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There are a few necessary properties of systems that need to be stated before proceeding. The most basic of these is that systems exhibit some degree of stability, or constancy. If they do not, it would not be possible to identify them as the same system over time. A system may be closed, which means that it is a self‑contained, self‑regulating entity that is insulated from, and does not interact with, other systems. Or, it may be open, or interactive. For open systems to be stable they must exhibit equilibrium through negative, or compensating, feedback, because if they do not, their form would change and the necessary property of stability over time would be lost. The "hunting" of servo‑mechanisms and the homeostasis of vegetative biological functions in animals are examples of open systems maintaining equilibrium through negative feedback.
The individual's psychological resources for coping with the social world‑what we call in everyday terms the person‑can be construed as a system in this sense. Analogies with systems in the social sciences usually concentrate on the behavior of thermodynamic systems. The second law of thermodynamics states that all closed systems are subject to increasing entropy. The entropy of a system is the measure of unavailable energy; energy that still exists but which is lost for the purpose of doing work. In thermodynamics, of course, the energy referred to is heat and the law can be roughly understood as the idea that all (hot and cold) material within a thermally insulated area will eventually come to have the same temperature. The more general version of this principle is that all closed systems are subject to loss of differentiation. A correspondence has been established between the entropy of a system and the loss of information in that system in the sense of information theory. So, this principle can also be taken to mean that the information in a closed system diminishes over time.
Schrödinger, applying the concept of entropy to living organisms, writes: "Thus a living organism continually increases its entropy‑or, as you might say, produces positive entropy‑and thus tends to approach the dangerous state of maximum entropy, which is death. It can only keep aloof from it, i.e., alive, by continually drawing from its environment negative entropy".