The systems approach integrates the analytic and the synthetic method, encompassing both holism and reductionism. It was first proposed under the name of "General System Theory" by the biologist Ludwig von Bertalanffy. von Bertalanffy noted that all systems studied by physicists are closed: they do not interact with the outside world. When a physicist makes a model of the solar system, of an atom, or of a pendulum, he or she assumes that all masses, particles, forces that affect the system are included in the model. It is as if the rest of the universe does not exist. This makes it possible to calculate future states with perfect accuracy, since all necessary information is known.
However, as a biologist von Bertalanffy knew that such an assumption is simply impossible for most practical phenomena. Separate a living organism from its surroundings and it will die shortly because of lack of oxygen, water and food. Organisms are open systems: they cannot survive without continuously exchanging matter and energy with their environment. The peculiarity of open systems is that they interact with other systems outside of themselves. This interaction has two components: input, that what enters the system from the outside, and output, that what leaves the system for the environment. In order to speak about the inside and the outside of a system, we need to be able to distinguish between the system itself and its environment. System and environment are in general separated by a boundary. For example, for living systems the skin plays the role of the boundary. The output of a system is in general a direct or indirect result from the input. What comes out, needs to have gotten in first. However, the output is in general quite different from the input: the system is not just a passive tube, but an active processor. For example, the food, drink and oxygen we take in, leave our body as urine, excrements and carbon dioxide. The transformation of input into output by the system is usually called throughput. This has given us all the basic components of a system as it is understood in systems theory (see Fig. )