System purposes need not be human purposes and are not necessarily those intended by any single actor within the system (…) Systems can be nested within systems (…) A system generally goes on being itself, changing only slowly if at all, even with complete substitutions of its elements—as long as its interconnections and purposes remain intact. If the interconnections change, the system may be greatly altered (…) To ask whether elements, interconnections, or purposes are most important in a system is to ask an un-systemic question. All are essential. All interact. All have their roles. But the least obvious part of the system, its function or purpose, is often the most crucial determinant of the system’s behavior.
Donella H. Meadows, Thinking in Systems: A primer, (Diana Wright, ed.). London; Sterling: VA
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