Systems theory usually is the interdisciplinary study of complex structures that may occur in nature, society, or science. Examples of complex structures can vary widely, entailing everything from ant colonies to political parties to languages. Systems theorists may analyze how such systems are formed, how they function, or what is the intended goal of a system.
The field of systems theory emerged as a corollary of biological studies. Following the analysis of ecological systems in nature, system theorists applied the basic logic of systems to non-ecological phenomenon. Pioneers in the field, such as the Hungarian scientist Bela H. Banathy, argued that a clear definition of the concept of system is integral to establishing the foundations of any science, while concomitantly determining the possibility of its growth.
Many system theorists have attempted to develop a general theory that would explain the function of any conceivable system. This approach would explicate systems as diverse as a beehive or a government according to the same general principles and laws. Such a hypothesis infers that there is a basic logic integral to all systems.
According to the debate in the academic literature over what exactly constitutes a system, many different approaches to systems theory have been proposed. Commonly, the key characteristics of a system have been identified as the relationship between perceived individual parts to form a holistic entity. This approach defines the consistency of a system according to uniform sets of relations and patterns of behavior, also known as a closed system. For example, the system of an ant colony is constituted by the various patterns of behavior of the specific ants that are its members. These differing behaviors will in turn be related to each other, yielding the notion of an overall homogeneity of a system defined in terms of its consistent function.
Some academics oppose the notion of the closed system with the concept of the open system. They argue that the definition of any closed system involves a fallacy of reduction that simplifies the nature of a system. In contrast, open system theorists emphasize that many times systems can be defined in terms of what they exclude, as opposed to what they include, or that parts, which are not viewed as part of a system, may be integrated into it.
Concepts such as emergence also are an important part of systems theory. Emergence describes the phenomenon of how a simple set of relations between parts can create something entirely heterogeneous to these parts. Common examples of emergence include the Internet and the stock market.