Constructing a System Theory (ST) is a method to establish a logical, mathematical, self-consistent, self-existing, coherent model to explain the interactions of the elements, functions and development of a closed or open system. System Theory (ST) is very important to define, organize, evaluate, control, regulate the systems and form mathematical models in a set of elements of that particular system. General Systems Theory (GST) is a name which has been adopted to describe a level of theoretical model-building which lies somewhere between the highly generalized constructions of pure mathematics & logic and the specific theories of the specialized disciplines. An ST can be universal, perfect, imperfect or defective; while the defective STs cannot survive. GST is a series of related definitions, assumptions, and postulates about all levels of systems from atomic particles through atoms, molecules, crystals, viruses, cells, organs, individuals, small groups, companies, societies, planets, solar systems, and galaxies. General Behavior ST is a subcategory of such a theory, dealing with living systems, extending roughly from viruses through societies. A significant fact about living things is that they are open systems, with important inputs and outputs. Laws which apply to them differ from those applying to relatively closed systems. Ludwig von Bertalanffy, the founder of ST, described two types of systems: open systems and closed systems. The open systems are systems that allow interactions between its internal elements and the environment. An open system, like space, is defined as a "system in exchange of matter and energy with its environment, presenting import and export, building-up and breaking-down of its material components." Closed systems, on the other hand, are held to be isolated from their environment. Equilibrium thermodynamics, for example, is a field of study that applies to closed systems; so are the biological cellular structures and neuroscience systems. Brain and central nervous system (CNS) are also closed systems. Establishing, for instance, an ST on CNS, will help us to use that ST not only in neuroscience, to explain the interactions of neurons, but also it will be a good aid to make new models in many other fields such as, biology, computer science, electronics, and social sciences etc., as well.