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There are a number of branches of knowledge that view systems as objects of their attention. For example, there are theories of open, stochastic, antagonistic, simple, complex, self-organizing, and other systems. But all these are special cases of the  theory of systems.
 "The system is a set of interacting elements," said von Bertalanffy, one of the founders of modern theory of systems, emphasizing that the system is a structure in which the elements somehow act on each other (interact).
Is this definition sufficient to distinguish a system from a non-system? Obviously not, because in any structure its elements one way or another, passively or actively act on each other (press, push, attract, induce, heat, act on nerves, nervous, deceive, absorb, obstruct light, etc.). Any set of elements always works in one way or another and it is impossible to find an object that does not perform any actions. However, these actions can be random, without a goal, although accidentally and not predictably they can contribute to the achievement of any goal. For example, a fork triggered by a mischievous grandson can get a grandmother into the eye and tear off the old thorn, but in such a way that the eye itself will not be damaged and its vision restored (the case described in the novel is theoretically possible). In this case, although a beneficial effect was obtained, the fork in combination with the grandson is not a system for removing the thorn, and this strange incident was accidental and unpredictable. Thus, although the sign of action is the main one, it determines not one concept of the system, but one of the necessary conditions for this concept.
"The system is an isolated part, a fragment of the world, of the universe, possessing a special quality (emergence) relative self-sufficiency (thermodynamic isolation)," said Etkins P.
But any object is a part or a fragment of the world, the universe, and each object differs from the others in some special quality (emergence*), including its location, time of existence, etc. And each object is to some extent thermodynamically independent, as well as dependent from his entourage. Consequently, this definition also determines not the system itself, but some of the consequences of systemic nature.
A full definition of the concept of "system" probably is not because the role of the concept "purpose" has been underestimated. Any properties of systems, in the final analysis, are connected with the concept of a purpose, because any system differs from other systems by the constancy of its actions. And her desire to maintain this constancy is the distinctive quality of any system - her purpose.
In ancient philosophy, the doctrine of purpose developed Aristotle, who interpreted the goal as "something for what" something exists. Spreading the idea of ​​the purpose, characteristic of human activity, on nature, Aristotle interpreted the goal as the ultimate cause of being (causa finalis).
In medieval philosophy, the true purpose of being was seen in the purpose of the eternal divine intelligence; Theological interpretation of history and nature prevailed as carrying out the divine purpose (theology).
Today, the purpose is treated as one of the elements of human behavior and conscious activity that characterizes the anticipation in the thinking of the result of activity and the ways of its realization with the help of certain means. The purpose is a way of integrating various human actions into a certain sequence or system. Thus, the purpose is treated as a purely human factor, inherent only to man.
It remains nothing - to tighten the notion of "purpose" not only to the psychological activity of man, but also to the very concept of "system", because the main distinguishing feature of any system is its purpose for any purpose. Any system is always designed for something, purposeful and serves for some specific purpose. Therefore, the goal is put not only before the person, but also before each system, regardless of its complexity.
The goal is a system-forming factor, not signs of action, emergence or anything else. There are no systems without a purpose and to achieve this purpose, a group of elements is combined into a system and acts. Purposefulness is determined by the question: - "What can this object do?".
A system is a complex of selectively involved elements that contribute to the achievement of a given useful result, which is accepted as the main systemically forming factor.

[Emergence](https://en.wikipedia.org/wiki/Emergence)*