The study of complex realities, in which the whole is notoriously more than the sum of the parts, compels to go beyond the traditional analytic method based on the separated study of the different parts of an object. On the contrary, the systemic approach put in the foreground the study of the interaction between the parts and among these and its environment.
        Through this study, you find that certain relation appear repeatedly in systems of different nature. The view on the structure of the relations over the nature of the involved systems carry us to the construction of General Systems: you can consider a General System as a class of Particular Systems with the same structure of relations, so that any of them can be taken as a model of the rest.
        Different Theories for different General Systems are so built. These Theories can have a mathematical form, because to take the abstract mathematical systems of its relations as a representative of the class is usual. But its content is not only formal, but it refers to the materiality of the common properties of the Particular Systems of that class.
        Now then, we can also build a General Systems Theory as a General Theory of Systems for the systematic treatment of the properties of any General System. This will be a formal mathematical theory, without specific material content.
        A General Systems Theory, ideally applicable to any real or imaginable system, would have to be able to treat systems with any number of variables (even with infinite variables), of continuous or discrete character.  So, for example, according to Mesarovic, a System is any subset of a generalized cartesian product (perhaps we will have to resort to the Axiom of Election to its construction).
        The importance of the interactions in the systemic approach will make that we are interested in distinguish between the variables of input generated by the environment and the variables of output generated by the System under consideration.
        In some cases, the value of the variables of output will univocally depend on the value of the variables of input. But, normally, these will be trivial cases which would have to be treated without using the Systems Theory. In other case, the different outputs with the same input will be able to be explained by the existence of different internal states of the System. And the change of these internal states carry us to take under consideration the temporal transition, these processes would be either deterministic or probabilistic .
        In the cases of more systemic interest, the output of a System reacts on its input, through a loop
of feedback which produces a non linear process. Therefore, the derived processes of regulation
and equilibrium which are usual in live or electronic open systems are of special interest of the
General System Theory.